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A planet is a large, rounded
astronomical body An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists in the observable universe The observable universe is a Ball (mathematics), ball-shap ...
that is neither a
star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

star
nor its
remnant Remnant or remnants may refer to: Religion * Remnant (Bible), a recurring theme in the Bible * Remnant (Seventh-day Adventist belief), the remnant theme in the Seventh-day Adventist Church * ''The Remnant'' (newspaper), a traditional Catholic ...
. The best available theory of planet formation is the
nebular hypothesis The nebular hypothesis is the most widely accepted model in the field of cosmogony to explain the formation and evolution of the Solar System (as well as other planetary systems). It suggests the Solar System is formed from gas and dust orbiting t ...

nebular hypothesis
, which posits that an
interstellar cloud An interstellar cloud is generally an accumulation of gas, plasma (physics), plasma, and Cosmic dust, dust in Milky Way, our and other Galaxy, galaxies. Put differently, an interstellar cloud is a denser-than-average region of the interstellar me ...
collapses out of a
nebula A nebula ('cloud' or 'fog' in Latin; pl. nebulae, nebulæ or nebulas) is a distinct luminescent part of interstellar medium, which can consist of ionized, neutral or molecular hydrogen and also cosmic dust. Nebulae are often star-forming region ...

nebula
to create a young
protostar A protostar is a very young star that is still gathering mass from its parent molecular cloud. The protostellar phase is the earliest one in the process of stellar evolution. For a low-mass star (i.e. that of the Sun or lower), it lasts about 50 ...
orbited by a
protoplanetary disk A protoplanetary disk is a rotating circumstellar disc of dense gas and dust surrounding a stellar evolution, young newly formed star, a T Tauri star, or Herbig Ae/Be star. The protoplanetary disk may also be considered an accretion disk for th ...

protoplanetary disk
. Planets grow in this disk by the gradual accumulation of material driven by gravity, a process called accretion. The
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...

Solar System
has at least eight planets: the
terrestrial planet A terrestrial planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate Rock (geology), rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the S ...
s
Mercury
Mercury
,
Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an Inferior and superior planets, interior planet to Earth, Venus (like Mercury (pl ...

Venus
,
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large list of largest lakes and seas in the Solar System, volumes of water can be found throughout the Solar System, only water distributi ...

Earth
and
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin atmosph ...

Mars
, and the
giant planet The giant planets constitute a diverse type of planet much larger than Earth. They are usually primarily composed of low-boiling-point materials (volatiles), rather than Rock (geology), rock or other solid matter, but massive solid planets can ...
s
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...

Jupiter
,
Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...

Saturn
,
Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus (mythology), Uranus (Caelus), who, according to Greek mythology, was the great-grandfather of Ares (Mars (mythology), Mars), grandfather ...

Uranus
and
Neptune Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...

Neptune
. These planets each rotate around an axis tilted with respect to its orbital pole. All of them possess an
atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...

atmosphere
, although that of Mercury is tenuous, and some share such features as
ice cap In glaciology, an ice cap is a mass of ice that covers less than of land area (usually covering a highland area). Larger ice masses covering more than are termed ice sheets. Description Ice caps are not constrained by topographical feat ...
s,
season A season is a division of the year based on changes in weather, ecology, and the number of daylight hours in a given region. On Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While ...

season
s,
volcanism Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evi ...

volcanism
,
hurricane A tropical cyclone is a rapidly rotating storm, storm system characterized by a Low-pressure area, low-pressure center, a closed low-level atmospheric circulation, Beaufort scale, strong winds, and a spiral arrangement of thunderstorms tha ...

hurricane
s,
tectonics Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of orogeny, mountain building, the growth and behavior of the strong, old cores of con ...
, and even
hydrology Hydrology () is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and environmental watershed sustainability. A practitioner of hydrology is calle ...
. Apart from Venus and Mars, the Solar System planets generate
magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to t ...

magnetic field
s, and all except Venus and Mercury have
natural satellite A natural satellite is, in the most common usage, an astronomical object, astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially ref ...

natural satellite
s. The giant planets bear planetary rings, the most prominent being
those of Saturn
those of Saturn
. The word ''planet'' probably comes from the Greek '' planḗtai,'' meaning "wanderers". In antiquity, this word referred to the
Sun The Sun is the star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other st ...

Sun
,
Moon The Moon is Earth's only natural satellite. It is the List of natural satellites, fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth ( ...

Moon
, and five points of light visible by the naked eye that moved across the background of the stars—namely, Mercury, Venus, Mars, Jupiter and Saturn. Planets have historically had religious associations: multiple cultures identified celestial bodies with gods, and these connections with mythology and folklore persist in the schemes for naming newly discovered Solar System bodies. Earth itself was recognized as a planet when
heliocentrism Heliocentrism (also known as the Heliocentric model) is the astronomical model in which the Earth and planets revolve around the Sun at the center of the universe. Historically, heliocentrism was opposed to Geocentric model, geocentrism, which p ...
supplanted
geocentrism In astronomy, the geocentric model (also known as geocentrism, often exemplified specifically by the Ptolemaic system) is a superseded scientific theories, superseded description of the Universe with Earth at the center. Under most geocentric mo ...

geocentrism
during the 16th and 17th centuries. With the development of the
telescope A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally meaning only an optical instrument usin ...

telescope
, the meaning of ''planet'' broadened to include objects only visible with assistance: the
ice giant An ice giant is a giant planet composed mainly of elements heavier than hydrogen and helium, such as oxygen, carbon, nitrogen, and sulfur. There are two ice giants in the Solar System: Uranus and Neptune. In astrophysics and planetary science th ...
s Uranus and Neptune; Ceres and other bodies later recognized to be part of the
asteroid belt The asteroid belt is a torus In geometry, a torus (plural tori, colloquially donut or doughnut) is a surface of revolution generated by revolving a circle in three-dimensional space about an axis that is coplanarity, coplanar with the ...

asteroid belt
; and
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the S ...
, later found to be the largest member of the collection of icy bodies known as the
Kuiper belt The Kuiper belt () is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical unit, astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larg ...
. The discovery of other large objects in the Kuiper belt, particularly Eris, spurred debate about how exactly to define a planet. The
International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach ...

International Astronomical Union
(IAU) adopted a standard by which the four terrestrials and four giants qualify, placing Ceres, Pluto and Eris in the category of
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to pl ...
, although many planetary scientists have continued to apply the term ''planet'' more broadly. Further advances in astronomy led to the discovery of over five thousand planets outside the Solar System, termed
exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...

exoplanet
s. These include
hot Jupiter Hot Jupiters (sometimes called hot Saturns) are a class of gas giant A gas giant is a giant planet composed mainly of hydrogen and helium. Gas giants are also called failed stars because they contain the same basic elements as a star. Jupiter ...
s—giant planets that orbit close to their parent stars—like
51 Pegasi b 51 Pegasi b, officially named Dimidium , and formerly unofficially dubbed Bellerophon , is an extrasolar planet approximately away in the constellation of Pegasus (constellation), Pegasus. It was the first exoplanet to be discovered orbiting a m ...
,
super-Earth A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below those of the Solar System's ice giants, Uranus and Neptune, which are 14.5 and 17 times Earth's, respectively. The term "super-Earth" refers only to t ...
s like
Gliese 581c
Gliese 581c
that have masses in between that of Earth and Neptune; and planets smaller than Earth, like
Kepler-20e
Kepler-20e
. Multiple exoplanets have been found to orbit in the habitable zones of their stars, but Earth remains the only planet known to support
life Life is a quality that distinguishes matter that has biological processes, such as Cell signaling, signaling and self-sustaining processes, from that which does not, and is defined by the capacity for Cell growth, growth, reaction to Stimu ...

life
.


History

The idea of planets has evolved over its history, from the divine lights of antiquity to the earthly objects of the scientific age. The concept has expanded to include worlds not only in the Solar System, but in multitudes of other extrasolar systems. The consensus definition as to what counts as a planet vs. other objects orbiting the Sun has changed several times, previously encompassing
asteroid An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere ...

asteroid
s,
moons A natural satellite is, in the most common usage, an astronomical object, astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially ref ...

moons
, and
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to pl ...
s like
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the S ...

Pluto
, and there continues to be some disagreement today. The five
classical planet In classical antiquity, the seven classical planets or seven luminaries are the seven moving astronomical objects in the sky visible to the naked eye: the Moon, Mercury (planet), Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. The word ''pla ...
s of the
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...

Solar System
, being visible to the naked eye, have been known since ancient times and have had a significant impact on
mythology Myth is a folklore genre consisting of narratives that play a fundamental role in a society, such as foundational tales or origin myths. Since "myth" is widely used to imply that a story is not objectively true, the identification of a narra ...

mythology
,
religious cosmology Religious cosmology is an explanation of the origin, evolution, and eventual fate of the universe from a religious perspective. This may include beliefs on origin in the form of a creation myth, subsequent evolution, current organizational for ...
, and ancient
astronomy Astronomy () is a natural science that studies astronomical object, celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and chronology of the Universe, evolution. Objects of interest ...
. In ancient times, astronomers noted how certain lights moved across the sky, as opposed to the "
fixed stars In astronomy, fixed stars ( la, stellae fixae) is a term to name the full set of glowing points, astronomical objects actually and mainly stars, that appear not to move relative to one another against the darkness of the night sky in the backgro ...
", which maintained a constant relative position in the sky. Ancient Greeks called these lights (, "wandering stars") or simply (, "wanderers"), from which today's word "planet" was derived. In
ancient Greece Ancient Greece ( el, Ἑλλάς, Hellás) was a northeastern Mediterranean Sea, Mediterranean civilization, existing from the Greek Dark Ages of the 12th–9th centuries BC to the end of Classical Antiquity, classical antiquity ( AD 600), th ...
,
China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's List of countries and dependencies by population, most populous country, with a Population of China, population exceeding 1.4 billion, slig ...
,
Babylon ''Bābili(m)'' * sux, 𒆍𒀭𒊏𒆠 * arc, 𐡁𐡁𐡋 ''Bāḇel'' * syc, ܒܒܠ ''Bāḇel'' * grc-gre, Βαβυλών ''Babylṓn'' * he, בָּבֶל ''Bāvel'' * peo, 𐎲𐎠𐎲𐎡𐎽𐎢 ''Bābiru'' * elx, 𒀸𒁀𒉿𒇷 ''Babi ...

Babylon
, and indeed all pre-modern civilizations, it was almost universally believed that Earth was the and that all the "planets" circled Earth. The reasons for this perception were that stars and planets appeared to revolve around Earth each day and the apparently common-sense perceptions that Earth was solid and stable and that it was not moving but at rest.


Babylon

The first civilization known to have a functional theory of the planets were the
Babylonia Babylonia (; Akkadian: , ''māt Akkadī'') was an ancient Akkadian-speaking state and cultural area based in the city of Babylon in central-southern Mesopotamia (present-day Iraq and parts of Syria). It emerged as an Amorites, Amorite-ruled ...
ns, who lived in
Mesopotamia Mesopotamia ''Mesopotamíā''; ar, بِلَاد ٱلرَّافِدَيْن or ; syc, ܐܪܡ ܢܗܪ̈ܝܢ, or , ) is a historical region of Western Asia situated within the Tigris–Euphrates river system, in the northern part of the F ...

Mesopotamia
in the first and second millennia BC. The oldest surviving planetary astronomical text is the Babylonian
Venus tablet of Ammisaduqa The Venus tablet of Ammisaduqa ('' Enuma Anu Enlil'' Tablet 63) is the record of astronomical positions for Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large a ...

Venus tablet of Ammisaduqa
, a 7th-century BC copy of a list of observations of the motions of the planet Venus, that probably dates as early as the second millennium BC. The MUL.APIN is a pair of
cuneiform Cuneiform is a logo- syllabic script that was used to write several languages of the Ancient Middle East. The script was in active use from the early Bronze Age The Bronze Age is a historic period, lasting approximately from 3300 B ...
tablets dating from the 7th century BC that lays out the motions of the Sun, Moon, and planets over the course of the year. Late Babylonian astronomy is the origin of Western astronomy and indeed all Western efforts in the
exact science The exact sciences, sometimes called the exact mathematical sciences, are those sciences "which admit of absolute precision in their results"; especially the mathematical sciences. Examples of the exact sciences are mathematics, optics O ...
s. The ''
Enuma anu enlil Enuma Anu Enlil ( ,'' The Assyrian Dictionary'', volume 7 (I/J) – ''inūma'', The Oriental Institute, Chicago 1960, s. 160. ''When he gods Anu and Enlil Enlil, , "Lord f theWind" later known as Elil, is an ancient Mesopotamian god asso ...
'', written during the
Neo-Assyrian The Neo-Assyrian Empire was the fourth and penultimate stage of ancient Assyrian history and the final and greatest phase of Assyria as an independent state. Beginning with the accession of Adad-nirari II in 911 BC, the Neo-Assyrian Empire grew t ...
period in the 7th century BC, comprises a list of
omen An omen (also called ''portent'') is a phenomenon that is believed to prediction, foretell the future, often signifying the advent of change. It was commonly believed in ancient times, and still believed by some today, that omens bring divine ...
s and their relationships with various celestial phenomena including the motions of the planets.
Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an Inferior and superior planets, interior planet to Earth, Venus (like Mercury (pl ...

Venus
, , and the outer planets
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin atmosph ...

Mars
,
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...

Jupiter
, and
Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...

Saturn
were all identified by Babylonian astronomers. These would remain the only known planets until the invention of the
telescope A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally meaning only an optical instrument usin ...

telescope
in early modern times.


Greco-Roman astronomy

The ancient Greeks initially did not attach as much significance to the planets as the Babylonians. The
Pythagoreans Pythagoreanism originated in the 6th century BC, based on and around the teachings and beliefs held by Pythagoras and his followers, the Pythagoreans. Pythagoras established the first Pythagorean community in the Ancient Greece, ancient Greek col ...
, in the 6th and 5th centuries BC appear to have developed their own independent planetary theory, which consisted of the Earth, Sun, Moon, and planets revolving around a "Central Fire" at the center of the Universe.
Pythagoras Pythagoras of Samos ( grc, Πυθαγόρας ὁ Σάμιος, Pythagóras ho Sámios, Pythagoras the Samos, Samian, or simply ; in Ionian Greek; ) was an ancient Ionians, Ionian Ancient Greek philosophy, Greek philosopher and the eponymou ...
or
Parmenides Parmenides of Elea (; grc-gre, Παρμενίδης ὁ Ἐλεάτης; ) was a pre-Socratic Greek philosopher from Elea in Magna Graecia. Parmenides was born in the Greek colony of Elea, from a wealthy and illustrious family. His date ...
is said to have been the first to identify the evening star ( Hesperos) and morning star (
Phosphoros Phosphorus () is one of the ''Astra Planeta'', specifically the god of the planet Venus in its appearance as the Morning Star. Another Greek name for the Morning Star is "Eosphorus" ( grc, Ἑωσφόρος, Heōsphoros, link=no), which means ...
) as one and the same (
Aphrodite Aphrodite ( ; grc-gre, Ἀφροδίτη, Aphrodítē; , , ) is an ancient Greek religion, ancient Greek goddess associated with love, lust, beauty, pleasure, passion (emotion), passion, and procreation. She was syncretized with the Roman god ...
, Greek corresponding to Latin
Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an Inferior and superior planets, interior planet to Earth, Venus (like Mercury (pl ...

Venus
), though this had long been known in Mesopotamia. In the 3rd century BC,
Aristarchus of Samos Aristarchus of Samos (; grc-gre, Ἀρίσταρχος ὁ Σάμιος, ''Aristarkhos ho Samios''; ) was an ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, anci ...
proposed a
heliocentric Heliocentrism (also known as the Heliocentric model) is the astronomical model in which the Earth and planets revolve around the Sun at the center of the universe. Historically, heliocentrism was opposed to Geocentric model, geocentrism, which p ...
system, according to which Earth and the planets revolved around the Sun. The geocentric system remained dominant until the
Scientific Revolution The Scientific Revolution was a series of events that marked the emergence of modern science during the early modern period, when developments in mathematics Mathematics is an area of knowledge that includes the topics of numbers, fo ...
. By the 1st century BC, during the
Hellenistic period In Classical antiquity, the Hellenistic period covers the time in History of the Mediterranean region, Mediterranean history after Classical Greece, between the death of Alexander the Great in 323 BC and the emergence of the Roman Empire, as sig ...
, the Greeks had begun to develop their own mathematical schemes for predicting the positions of the planets. These schemes, which were based on geometry rather than the arithmetic of the Babylonians, would eventually eclipse the Babylonians' theories in complexity and comprehensiveness, and account for most of the astronomical movements observed from Earth with the naked eye. These theories would reach their fullest expression in the ''
Almagest The ''Almagest'' is a 2nd-century Greek-language mathematical and astronomical treatise on the apparent motions of the stars and planetary paths, written by Claudius Ptolemy ( ). One of the most influential scientific texts in history, it ca ...
'' written by
Ptolemy Claudius Ptolemy (; grc-gre, wikt:Πτολεμαῖος, Πτολεμαῖος, ; la, Claudius Ptolemaeus; AD) was a mathematician, astronomer, astrologer, geographer, and music theorist, who wrote about a dozen scientific Treatise, treatis ...
in the 2nd century CE. So complete was the domination of Ptolemy's model that it superseded all previous works on astronomy and remained the definitive astronomical text in the Western world for 13 centuries. To the Greeks and Romans there were seven known planets, each presumed to be circling Earth according to the complex laws laid out by Ptolemy. They were, in increasing order from Earth (in Ptolemy's order and using modern names): the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. ''Note: select the Etymology tab ''


Medieval astronomy

After the
fall of the Western Roman Empire The fall of the Western Roman Empire (also called the fall of the Roman Empire or the fall of Ancient Rome, Rome) was the loss of central political control in the Western Roman Empire, a process in which the Empire failed to enforce its rul ...
, astronomy developed further in India and the medieval Islamic world. In 499 CE, the Indian astronomer
Aryabhata Aryabhata (ISO 15919, ISO: ) or Aryabhata I (476–550 Common Era, CE) was an Indian mathematician and astronomer of the classical age of Indian mathematics and Indian astronomy. He flourished in the Gupta era, Gupta Era and produced works su ...
propounded a planetary model that explicitly incorporated
Earth's rotation Earth's rotation or Earth's spin is the rotation of planet Earth around its own Rotation around a fixed axis, axis, as well as changes in the orientation (geometry), orientation of the rotation axis in space. Earth rotates eastward, in retrograd ...
about its axis, which he explains as the cause of what appears to be an apparent westward motion of the stars. He also theorised that the orbits of planets were elliptical. Aryabhata's followers were particularly strong in
South India South India, also known as Dakshina Bharata or Peninsular India, consists of the peninsular southern part of India. It encompasses the States and union territories of India, Indian states of Andhra Pradesh, Karnataka, Kerala, Tamil Nadu, and T ...
, where his principles of the diurnal rotation of Earth, among others, were followed and a number of secondary works were based on them. The astronomy of the
Islamic Golden Age The Islamic Golden Age was a period of cultural, economic, and scientific flourishing in the history of Islam, traditionally dated from the 8th century to the 14th century. This period is traditionally understood to have begun during the reign ...
mostly took place in the
Middle East The Middle East ( ar, الشرق الأوسط, ISO 233: ) is a geopolitical region commonly encompassing Arabian Peninsula, Arabia (including the Arabian Peninsula and Bahrain), Anatolia, Asia Minor (Asian part of Turkey except Hatay Pro ...
,
Central Asia Central Asia, also known as Middle Asia, is a region of Asia Asia (, ) is one of the world's most notable geographical regions, which is either considered a continent in its own right or a subcontinent of Eurasia, which shares the c ...
,
Al-Andalus Al-Andalus translit. ; an, al-Andalus; ast, al-Ándalus; eu, al-Andalus; ber, ⴰⵏⴷⴰⵍⵓⵙ, label=Berber, translit=Andalus; ca, al-Àndalus; gl, al-Andalus; oc, Al Andalús; pt, al-Ândalus; es, al-Ándalus () was the Mus ...
, and
North Africa North Africa, or Northern Africa is a region encompassing the northern portion of the African continent. There is no singularly accepted scope for the region, and it is sometimes defined as stretching from the Atlantic shores of Mauritania in t ...
, and later in the
Far East The ''Far East'' was a European term to refer to the geographical regions that includes East and Southeast Asia Southeast Asia, also spelled South East Asia and South-East Asia, and also known as Southeastern Asia, South-eastern Asia or S ...
and India. These astronomers, like the polymath
Ibn al-Haytham Ḥasan Ibn al-Haytham, Latinization of names, Latinized as Alhazen (; full name ; ), was a medieval Mathematics in medieval Islam, mathematician, Astronomy in the medieval Islamic world, astronomer, and Physics in the medieval Islamic world, ...
, generally accepted geocentrism, although they did dispute Ptolemy's system of epicycles and sought alternatives. The 10th-century astronomer Abu Sa'id al-Sijzi accepted that the Earth rotates around its axis. In the 11th century, the
transit of Venus file:Venus transit symbol.svg, frameless, upright=0.5 A transit of Venus across the Sun takes place when the planet Venus passes directly between the Sun and a inferior and superior planets, superior planet, becoming visible against (and hence ...
was observed by
Avicenna Ibn Sina ( fa, ابن سینا; 980 – June 1037 CE), commonly known in the West as Avicenna (), was a Persians, Persian polymath who is regarded as one of the most significant physicians, astronomers, philosophers, and writers of the ...
. His contemporary
Al-Biruni Abu Rayhan Muhammad ibn Ahmad al-Biruni (973 – after 1050) commonly known as al-Biruni, was a Khwarazmian Iranian peoples, Iranian in scholar and polymath during the Islamic Golden Age. He has been called variously the "founder of Indolog ...
devised a method of determining the Earth's radius using
trigonometry Trigonometry () is a branch of mathematics that studies relationships between side lengths and angles of triangles. The field emerged in the Hellenistic period, Hellenistic world during the 3rd century BC from applications of geometry to Astr ...
that, unlike the older method of
Eratosthenes Eratosthenes of Cyrene (; grc-gre, wikt:Ἐρατοσθένης, Ἐρατοσθένης ;  – ) was a Greek polymath: a Greek mathematics, mathematician, geographer, poet, astronomer, and music theory, music theorist. He was a man of lear ...
, only required observations at a single mountain.


Scientific Revolution and new planets

With the advent of the
Scientific Revolution The Scientific Revolution was a series of events that marked the emergence of modern science during the early modern period, when developments in mathematics Mathematics is an area of knowledge that includes the topics of numbers, fo ...
and the
heliocentric model Heliocentrism (also known as the Heliocentric model) is the astronomical model in which the Earth and planets revolve around the Sun at the center of the universe. Historically, heliocentrism was opposed to Geocentric model, geocentrism, which p ...
of
Copernicus Nicolaus Copernicus (; pl, Mikołaj Kopernik; gml, Niklas Koppernigk, german: Nikolaus Kopernikus; 19 February 1473 – 24 May 1543) was a Renaissance The Renaissance ( , ) , from , with the same meanings. is a Periodization, ...
,
Galileo Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...
and
Kepler Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe ...
, use of the term "planet" changed from something that moved around the sky relative to the fixed star to a body that orbited the Sun, directly (a primary planet) or indirectly (a secondary or satellite planet). Thus the Earth was added to the roster of planets and the Sun was removed. The Copernican count of primary planets stood until 1781, when
William Herschel Frederick William Herschel (; german: Friedrich Wilhelm Herschel; 15 November 1738 – 25 August 1822) was a German-born British astronomer and composer. He frequently collaborated with his younger sister and fellow astronomer Caroline H ...
discovered
Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus (mythology), Uranus (Caelus), who, according to Greek mythology, was the great-grandfather of Ares (Mars (mythology), Mars), grandfather ...

Uranus
. When four satellites of Jupiter (the
Galilean moons The Galilean moons (), or Galilean satellites, are the four largest moons of Jupiter: Io (moon), Io, Europa (moon), Europa, Ganymede (moon), Ganymede, and Callisto (moon), Callisto. They were first seen by Galileo Galilei in December 1609 or Jan ...
) and five of Saturn were discovered in the 17th century, they were thought of as "satellite planets" or "secondary planets" orbiting the primary planets, though in the following decades they would come to be called simply "satellites" for short. Scientists generally considered planetary satellites to also be planets until about the 1920s, although this usage was not common among non-scientists. In the first decade of the 19th century, four new planets were discovered: Ceres (in 1801), Pallas (in 1802), Juno (in 1804), and Vesta (in 1807). It soon became apparent that they were rather different from previously known planets: they shared the same general region of space, between Mars and Jupiter (the
asteroid belt The asteroid belt is a torus In geometry, a torus (plural tori, colloquially donut or doughnut) is a surface of revolution generated by revolving a circle in three-dimensional space about an axis that is coplanarity, coplanar with the ...

asteroid belt
), with sometimes overlapping orbits. This was an area where only one planet had been expected, and they were much much smaller than all other planets; indeed, it was suspected that they might be shards of a larger planet that had broken up. Herschel called them ''
asteroid An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere ...

asteroid
s'' (from the Greek for "starlike") because even in the largest telescopes they resembled stars, without a resolvable disk. The situation was stable for four decades, but in the mid-1840s several additional asteroids were discovered ( Astraea in 1845, Hebe in 1847,
Iris Iris most often refers to: *Iris (anatomy) In humans and most mammals and birds, the iris (plural: ''irides'' or ''irises'') is a thin, annular structure in the eye, responsible for controlling the diameter and size of the pupil, and thus the a ...
in 1847,
Flora Flora is all the plant life present in a particular region or time, generally the naturally occurring (indigenous (ecology), indigenous) native plant, native plants. Sometimes bacteria and fungi are also referred to as flora, as in the terms '' ...
in 1848, Metis in 1848, and Hygiea in 1849), and soon new "planets" were discovered every year. As a result, astronomers began tabulating the asteroids (
minor planet According to the International Astronomical Union (IAU), a minor planet is an astronomical object in direct orbit around the Sun that is exclusively classified as neither a planet nor a comet. Before 2006, the IAU officially used the term ''minor ...
s) separately from the major planets, and assigning them numbers instead of abstract planetary symbols, although they continued to be considered as small planets. Neptune was discovered in 1846, its position having been predicted thanks to its gravitational influence upon Uranus. Because the orbit of Mercury appeared to be affected in a similar way, it was believed in the late 19th century that there might be another planet even closer to the Sun. However, the discrepancy between Mercury's orbit and the predictions of Newtonian gravity was instead explained by an improved theory of gravity, Einstein's
general relativity General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the differential geometry, geometric scientific theory, theory of gravitation published by Albert Einstein in 1915 and is the current descr ...
.


20th century

Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the S ...

Pluto
was discovered in 1930. After initial observations led to the belief that it was larger than Earth, the object was immediately accepted as the ninth major planet. Further monitoring found the body was actually much smaller: in 1936, Ray Lyttleton suggested that Pluto may be an escaped satellite of
Neptune Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...

Neptune
, and Fred Whipple suggested in 1964 that Pluto may be a comet. The discovery of its large moon
Charon In Greek mythology, Charon or Kharon (; grc, Χάρων) is a psychopomp, the ferryman of Greek underworld, Hades, the Greek underworld. He carries the souls of those who have been given funeral rites across the rivers Acheron and Styx, which ...
in 1978 showed that Pluto was only 0.2% the mass of Earth. As this was still substantially more massive than any known asteroid, and because no other
trans-Neptunian objects A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has a semi-major axis of 30.1 astronomical units (au). Typically, ...
had been discovered at that time, Pluto kept its planetary status, only officially losing it in 2006. In the 1950s,
Gerard Kuiper Gerard Peter Kuiper (; ; born Gerrit Pieter Kuiper; 7 December 1905 – 23 December 1973) was a Dutch astronomer, planetary scientist, selenographer, author and professor. He is the eponymous namesake of the Kuiper belt. Kuiper is co ...
published papers on the origin of the asteroids. He recognised that asteroids were typically not spherical, as had previously been thought, and that the
asteroid families An asteroid is a minor planet According to the International Astronomical Union (IAU), a minor planet is an astronomical object in direct orbit around the Sun that is exclusively classified as neither a planet nor a comet. Before 2006, the I ...
were remnants of collisions. Thus he differentiated between the largest asteroids as "true planets" versus the smaller ones as collisional fragments. From the 1960s onwards, the term "minor planet" was mostly displaced by the term "asteroid", and references to the asteroids as planets in the literature became scarce, except for the geologically evolved largest three: Ceres, and less often Pallas and Vesta. The beginning of Solar System exploration by space probes in the 1960s spurred a renewed interest in planetary science. A split in definitions regarding satellites occurred around then: planetary scientists began to reconsider the large moons as also being planets, but astronomers who were not planetary scientists generally did not. In 1992, astronomers Aleksander Wolszczan and Dale Frail announced the discovery of planets around a
pulsar A pulsar (from ''pulsating radio source'') is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its Poles of astronomical bodies#Magnetic poles, magnetic poles. This radiation can be observed only w ...
,
PSR B1257+12 PSR B1257+12, previously designated PSR 1257+12, alternatively designated PSR J1300+1240, is a millisecond pulsar located 2,300 light-years from the Sun in the constellation of Virgo (constellation), Virgo, rotating at about 161 times per ...
. This discovery is generally considered to be the first definitive detection of a planetary system around another star. Then, on 6 October 1995,
Michel Mayor Michel Gustave Édouard Mayor (; born 12 January 1942) is a Switzerland, Swiss astrophysicist and professor emeritus at the University of Geneva's Department of Astronomy. He formally retired in 2007, but remains active as a researcher at the O ...
and
Didier Queloz Didier Patrick Queloz (; born 23 February 1966) is a Swiss astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical ob ...
of the
Geneva Observatory The Geneva Observatory (french: Observatoire de Genève, german: Observatorium von Genf) is an astronomical observatory at Sauverny (CH) in the municipality of Versoix, Canton of Geneva, in Switzerland. It shares its buildings with the astronomy d ...
announced the first definitive detection of an exoplanet orbiting an ordinary
main-sequence In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color index, color versus absolute magnitude, brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams afte ...
star (
51 Pegasi 51 Pegasi (abbreviated 51 Peg), formally named Helvetios , is a Solar analog, Sun-like star located from Earth in the constellation of Pegasus (constellation), Pegasus. It was the first main sequence, main-sequence star found to have an ...
). The discovery of extrasolar planets led to another ambiguity in defining a planet: the point at which a planet becomes a star. Many known extrasolar planets are many times the mass of Jupiter, approaching that of stellar objects known as
brown dwarf Brown dwarfs (also called failed stars) are substellar objects that are not massive enough to sustain nuclear fusion of ordinary hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen ...
s. Brown dwarfs are generally considered stars due to their theoretical ability to fuse
deuterium Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two Stable isotope ratio, stable isotopes of hydrogen (the other being Hydrogen atom, protium, or hydrogen-1). The atomic nucleus, nucleus of a deuterium ato ...
, a heavier isotope of
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the chemical ...
. Although objects more massive than 75 times that of Jupiter fuse simple hydrogen, objects of 13 Jupiter masses can fuse deuterium. Deuterium is quite rare, constituting less than 0.0026% of the hydrogen in the galaxy, and most brown dwarfs would have ceased fusing deuterium long before their discovery, making them effectively indistinguishable from supermassive planets.


21st century

With the discovery during the latter half of the 20th century of more objects within the Solar System and large objects around other stars, disputes arose over what should constitute a planet. There were particular disagreements over whether an object should be considered a planet if it was part of a distinct population such as a belt, or if it was large enough to generate energy by the
thermonuclear fusion Thermonuclear fusion is the process of atomic nuclei combining or “fusing” using high temperatures to drive them close enough together for this to become possible. There are two forms of thermonuclear fusion: ''uncontrolled'', in which the re ...
of
deuterium Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two Stable isotope ratio, stable isotopes of hydrogen (the other being Hydrogen atom, protium, or hydrogen-1). The atomic nucleus, nucleus of a deuterium ato ...
. Complicating the matter even further, bodies too small to generate energy by fusing deuterium can form by gas-cloud collapse just like stars and brown dwarfs, even down to the mass of Jupiter: there was thus disagreement about whether how a body formed should be taken into account. A growing number of astronomers argued for Pluto to be declassified as a planet, because many similar objects approaching its size had been found in the same region of the Solar System (the
Kuiper belt The Kuiper belt () is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical unit, astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larg ...
) during the 1990s and early 2000s. Pluto was found to be just one small body in a population of thousands. They often referred to the demotion of the asteroids as a precedent, although that had been done based on their geophysical differences from planets rather than their being in a belt. Some of the larger
trans-Neptunian object A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has a semi-major axis of 30.1 astronomical units (au). Typically, ...
s, such as Quaoar, Sedna, Eris, and
Haumea Haumea (minor-planet designation 136108 Haumea) is a dwarf planet located trans-Neptunian object, beyond Neptune's orbit. It was discovered in 2004 by a team headed by Michael E. Brown, Mike Brown of Caltech at the Palomar Observatory in the United ...
were heralded in the popular press as the tenth planet. The announcement of Eris in 2005, an object 27% more massive than Pluto, created the impetus for an official definition of a planet, as considering Pluto a planet would logically have demanded that Eris be considered a planet as well. Since different procedures were in place for naming planets versus non-planets, this created an urgent situation because under the rules Eris could not be named without defining what a planet was. At the time, it was also thought that the size required for a trans-Neptunian object to become round was about the same as that required for the moons of the giant planets (about 400 km diameter), a figure that would have suggested about 200 round objects in the Kuiper belt and thousands more beyond. Many astronomers argued that the public would not accept a definition creating a large number of planets. To acknowledge the problem, the IAU set about creating the definition of planet, and produced one in August 2006. Their definition dropped to the eight significantly larger bodies that had cleared their orbit (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), and a new class of
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to pl ...
s was created, initially containing three objects (Ceres, Pluto and Eris). This definition has not been universally used or accepted. In
planetary geology Planetary geology, alternatively known as astrogeology or exogeology, is a planetary science discipline concerned with the geology of the celestial bodies such as the planets and their moons, asteroids, comets, and meteorites. Although ...
celestial objects have been assessed and defined as planets by geophysical characteristics. Planetary scientists are more interested in planetary geology than dynamics, so they classify planets based on their geological properties. A celestial body may acquire a dynamic (planetary) geology at approximately the mass required for its mantle to become plastic under its own weight. This leads to a state of
hydrostatic equilibrium In fluid mechanics Fluid mechanics is the branch of physics concerned with the mechanics of fluids (liquids, gases, and plasma (physics), plasmas) and the forces on them. It has applications in a wide range of disciplines, including mechanic ...
where the body acquires a stable, round shape, which is adopted as the hallmark of planethood by geophysical definitions. For example: See p. 208. In the Solar System, this mass is generally less than the mass required for a body to clear its orbit, and thus some objects that are considered "planets" under geophysical definitions are not considered as such under the IAU definition, such as Ceres and Pluto. Proponents of such definitions often argue that location should not matter and that planethood should be defined by the intrinsic properties of an object. Dwarf planets had been proposed as a category of small planet (as opposed to
planetoid According to the International Astronomical Union (IAU), a minor planet is an astronomical object in direct orbit around the Sun that is exclusively classified as neither a planet nor a comet. Before 2006, the IAU officially used the term ''minor ...
s as sub-planetary objects) and planetary geologists continue to treat them as planets despite the IAU definition. The number of dwarf planets even among known objects is not certain. In 2019, Grundy et al. argued based on the low densities of some mid-sized trans-Neptunian objects that the limiting size required for a trans-Neptunian object to reach equilibrium was in fact much larger than it is for the icy moons of the giant planets, being about 900 km diameter. There is general consensus on Ceres in the asteroid belt and on the eight trans-Neptunians that probably cross this threshold: Quaoar, Sedna, Orcus, Pluto,
Haumea Haumea (minor-planet designation 136108 Haumea) is a dwarf planet located trans-Neptunian object, beyond Neptune's orbit. It was discovered in 2004 by a team headed by Michael E. Brown, Mike Brown of Caltech at the Palomar Observatory in the United ...
, Eris,
Makemake Makemake (minor-planet designation 136472 Makemake) is a dwarf planet and – depending on how they are defined – the second-largest Kuiper belt object in the Classical Kuiper belt object, classical population, with a diameter approxim ...
, and
Gonggong Gonggong () is a Chinese water god who is depicted in Chinese mythology and folktales as having a copper human head with an iron forehead, red hair, and the body of a serpent, or sometimes the head and torso are human, with the tail of a serpen ...
. Planetary geologists may include the twenty known planetary-mass moons as "satellite planets", including Earth's Moon and Pluto's
Charon In Greek mythology, Charon or Kharon (; grc, Χάρων) is a psychopomp, the ferryman of Greek underworld, Hades, the Greek underworld. He carries the souls of those who have been given funeral rites across the rivers Acheron and Styx, which ...
, like the early modern astronomers. Some go even further and include relatively large, geologically evolved bodies that are nonetheless not very round today, such as Pallas and Vesta, or rounded bodies that were completely disrupted by impacts and re-accreted like Hygiea, as planets. The 2006 IAU definition presents some challenges for
exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...

exoplanet
s because the language is specific to the Solar System and the criteria of roundness and orbital zone clearance are not presently observable for exoplanets. There is no official definition of exoplanets, but the IAU's working group on the topic adopted a provisional statement in 2018. Astronomer
Jean-Luc Margot Jean-Luc Margot (born 1969) is a Belgian-born astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as st ...
proposed a mathematical criterion that determines whether an object can clear its orbit during the lifetime of its host star, based on the mass of the planet, its semimajor axis, and the mass of its host star. The formula produces a value called that is greater than 1 for planets. The eight known planets and all known exoplanets have values above 100, while Ceres, Pluto, and Eris have values of 0.1, or less. Objects with values of 1 or more are expected to be approximately spherical, so that objects that fulfill the orbital zone clearance requirement automatically fulfill the roundness requirement.


Definition and similar concepts

At the 2006 meeting of the IAU's General Assembly, after much debate and one failed proposal, the following definition was passed in a resolution voted for by a large majority of those remaining at the meeting, addressing particularly the issue of the lower limits for a celestial object to be defined as a planet. The 2006 resolution defines planets within the Solar System as follows: Under this definition, the Solar System is considered to have eight planets. Bodies that fulfill the first two conditions but not the third are classified as
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to pl ...
s, provided they are not
natural satellite A natural satellite is, in the most common usage, an astronomical object, astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially ref ...

natural satellite
s of other planets. Originally an IAU committee had proposed a definition that would have included a larger number of planets as it did not include (c) as a criterion. After much discussion, it was decided via a vote that those bodies should instead be classified as dwarf planets. This definition is based in modern theories of planetary formation, in which planetary embryos initially clear their orbital neighborhood of other smaller objects. As described below, planets form by material accreting together in a disk of matter surrounding a
protostar A protostar is a very young star that is still gathering mass from its parent molecular cloud. The protostellar phase is the earliest one in the process of stellar evolution. For a low-mass star (i.e. that of the Sun or lower), it lasts about 50 ...
. This process results in a collection of relatively substantial objects, each of which has either "swept up" or scattered away most of the material that had been orbiting near it. These objects do not collide with one another because they are too far apart, sometimes in
orbital resonance In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relationsh ...
.


Exoplanet

The 2006 IAU definition presents some challenges for exoplanets because the language is specific to the Solar System and the criteria of roundness and orbital zone clearance are not presently observable for exoplanets. The IAU working group on extrasolar planets (WGESP) issued a working definition in 2001 and amended it in 2003. In 2018, this definition was reassessed and updated as knowledge of exoplanets increased. The current official working definition of an exoplanet is as follows: The IAU noted that this definition could be expected to evolve as knowledge improves. A 2022 review article discussing the history and rationale of this definition suggested that the words "in young star clusters" should be deleted in clause 3, as such objects have now been found elsewhere, and that the term "sub-brown dwarfs" should be replaced by the more current "free-floating planetary mass objects".


Planetary-mass object

Geoscientists often reject the IAU definition, preferring to consider round moons and dwarf planets as also being planets. Some scientists who accept the IAU definition of "planet" use other terms for bodies satisfying geophysical planet definitions, such as "world". The term "planetary mass object" has also been used to refer to ambiguous situations concerning exoplanets, such as objects with mass typical for a planet that are free-floating or orbit a brown dwarf instead of a star.


Mythology and naming

The names for the planets in the Western world are derived from the naming practices of the Romans, which ultimately derive from those of the Greeks and the Babylonians. In
ancient Greece Ancient Greece ( el, Ἑλλάς, Hellás) was a northeastern Mediterranean Sea, Mediterranean civilization, existing from the Greek Dark Ages of the 12th–9th centuries BC to the end of Classical Antiquity, classical antiquity ( AD 600), th ...
, the two great luminaries the Sun and the Moon were called ''
Helios In ancient Greek religion and Greek mythology, mythology, Helios (; grc, , , Sun; Homeric Greek: ) is the deity, god and personification of the Sun (Solar deity). His name is also Latinized as Helius, and he is often given the epithets Hyper ...
'' and ''
Selene In ancient Greek mythology and Ancient Greek religion, religion, Selene (; grc-gre, wikt:Σελήνη, Σελήνη , meaning "Moon"''A Greek–English Lexicon's.v. σελήνη) is the goddess and the personification of the Moon. Also known as ...
'', two ancient
Titanic RMS ''Titanic'' was a British passenger Ocean liner, liner, operated by the White Star Line, which Sinking of the Titanic, sank in the North Atlantic Ocean on 15 April 1912 after striking an iceberg during her maiden voyage from Southampton ...
deities; the slowest planet (Saturn) was called '' Phainon'', the shiner; followed by ''
Phaethon Phaethon (; grc, Φαέθων, Phaéthōn, ), also spelled Phaëthon, was the son of the Oceanid Clymene (mother of Phaethon), Clymene and the solar deity, sun-god Helios in Greek mythology. According to most authors, Phaethon is the son of He ...
'' (Jupiter), "bright"; the red planet (Mars) was known as ''
Pyroeis Pyroeis ( grc, Πυρόεις) in ancient Greek religion is the god of the wandering star (''Aster Planetos'' / ''Astra Planeta'') Areios, the planet Mars. He is also known as Mesonyx (Μεσονυξ; "midnight"). His parents are Astraeus and Eos ...
'', the "fiery"; the brightest (Venus) was known as ''
Phosphoros Phosphorus () is one of the ''Astra Planeta'', specifically the god of the planet Venus in its appearance as the Morning Star. Another Greek name for the Morning Star is "Eosphorus" ( grc, Ἑωσφόρος, Heōsphoros, link=no), which means ...
'', the light bringer; and the fleeting final planet (Mercury) was called '' Stilbon'', the gleamer. The Greeks assigned each planet to one among their pantheon of gods, the Olympians and the earlier Titans: * Helios and Selene were the names of both planets and gods, both of them Titans (later supplanted by Olympians
Apollo Apollo, grc, Ἀπόλλωνος, Apóllōnos, label=genitive , ; , grc-dor, Ἀπέλλων, Apéllōn, ; grc, Ἀπείλων, Apeílōn, label=Arcadocypriot Greek Arcadocypriot, or southern Achaeans (tribe), Achaean, was an ancient ...
and
Artemis In ancient Greek mythology and Ancient Greek religion, religion, Artemis (; grc-gre, Ἄρτεμις) is the goddess of the hunt, the wilderness, wild animals, nature, vegetation, childbirth, Kourotrophos, care of children, and chastity. S ...
); * Phainon was sacred to
Cronus In Ancient Greek religion and Greek mythology, mythology, Cronus, Cronos, or Kronos ( or , from el, Κρόνος, ''Krónos'') was the leader and youngest of the first generation of Titans, the divine descendants of the Greek primordial de ...
, the Titan who fathered the Olympians; * Phaethon was sacred to
Zeus Zeus or , , ; grc, Δῐός, ''Diós'', label=Genitive case, genitive Aeolic Greek, Boeotian Aeolic and Doric Greek#Laconian, Laconian grc-dor, Δεύς, Deús ; grc, Δέος, ''Déos'', label=Genitive case, genitive el, Δίας, ''D ...
, Cronus's son who deposed him as king; * Pyroeis was given to
Ares Ares (; grc, Ἄρης, ''Árēs'' ) is the List of Greek mythological figures, Greek god of war god, war and courage. He is one of the Twelve Olympians, and the son of Zeus and Hera. The Greeks were ambivalent towards him. He embodies the p ...
, son of Zeus and god of war; * Phosphoros was ruled by
Aphrodite Aphrodite ( ; grc-gre, Ἀφροδίτη, Aphrodítē; , , ) is an ancient Greek religion, ancient Greek goddess associated with love, lust, beauty, pleasure, passion (emotion), passion, and procreation. She was syncretized with the Roman god ...
, the goddess of love; and * Stilbon with its speedy motion, was ruled over by
Hermes Hermes (; grc-gre, wikt:Ἑρμῆς, Ἑρμῆς) is an Olympian deity in ancient Greek religion and Greek mythology, mythology. Hermes is considered the herald of the gods. He is also considered the protector of human heralds, travelle ...
, messenger of the gods and god of learning and wit. The Greek practice of grafting their gods' names onto the planets was almost certainly borrowed from the Babylonians. The Babylonians named Venus after their goddess of love,
Ishtar Inanna, also sux, 𒀭𒊩𒌆𒀭𒈾, nin-an-na, label=none is an List of Mesopotamian deities, ancient Mesopotamian goddess of love, war, and fertility. She is also associated with beauty, sex, Divine law, divine justice, and political p ...
; Mars after their god of war,
Nergal Nergal (Sumerian language, Sumerian: Dingir, d''KIŠ.UNU'' or ; ; Aramaic Language, Aramaic: ܢܸܪܓܲܠ; la, Nirgal) was a List of Mesopotamian deities, Mesopotamian god worshiped through all periods of Mesopotamian history, from Early Dynast ...
; Mercury after their god of wisdom
Nabu Nabu ( akk, cuneiform: 𒀭𒀝 Nabû syr, ܢܵܒܼܘܼ\ܢܒܼܘܿ\ܢܵܒܼܘܿ Nāvū or Nvō or Nāvō) is the ancient Mesopotamian religion, ancient Mesopotamian patron god of literacy, the Science, rational arts, scribes, and wisdom. Et ...
; and Jupiter after their chief god,
Marduk Marduk (Cuneiform: dAMAR.UTU; Sumerian language, Sumerian: ''amar utu.k'' "calf of the sun; solar calf"; ) was a god from ancient Mesopotamia and patron deity of the city of Babylon. When Babylon became the political center of the Euphrates vall ...
. There are too many concordances between Greek and Babylonian naming conventions for them to have arisen separately. Given the differences in mythology, the correspondence was not perfect. For instance, the Babylonian Nergal was a god of war, and thus the Greeks identified him with Ares. Unlike Ares, Nergal was also a god of pestilence and ruler of the underworld. Today, most people in the western world know the planets by names derived from the Olympian pantheon of gods. Although modern Greeks still use their ancient names for the planets, other European languages, because of the influence of the
Roman Empire The Roman Empire ( la, Imperium Romanum ; grc-gre, Βασιλεία τῶν Ῥωμαίων, Basileía tôn Rhōmaíōn) was the post-Roman Republic, Republican period of ancient Rome. As a polity, it included large territorial holdings aro ...
and, later, the
Catholic Church The Catholic Church, also known as the Roman Catholic Church, is the List of Christian denominations by number of members, largest Christian church, with 1.3 billion baptized Catholics Catholic Church by country, worldwide . It is am ...
, use the Roman (Latin) names rather than the Greek ones. The Romans inherited
Proto-Indo-European mythology Proto-Indo-European mythology is the body of myths and deities associated with the Proto-Indo-Europeans, the hypothetical speakers of the reconstructed Proto-Indo-European language. Although the mythological motifs are not directly attested ...
as the Greeks did and shared with them a common pantheon under different names, but the Romans lacked the rich narrative traditions that Greek poetic culture had given their gods. During the later period of the
Roman Republic The Roman Republic ( la, Res publica Romana ) was a form of government of Rome and the era of the ancient Rome, classical Roman civilization when it was run through res publica, public Representation (politics), representation of the Roman peo ...
, Roman writers borrowed much of the Greek narratives and applied them to their own pantheon, to the point where they became virtually indistinguishable. When the Romans studied Greek astronomy, they gave the planets their own gods' names: '' Mercurius'' (for Hermes), ''
Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an Inferior and superior planets, interior planet to Earth, Venus (like Mercury (pl ...
'' (Aphrodite), ''
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin atmosph ...
'' (Ares), '' Iuppiter'' (Zeus) and '' Saturnus'' (Cronus). Some Romans, following a belief possibly originating in
Mesopotamia Mesopotamia ''Mesopotamíā''; ar, بِلَاد ٱلرَّافِدَيْن or ; syc, ܐܪܡ ܢܗܪ̈ܝܢ, or , ) is a historical region of Western Asia situated within the Tigris–Euphrates river system, in the northern part of the F ...

Mesopotamia
but developed in Hellenistic Egypt, believed that the seven gods after whom the planets were named took hourly shifts in looking after affairs on Earth. The order of shifts went Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon (from the farthest to the closest planet). Therefore, the first day was started by Saturn (1st hour), second day by Sun (25th hour), followed by Moon (49th hour), Mars, Mercury, Jupiter and Venus. Because each day was named by the god that started it, this became the order of the days of the week in the
Roman calendar The Roman calendar was the calendar used by the Roman Kingdom and Roman Republic. The term often includes the Julian calendar established by the reforms of the Roman dictator, dictator Julius Caesar and Roman emperor, emperor Augustus in the ...
. In English, ''Saturday'', ''Sunday'', and ''Monday'' are straightforward translations of these Roman names. The other days were renamed after '' Tīw'' (Tuesday), '' Wōden'' (Wednesday), '' Þunor'' (Thursday), and '' Frīġ'' (Friday), the Anglo-Saxon gods considered similar or equivalent to Mars, Mercury, Jupiter, and Venus, respectively. Earth's name in English is not derived from Greco-Roman mythology. Because it was only generally accepted as a planet in the 17th century, there is no tradition of naming it after a god. (The same is true, in English at least, of the Sun and the Moon, though they are no longer generally considered planets.) The name originates from the
Old English Old English (, ), or Anglo-Saxon, is the earliest recorded form of the English language English is a West Germanic languages, West Germanic language of the Indo-European language family, with its earliest forms spoken by the inhabita ...
word ''eorþe'', which was the word for "ground" and "dirt" as well as the world itself. As with its equivalents in the other
Germanic languages The Germanic languages are a branch of the Indo-European languages, Indo-European language family spoken natively by a population of about 515 million people mainly in Europe, North America, Oceania and Southern Africa. The most widely spoken ...
, it derives ultimately from the
Proto-Germanic Proto-Germanic (abbreviated PGmc; also called Common Germanic) is the linguistic reconstruction, reconstructed proto-language of the Germanic languages, Germanic branch of the Indo-European languages. Proto-Germanic eventually developed from ...
word ''erþō'', as can be seen in the English ''earth'', the German ''Erde'', the Dutch ''aarde'', and the Scandinavian ''jord''. Many of the
Romance languages The Romance languages, sometimes referred to as Latin languages or Neo-Latin languages, are the various modern languages that evolved from Vulgar Latin. They are the only extant subgroup of the Italic languages in the Indo-European languages, I ...
retain the old Roman word '' terra'' (or some variation of it) that was used with the meaning of "dry land" as opposed to "sea". The non-Romance languages use their own native words. The Greeks retain their original name, '' Γή'' ''(Ge)''. Non-European cultures use other planetary-naming systems.
India India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the List of countries and dependencies by area, seventh-largest country by area, the List of countries and dependencies by population, second-most populous ...
uses a system based on the
Navagraha Navagraha are nine heavenly bodies and deities that influence human life on Earth according to Hinduism and Hindu astrology. The term is derived from ''nava'' ( sa, नव "nine") and ''graha'' ( sa, ग्रह "planet, seizing, laying hold of, ...
, which incorporates the seven traditional planets (''
Surya Surya (; sa, सूर्य, ) is the sun as well as the solar deity in Hinduism. He is traditionally one of the major five deities in the Smarta tradition, Smarta tradition, all of whom are considered as equivalent deities in the Panchaya ...
'' 'Sun', ''
Chandra Chandra ( sa, चन्द्र, Candra, shining' or 'moon), also known as Soma ( sa, सोम), is the Hindu god of the Moon, and is associated with the night, plants and vegetation. He is one of the Navagraha (nine planets of Hinduism) and ...
'' 'Moon', ''
Budha Budha ( sa, बुध) is a Sanskrit word that connotes the planet Mercury (planet), Mercury. Budha, in Hindu legends, is also a deity. He is also known as Soumya (Sanskrit: सौम्य, lit. ''son of Moon''), Rauhineya and Tunga and is t ...
'' for Mercury, ''
Shukra Shukra (Sanskrit Sanskrit (; attributively , ; nominalization, nominally , , ) is a classical language belonging to the Indo-Aryan languages, Indo-Aryan branch of the Indo-European languages. It arose in South Asia after its predecessor l ...
'' ('bright') for Venus, ''
Mangala Mangala (Devanagari, Sanskrit: मङ्गल, IAST: ) is the personification, as well as the name for the planet Mars, in Hindu texts, Hindu literature. Also known as Lohita (), he is the celibate deity of anger, aggression, as well as war. A ...
'' (the god of war) for Mars, '' '' (councilor of the gods) for Jupiter, and ''
Shani Shani ( sa, शनि, ), or Shanaishchara ( sa, शनैश्चर, ), refers to the divine personification of the planet Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It ...
'' (symbolic of time) for Saturn) and the ascending and descending
lunar node A lunar node is either of the two orbital nodes of the Moon, that is, the two points at which the orbit of the Moon intersects the ecliptic The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the pers ...
s ''
Rahu Rāhu (Sanskrit: :wikt:राहु#Sanskrit, राहु, file:ascending node (bold).svg, 16px, ☊) is one of the nine major celestial bodies (navagraha) in Hindu texts and the king of meteors. It represents the ascension of the moon in it ...
'' and ''Ketu''. China and the countries of eastern Asia historically subject to Chinese cultural influence (such as Japan,
Korea Korea ( ko, 한국, or , ) is a peninsular region in East Asia. Since 1945, it has been divided at or near the 38th parallel north, 38th parallel, with North Korea (Democratic People's Republic of Korea) comprising its northern half and Sout ...
and
Vietnam Vietnam or Viet Nam ( vi, Việt Nam, ), officially the Socialist Republic of Vietnam,., group="n" is a country in Southeast Asia, at the eastern edge of mainland Southeast Asia, with an area of and population of 96 million, making it ...
) use a naming system based on the five Chinese elements:
water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
(Mercury 星 "water star"),
metal A metal (from ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, e ...
(Venus 星 "metal star"),
fire Fire is the rapid oxidation of a material (the fuel) in the exothermic chemical process of combustion, releasing heat, light, and various reaction Product (chemistry), products. At a certain point in the combustion reaction, called the ignition ...
(Mars 星 "fire star"),
wood Wood is a porous and fibrous structural tissue found in the Plant stem, stems and roots of trees and other woody plants. It is an organic materiala natural composite material, composite of cellulose fibers that are strong in tension and emb ...
(Jupiter 星 "wood star") and
earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large list of largest lakes and seas in the Solar System, volumes of water can be found throughout the Solar System, only water distributi ...
(Saturn 星 "earth star"). The names of Uranus ( 王星 "sky king star"), Neptune ( 王星 "sea king star"), and Pluto ( 王星 "underworld king star") in Chinese, Korean, and Japanese are
calque In linguistics, a calque () or loan translation is a word or phrase borrowed from another language by literal translation, literal word-for-word or root-for-root translation. When used as a verb, "to calque" means to borrow a word or phrase from ...
s based on the roles of those gods in Roman and Greek mythology. Chinese uses calques for the dwarf planets and many asteroids as well, e.g. Eris ( 神星 "quarrel goddess star"), Ceres ( 神星 "grain goddess star"), and Pallas ( 神星 "wisdom goddess star"). In traditional Hebrew astronomy, the seven traditional planets have (for the most part) descriptive names – the Sun is חמה ''Ḥammah'' or "the hot one", the Moon is לבנה ''Levanah'' or "the white one", Venus is כוכב נוגה ''Kokhav Nogah'' or "the bright planet", Mercury is כוכב ''Kokhav'' or "the planet" (given its lack of distinguishing features), Mars is מאדים ''Ma'adim'' or "the red one", and Saturn is שבתאי ''Shabbatai'' or "the resting one" (in reference to its slow movement compared to the other visible planets). The odd one out is Jupiter, called צדק ''Tzedeq'' or "justice". Hebrew names were chosen for Uranus (אורון ''Oron'', "small light") and Neptune (רהב ''Rahab'', a Biblical sea monster) in 2009; prior to that the names "Uranus" and "Neptune" had simply been borrowed. The etymologies for the Arabic names of the planets are less well understood. Mostly agreed among scholars are Venus الزهرة (''az-Zuhara'', "the bright one"), Earth الأرض (''al-ʾArḍ'', from the same root as eretz), and Saturn زُحَل (''Zuḥal'', "withdrawer"). Multiple suggested etymologies exist for Mercury عُطَارِد (''ʿUṭārid''), Mars اَلْمِرِّيخ (''al-Mirrīkh''), and Jupiter المشتري (''al-Muštarī''), but there is no agreement among scholars. When subsequent planets were discovered in the 18th and 19th centuries, Uranus was named for a Greek deity and Neptune for a Roman one (the counterpart of
Poseidon Poseidon (; grc-gre, wikt:Ποσειδῶν, Ποσειδῶν) was one of the Twelve Olympians in Religion in ancient Greece, ancient Greek religion and Greek mythology, myth, god of the sea, storms, earthquakes and horses.Burkert 1985pp. 1 ...
). The asteroids were initially named from mythology as well – Ceres, Juno, and Vesta are major Roman goddesses, and Pallas is an epithet of the Greek goddess
Athena Athena or Athene, often given the epithet Pallas, is an ancient Greek religion, ancient Greek goddess associated with wisdom, warfare, and handicraft who was later syncretism, syncretized with the Roman goddess Minerva. Athena was regarded ...
– but as more and more were discovered, the mythological restriction was dropped starting from
Massalia Massalia (Ancient Greek, Greek: Μασσαλία; Latin: Massilia; modern Marseille) was an ancient Greek colony founded ca. 600 BC on the Mediterranean Sea, Mediterranean coast of present-day France, east of the river Rhône, by Ionian Greeks, ...
in 1852. Pluto was given a classical name, as it was considered a major planet when it was discovered. After more objects were discovered beyond Neptune, naming conventions depending on their orbits were put in place: those in the 2:3 resonance with Neptune (the
plutino In astronomy Astronomy () is a natural science that studies astronomical object, celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and chronology of the Universe, evolution. Obj ...
s) are given names from underworld myths, while others are given names from creation myths. Most of the trans-Neptunian dwarf planets are named after gods and goddesses from other cultures (e.g. Quaoar is named after a
Tongva The Tongva ( ) are an Indigenous people of California The indigenous peoples of California (known as Native Californians) are the Native Americans in the United States, indigenous inhabitants who have lived or currently live in the geograph ...
god), except for Orcus and Eris which continued the Roman and Greek scheme. The moons (including the planetary-mass ones) are generally given names with some association with their parent planet. The planetary-mass moons of Jupiter are named after four of Zeus' lovers (or other sexual partners); those of Saturn are named after Cronus' brothers and sisters, the Titans; those of Uranus are named after characters from
Shakespeare William Shakespeare ( 26 April 1564 – 23 April 1616) was an English playwright, poet and actor. He is widely regarded as the greatest writer in the English language and the world's pre-eminent dramatist. He is often called England's nation ...
and
Pope The pope ( la, papa, from el, πάππας, translit=pappas, 'father'), also known as supreme pontiff ( or ), Roman pontiff () or sovereign pontiff, is the bishop of Rome (or historically the patriarch of Rome), head of the worldwide Cathol ...
(originally specifically from fairy mythology, but that ended with the naming of Miranda). Neptune's planetary-mass moon Triton is named after the god's son; Pluto's planetary-mass moon Charon is named after the ferryman of the dead, who carries the souls of the newly deceased to the underworld (Pluto's domain); and Eris' only known moon Dysnomia is named after one of Eris' daughters, the spirit of lawlessness.


Symbols

The written symbols for Mercury, Venus, Jupiter, Saturn and possibly Mars have been traced to forms found in late Greek papyrus texts. The symbols for Jupiter and Saturn are identified as
monogram A monogram is a motif (art), motif made by overlapping or combining two or more letters or other graphemes to form one symbol. Monograms are often made by combining the initials of an individual or a company, used as recognizable symbols or logos ...
s of the corresponding Greek names, and the symbol for Mercury is a stylized
caduceus The caduceus (☤; ; la, cādūceus, from grc-gre, κηρύκειον "herald's wand, or staff") is the staff carried by Hermes in Greek mythology and consequently by Hermes Trismegistus in Greco-Egyptian mythology. The same staff was also ...
. According to Annie Scott Dill Maunder, antecedents of the planetary symbols were used in art to represent the gods associated with the classical planets. ''Bianchini's
planisphere In astronomy, a planisphere () is a star chart Analog computer, analog computing instrument in the form of two adjustable disks that rotate on a common pivot. It can be adjusted to display the visible stars for any time and date. It is an instru ...
'', discovered by Francesco Bianchini in the 18th century but produced in the 2nd century, shows Greek personifications of planetary gods charged with early versions of the planetary symbols. Mercury has a
caduceus The caduceus (☤; ; la, cādūceus, from grc-gre, κηρύκειον "herald's wand, or staff") is the staff carried by Hermes in Greek mythology and consequently by Hermes Trismegistus in Greco-Egyptian mythology. The same staff was also ...
; Venus has, attached to her necklace, a cord connected to another necklace; Mars, a spear; Jupiter, a staff; Saturn, a scythe; the
Sun The Sun is the star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other st ...

Sun
, a
circlet A circlet is a piece of headwear that is similar to a diadem or a Corolla (headgear), corolla. The word 'circlet' is also used to refer to the base of a Crown (headgear), crown or a coronet, with or without a Cap (crown), cap. Diadem and circlet ar ...
with rays radiating from it; and the Moon, a headdress with a crescent attached. The modern shapes with the cross-marks first appeared around the 16th century. According to Maunder, the addition of crosses appears to be "an attempt to give a savour of Christianity to the symbols of the old pagan gods." Earth itself was not considered a classical planet; its symbol descends from a pre-heliocentric symbol for the four corners of the world. When further planets were discovered orbiting the Sun, symbols were invented for them. The most common astronomical symbol for Uranus, ⛢, was invented by Johann Gottfried Köhler, and was intended to represent the newly discovered metal
platinum Platinum is a chemical element with the Symbol (chemistry), symbol Pt and atomic number 78. It is a density, dense, malleable, ductility, ductile, highly unreactive, precious metal, precious, silverish-white transition metal. Its name origina ...
. An alternative symbol, ♅, was invented by Jérôme Lalande, and represents a globe with a H on top, for Uranus' discoverer Herschel. Today, ⛢ is mostly used by astronomers and ♅ by
astrologers Astrology is a range of Divination, divinatory practices, recognized as pseudoscientific since the 18th century, that claim to discern information about human affairs and terrestrial events by studying the apparent positions of Celestial o ...
, though it is possible to find each symbol in the other context. The first few asteroids were similarly given abstract symbols, but as their number rose further and further, this practice stopped in favour of numbering them instead. Neptune's symbol (♆) represents the god's trident. The astronomical symbol for Pluto is a P-L monogram (♇), though it has become less common since the IAU definition reclassified Pluto. Since Pluto's reclassification,
NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the US federal government responsible for the civil List of government space agencies, space program ...
has used the traditional astrological symbol of Pluto (⯓), a planetary orb over Pluto's bident. The IAU discourages the use of planetary symbols in modern journal articles in favour of one-letter or (to disambiguate Mercury and Mars) two-letter abbreviations for the major planets. The symbols for the Sun and Earth are nonetheless common, as
solar mass The solar mass () is a standard mass#Units of mass, unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as star cluster, stellar clusters, nebulae, galaxy, galaxies and Black hole, b ...
,
Earth mass An Earth mass (denoted as M_\mathrm or M_\oplus, where ⊕ is the standard astronomical Earth symbol, symbol for Earth), is a unit of mass equal to the mass of the planet Earth. The current best estimate for the mass of Earth is , with a relativ ...
and similar units are common in astronomy. Other planetary symbols today are mostly encountered in astrology. Astrologers have started reusing the old astronomical symbols for the first few asteroids, and continue to invent symbols for other objects, though most proposed symbols are only used by their proposers.
Unicode Unicode, formally The Unicode Standard,The formal version reference is is an information technology standard for the consistent encoding, representation, and handling of text expressed in most of the world's writing systems. The standard, ...
includes some relatively standard astrological symbols for some minor planets, including the dwarf planets discovered in the 21st century, though astronomical use of any of them is rare.


Formation

It is not known with certainty how planets are built. The prevailing theory is that they are formed during the collapse of a
nebula A nebula ('cloud' or 'fog' in Latin; pl. nebulae, nebulæ or nebulas) is a distinct luminescent part of interstellar medium, which can consist of ionized, neutral or molecular hydrogen and also cosmic dust. Nebulae are often star-forming region ...

nebula
into a thin disk of gas and dust. A
protostar A protostar is a very young star that is still gathering mass from its parent molecular cloud. The protostellar phase is the earliest one in the process of stellar evolution. For a low-mass star (i.e. that of the Sun or lower), it lasts about 50 ...
forms at the core, surrounded by a rotating
protoplanetary disk A protoplanetary disk is a rotating circumstellar disc of dense gas and dust surrounding a stellar evolution, young newly formed star, a T Tauri star, or Herbig Ae/Be star. The protoplanetary disk may also be considered an accretion disk for th ...

protoplanetary disk
. Through
accretion Accretion may refer to: Science * Accretion (astrophysics), the formation of planets and other bodies by collection of material through gravity * Accretion (meteorology), the process by which water vapor in clouds forms water droplets around nucl ...
(a process of sticky collision) dust particles in the disk steadily accumulate mass to form ever-larger bodies. Local concentrations of mass known as
planetesimal Planetesimals are solid objects thought to exist in protoplanetary disks and debris disks. Per the Chamberlin–Moulton planetesimal hypothesis, they are believed to form out of cosmic dust grains. Believed to have formed in the Solar System ab ...
s form, and these accelerate the accretion process by drawing in additional material by their gravitational attraction. These concentrations become ever denser until they collapse inward under gravity to form
protoplanet A protoplanet is a large planetary embryo that originated within a protoplanetary disc and has undergone internal melting to produce a differentiated interior. Protoplanets are thought to form out of kilometer-sized planetesimals that gravitationa ...
s. After a planet reaches a mass somewhat larger than Mars' mass, it begins to accumulate an extended atmosphere, greatly increasing the capture rate of the planetesimals by means of
atmospheric drag In fluid dynamics, drag (sometimes called air resistance, a type of friction, or fluid resistance, another type of friction or fluid friction) is a force acting opposite to the relative motion of any object moving with respect to a surrounding flu ...
. Depending on the accretion history of solids and gas, a
giant planet The giant planets constitute a diverse type of planet much larger than Earth. They are usually primarily composed of low-boiling-point materials (volatiles), rather than Rock (geology), rock or other solid matter, but massive solid planets can ...
, an
ice giant An ice giant is a giant planet composed mainly of elements heavier than hydrogen and helium, such as oxygen, carbon, nitrogen, and sulfur. There are two ice giants in the Solar System: Uranus and Neptune. In astrophysics and planetary science th ...
, or a
terrestrial planet A terrestrial planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate Rock (geology), rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the S ...
may result. It is thought that the
regular satellite In astronomy, a regular moon is a natural satellite A natural satellite is, in the most common usage, an astronomical object, astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural sate ...
s of Jupiter, Saturn, and Uranus formed in a similar way; however, Triton was likely captured by Neptune, and Earth's Moon and Pluto's Charon might have formed in collisions. When the protostar has grown such that it ignites to form a
star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

star
, the surviving disk is removed from the inside outward by
photoevaporation Photoevaporation denotes the process where energetic radiation ionises gas and causes it to disperse away from the ionising source. This typically refers to an astrophysical context where ultraviolet Ultraviolet (UV) is a form of electromagne ...
, the
solar wind The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the Stellar corona, corona. This Plasma (physics), plasma mostly consists of electrons, protons and alpha particles with kinetic energy betwee ...
, Poynting–Robertson drag and other effects. Thereafter there still may be many protoplanets orbiting the star or each other, but over time many will collide, either to form a larger, combined protoplanet or release material for other protoplanets to absorb. Those objects that have become massive enough will capture most matter in their orbital neighbourhoods to become planets. Protoplanets that have avoided collisions may become
natural satellite A natural satellite is, in the most common usage, an astronomical object, astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially ref ...

natural satellite
s of planets through a process of gravitational capture, or remain in belts of other objects to become either dwarf planets or small bodies. The energetic impacts of the smaller planetesimals (as well as
radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive Decay chain, disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nucl ...
) will heat up the growing planet, causing it to at least partially melt. The interior of the planet begins to differentiate by density, with higher density materials sinking toward the core. Smaller terrestrial planets lose most of their atmospheres because of this accretion, but the lost gases can be replaced by outgassing from the mantle and from the subsequent impact of
comet A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or Coma (cometary), coma, and sometimes also a Comet ta ...
s. (Smaller planets will lose any atmosphere they gain through various escape mechanisms.) With the discovery and observation of
planetary system A planetary system is a set of gravity, gravitationally bound non-Star, stellar objects in or out of orbit around a star or star system. Generally speaking, systems with one or more planets constitute a planetary system, although such systems m ...
s around stars other than the Sun, it is becoming possible to elaborate, revise or even replace this account. The level of
metallicity In astronomy, metallicity is the Abundance of the chemical elements, abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astron ...
—an astronomical term describing the abundance of
chemical element A chemical element is a species of atoms that have a given number of protons in their atomic nucleus, nuclei, including the pure Chemical substance, substance consisting only of that species. Unlike chemical compounds, chemical elements canno ...
s with an
atomic number The atomic number or nuclear charge number (symbol ''Z'') of a chemical element is the charge number of an atomic nucleus. For ordinary nuclei, this is equal to the proton number (''n''p) or the number of protons found in the nucleus of every ...
greater than 2 (
helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol (chemistry), symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas gr ...
)—appears to determine the likelihood that a star will have planets. Hence, a metal-rich population I star is more likely to have a substantial planetary system than a metal-poor, population II star.


Solar System

According to the IAU definition, there are eight planets in the Solar System, which are (in increasing distance from the Sun): Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Jupiter is the largest, at 318 Earth masses, whereas Mercury is the smallest, at 0.055 Earth masses. The planets of the Solar System can be divided into categories based on their composition. Terrestrials are similar to Earth, with bodies largely composed of
rock Rock most often refers to: * Rock (geology) In geology, rock (or stone) is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its Chemical compound, chemical compos ...
and metal: Mercury, Venus, Earth, and Mars. Earth is the largest terrestrial planet.
Giant planet The giant planets constitute a diverse type of planet much larger than Earth. They are usually primarily composed of low-boiling-point materials (volatiles), rather than Rock (geology), rock or other solid matter, but massive solid planets can ...
s are significantly more massive than the terrestrials: Jupiter, Saturn, Uranus, and Neptune. They differ from the terrestrial planets in composition. The
gas giant A gas giant is a giant planet composed mainly of hydrogen and helium. Gas giants are also called failed stars because they contain the same basic elements as a star. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" ...
s, Jupiter and Saturn, are primarily composed of hydrogen and helium and are the most massive planets in the Solar System. Saturn is one third as massive as Jupiter, at 95 Earth masses. The
ice giant An ice giant is a giant planet composed mainly of elements heavier than hydrogen and helium, such as oxygen, carbon, nitrogen, and sulfur. There are two ice giants in the Solar System: Uranus and Neptune. In astrophysics and planetary science th ...
s, Uranus and Neptune, are primarily composed of low-boiling-point materials such as water, methane, and ammonia, with thick atmospheres of hydrogen and helium. They have a significantly lower mass than the gas giants (only 14 and 17 Earth masses). Dwarf planets are gravitationally rounded, but have not cleared their orbits of other bodies. In increasing order of average distance from the Sun, the ones generally agreed among astronomers are , , , , , , , and . Ceres is the largest object in the
asteroid belt The asteroid belt is a torus In geometry, a torus (plural tori, colloquially donut or doughnut) is a surface of revolution generated by revolving a circle in three-dimensional space about an axis that is coplanarity, coplanar with the ...

asteroid belt
, located between the orbits of Mars and Jupiter. The other eight all orbit beyond Neptune. Orcus, Pluto, Haumea, Quaoar, and Makemake orbit in the
Kuiper belt The Kuiper belt () is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical unit, astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larg ...
, which is a second belt of small Solar System bodies beyond the orbit of Neptune. Gonggong and Eris orbit in the
scattered disc The scattered disc (or scattered disk) is a distant circumstellar disc in the Solar System that is sparsely populated by icy small solar system bodies, which are a subset of the broader family of trans-Neptunian objects. The scattered-disc objec ...
, which is somewhat further out and, unlike the Kuiper belt, is unstable towards interactions with Neptune. Sedna is the largest known
detached object Detached objects are a Orbital mechanics, dynamical class of minor planets in the outer reaches of the Solar System and belong to the broader family of trans-Neptunian objects (TNOs). These objects have orbits whose points of closest approach to ...
, a population that never comes close enough to the Sun to interact with any of the classical planets; the origins of their orbits are still being debated. All nine are similar to terrestrial planets in having a solid surface, but they are made of ice and rock, rather than rock and metal. Moreover, all of them are smaller than Mercury, with Pluto being the largest known dwarf planet, and Eris being the most massive known. There are at least twenty planetary-mass moons or satellite planets—moons large enough to take on ellipsoidal shapes (though Dysnomia's shape has never been measured, it is massive and dense enough to be a solid body). The twenty generally agreed are as follows. * One satellite of Earth: the
Moon The Moon is Earth's only natural satellite. It is the List of natural satellites, fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth ( ...

Moon
* Four satellites of Jupiter: Io, Europa, Ganymede, and Callisto * Seven satellites of Saturn: Mimas,
Enceladus Enceladus is the sixth-largest Moons of Saturn, moon of Saturn (19th largest in the Solar System). It is about in diameter, about a tenth of that of Saturn's largest moon, Titan (moon), Titan. Enceladus is mostly covered by fresh, clean ice, m ...
, Tethys, Dione, Rhea, Titan, and
Iapetus In Greek mythology, Iapetus (; ; grc, Ἰαπετός, Iapetós), also Japetus, is a Titan (mythology), Titan, the son of Uranus (mythology), Uranus and Gaia (mythology), Gaia and father of Atlas (mythology), Atlas, Prometheus, Epimetheus (myth ...
* Five satellites of Uranus: Miranda, Ariel, Umbriel, Titania, and Oberon * One satellite of Neptune: Triton * One satellite of Pluto:
Charon In Greek mythology, Charon or Kharon (; grc, Χάρων) is a psychopomp, the ferryman of Greek underworld, Hades, the Greek underworld. He carries the souls of those who have been given funeral rites across the rivers Acheron and Styx, which ...
* One satellite of Eris: Dysnomia The Moon, Io, and Europa have compositions similar to the terrestrial planets; the others are made of ice and rock like the dwarf planets, with Tethys being made of almost pure ice. (Europa is often considered an icy planet, though, because its surface ice layer makes it difficult to study its interior.) Ganymede and Titan are larger than Mercury by radius, and Callisto almost equals it, but all three are much less massive. Mimas is the smallest object generally agreed to be a geophysical planet, at about six millionths of Earth's mass, though there are many larger bodies that may not be geophysical planets (e.g. ).


Planetary attributes

The tables below summarise some properties of objects generally agreed to satisfy geophysical planet definitions. There are many smaller dwarf planet candidates, such as Salacia, that have not been included in the tables because astronomers disagree on whether or not they are dwarf planets. The diameters, masses, orbital periods, and rotation periods of the major planets are available from the
Jet Propulsion Laboratory The Jet Propulsion Laboratory (JPL) is a Federally funded research and development centers, federally funded research and development center and NASA field center in the City of La Cañada Flintridge, California, La Cañada Flintridge, California ...
. JPL also provides their semi-major axes, inclinations, and eccentricities of planetary orbits, and the axial tilts are taken from their Horizons database. Other information is summarized by NASA. The data for the dwarf planets and planetary-mass moons is taken from
list of gravitationally rounded objects of the Solar System This is a list of most likely gravitationally rounded objects of the Solar System, which are objects that have a rounded, ellipsoid An ellipsoid is a surface that may be obtained from a sphere by deforming it by means of directional Scaling (g ...
, with sources listed there. As all the planetary-mass moons exhibit synchronous rotation, their rotation periods equal their orbital periods.


Exoplanets

An exoplanet (extrasolar planet) is a planet outside the Solar System. Known exoplanets range in size from gas giants about twice as large as Jupiter down to just over the size of the Moon. Analysis of
gravitational microlensing Gravitational microlensing is an astronomical Astronomy () is a natural science that studies astronomical object, celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and chronology ...
data suggests a minimum average of 1.6 bound planets for every star in the Milky Way. In early 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the
pulsar A pulsar (from ''pulsating radio source'') is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its Poles of astronomical bodies#Magnetic poles, magnetic poles. This radiation can be observed only w ...
PSR 1257+12. This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. Researchers suspect they formed from a disk remnant left over from the
supernova A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last stellar evolution, evolutionary stages of a mass ...
that produced the pulsar. The first confirmed discovery of an extrasolar planet orbiting an ordinary main-sequence star occurred on 6 October 1995, when
Michel Mayor Michel Gustave Édouard Mayor (; born 12 January 1942) is a Switzerland, Swiss astrophysicist and professor emeritus at the University of Geneva's Department of Astronomy. He formally retired in 2007, but remains active as a researcher at the O ...
and
Didier Queloz Didier Patrick Queloz (; born 23 February 1966) is a Swiss astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical ob ...
of the
University of Geneva The University of Geneva (French: ''Université de Genève'') is a public university, public research university located in Geneva, Switzerland. It was founded in 1559 by John Calvin as a Theology, theological seminary. It remained focused on t ...
announced the detection of
51 Pegasi b 51 Pegasi b, officially named Dimidium , and formerly unofficially dubbed Bellerophon , is an extrasolar planet approximately away in the constellation of Pegasus (constellation), Pegasus. It was the first exoplanet to be discovered orbiting a m ...
, an exoplanet around
51 Pegasi 51 Pegasi (abbreviated 51 Peg), formally named Helvetios , is a Solar analog, Sun-like star located from Earth in the constellation of Pegasus (constellation), Pegasus. It was the first main sequence, main-sequence star found to have an ...
. From then until the Kepler mission most known extrasolar planets were gas giants comparable in mass to Jupiter or larger as they were more easily detected. The catalog of Kepler candidate planets consists mostly of planets the size of Neptune and smaller, down to smaller than Mercury. In 2011, the
Kepler Space Telescope The Kepler space telescope is a disused space telescope launched by NASA in 2009 to discover Earth-sized Exoplanet, planets orbiting other stars. Named after astronomer Johannes Kepler, the spacecraft was launched into an Earth-trailing heliocen ...
team reported the discovery of the first Earth-sized extrasolar planets orbiting a Sun-like star, and Kepler-20f. Since that time, more than 100 planets have been identified that are approximately the same size as Earth, 20 of which orbit in the
habitable zone In astronomy and astrobiology, the circumstellar habitable zone (CHZ), or simply the habitable zone, is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure.J. F. Kast ...
of their star – the range of orbits where a terrestrial planet could sustain liquid water on its surface, given enough atmospheric pressure. One in five Sun-like stars is thought to have an Earth-sized planet in its habitable zone, which suggests that the nearest would be expected to be within 12 light-years distance from Earth. The frequency of occurrence of such terrestrial planets is one of the variables in the
Drake equation The Drake equation is a probability theory, probabilistic argument used to estimate the number of active, communicative extraterrestrial life, extraterrestrial civilizations in the Milky Way Galaxy. The equation was formulated in 1961 by Frank ...
, which estimates the number of intelligent, communicating civilizations that exist in the
Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...
. There are types of planets that do not exist in the Solar System:
super-Earth A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below those of the Solar System's ice giants, Uranus and Neptune, which are 14.5 and 17 times Earth's, respectively. The term "super-Earth" refers only to t ...
s and
mini-Neptune A Mini-Neptune (sometimes known as a gas dwarf or transitional planet) is a planet less massive than Neptune Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in ...
s, which have masses between that of Earth and Neptune. Such planets could be rocky like Earth or a mixture of volatiles and gas like Neptune—the dividing line between the two possibilities is currently thought to occur at about twice the mass of Earth. The planet , with mass 5.5–10.4 times the mass of Earth, attracted attention upon its discovery for potentially being in the habitable zone, though later studies concluded that it is actually too close to its star to be habitable. Exoplanets have been found that are much closer to their parent star than any planet in the Solar System is to the Sun. Mercury, the closest planet to the Sun at 0.4 AU, takes 88 days for an orbit, but ultra-short period planets can orbit in less than a day. The Kepler-11 system has five of its planets in shorter orbits than Mercury's, all of them much more massive than Mercury. There are
hot Jupiter Hot Jupiters (sometimes called hot Saturns) are a class of gas giant A gas giant is a giant planet composed mainly of hydrogen and helium. Gas giants are also called failed stars because they contain the same basic elements as a star. Jupiter ...
s, such as 51 Pegasi b, that orbit very close to their star and may evaporate to become
chthonian planet Chthonian planets (, sometimes 'cthonian') are a hypothetical class of celestial objects resulting from the stripping away of a gas giant's hydrogen and helium celestial body atmosphere, atmosphere and outer layers, which is called hydrodynamic ...
s, which are the leftover cores. There are also exoplanets that are much farther from their star. Neptune is 30 AU from the Sun and takes 165 years to orbit, but there are exoplanets that are thousands of AU from their star and take more than a million years to orbit. e.g. COCONUTS-2b.


Attributes

Although each planet has unique physical characteristics, a number of broad commonalities do exist among them. Some of these characteristics, such as rings or natural satellites, have only as yet been observed in planets in the Solar System, whereas others are commonly observed in extrasolar planets.


Dynamic characteristics


Orbit

In the Solar System, all the planets orbit the Sun in the same direction as the Sun rotates: counter-clockwise as seen from above the Sun's north pole. At least one extrasolar planet, WASP-17b, has been found to orbit in the opposite direction to its star's rotation. The period of one revolution of a planet's orbit is known as its
sidereal period The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting pl ...
or ''year''. A planet's year depends on its distance from its star; the farther a planet is from its star, the longer the distance it must travel and the slower its speed, since it is less affected by its star's
gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the strong ...
. No planet's orbit is perfectly circular, and hence the distance of each from the host star varies over the course of its year. The closest approach to its star is called its
periastron An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary (astronomy), primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two ...
, or
perihelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any e ...
in the Solar System, whereas its farthest separation from the star is called its apastron (
aphelion An apsis (; ) is the farthest or nearest point in the orbit In celestial mechanics, an orbit is the curved trajectory of an physical body, object such as the trajectory of a planet around a star, or of a natural satellite around a ...
). As a planet approaches periastron, its speed increases as it trades
gravitational potential energy Gravitational energy or gravitational potential energy is the potential energy a massive object has in relation to another massive object due to gravity. It is the potential energy associated with the gravitational field, which is released (conver ...
for kinetic energy, just as a falling object on Earth accelerates as it falls. As the planet nears apastron, its speed decreases, just as an object thrown upwards on Earth slows down as it reaches the apex of its trajectory. Each planet's orbit is delineated by a set of elements: * The '' eccentricity'' of an orbit describes the elongation of a planet's elliptical (oval) orbit. Planets with low eccentricities have more circular orbits, whereas planets with high eccentricities have more elliptical orbits. The planets and large moons in the Solar System have relatively low eccentricities, and thus nearly circular orbits. The comets and many Kuiper belt objects, as well as several extrasolar planets, have very high eccentricities, and thus exceedingly elliptical orbits. * The ''
semi-major axis In geometry, the major axis of an ellipse is its longest diameter: a line segment that runs through the center and both focus (geometry), foci, with ends at the two most widely separated points of the perimeter. The semi-major axis (major wikt: ...
'' gives the size of the orbit. It is the distance from the midpoint to the longest diameter of its elliptical orbit. This distance is not the same as its apastron, because no planet's orbit has its star at its exact centre. * The ''
inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a Plane of reference, reference plane and the orbital plane or Axis of rotation, axis of direction of the orbiting object ...
'' of a planet tells how far above or below an established reference plane its orbit is tilted. In the Solar System, the reference plane is the plane of Earth's orbit, called the
ecliptic The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic agai ...
. For extrasolar planets, the plane, known as the ''sky plane'' or ''plane of the sky'', is the plane perpendicular to the observer's line of sight from Earth. The eight planets of the Solar System all lie very close to the ecliptic; comets and
Kuiper belt The Kuiper belt () is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical unit, astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larg ...
objects like Pluto are at far more extreme angles to it. The large moons are generally not very inclined to their parent planets' equators, but Earth's Moon, Saturn's Iapetus, and Neptune's Triton are exceptions. Triton is unique among the large moons in that it orbits retrograde, i.e. in the direction opposite to its parent planet's rotation. * The points at which a planet crosses above and below its reference plane are called its ascending and
descending node An orbital node is either of the two points where an orbit In celestial mechanics, an orbit is the curved trajectory of an physical body, object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or ...
s. The
longitude of the ascending node The longitude of the ascending node (☊ or Ω) is one of the orbital elements Orbital elements are the parameters required to uniquely identify a specific orbit. In celestial mechanics these elements are considered in two-body systems using a ...
is the angle between the reference plane's 0 longitude and the planet's ascending node. The
argument of periapsis The argument of periapsis (also called argument of perifocus or argument of pericenter), symbolized as ''ω'', is one of the orbital elements of an orbit In celestial mechanics, an orbit is the curved trajectory of an physical body, object ...
(or perihelion in the Solar System) is the angle between a planet's ascending node and its closest approach to its star.


Axial tilt

Planets have varying degrees of axial tilt; they spin at an angle to the
plane Plane(s) most often refers to: * Aero- or airplane An airplane or aeroplane (informally plane) is a fixed-wing aircraft that is propelled forward by thrust from a jet engine, Propeller (aircraft), propeller, or rocket engine. Airplanes co ...
of their stars' equators. This causes the amount of light received by each hemisphere to vary over the course of its year; when the northern hemisphere points away from its star, the southern hemisphere points towards it, and vice versa. Each planet therefore has seasons, resulting in changes to the climate over the course of its year. The time at which each hemisphere points farthest or nearest from its star is known as its
solstice A solstice is an event that occurs when the Sun appears to reach its most northerly or southerly sun path, excursion relative to the celestial equator on the celestial sphere. Two solstices occur annually, around June 21 and December 21. In man ...
. Each planet has two in the course of its orbit; when one hemisphere has its summer solstice with its day being the longest, the other has its winter solstice when its day is shortest. The varying amount of light and heat received by each hemisphere creates annual changes in weather patterns for each half of the planet. Jupiter's axial tilt is very small, so its seasonal variation is minimal; Uranus, on the other hand, has an axial tilt so extreme it is virtually on its side, which means that its hemispheres are either continually in sunlight or continually in darkness around the time of its solstices. Among extrasolar planets, axial tilts are not known for certain, though most hot Jupiters are believed to have a negligible axial tilt as a result of their proximity to their stars.


Rotation

The planets rotate around invisible axes through their centres. A planet's
rotation period The rotation period of a celestial object (e.g., star, gas giant, planet, moon, asteroid) may refer to its sidereal rotation period, i.e. the time that the object takes to complete a single revolution around its axis of rotation relative to the b ...
is known as a
stellar day Earth's rotation or Earth's spin is the rotation of planet Earth around its own Rotation around a fixed axis, axis, as well as changes in the orientation (geometry), orientation of the rotation axis in space. Earth rotates eastward, in retrograd ...
. Most of the planets in the Solar System rotate in the same direction as they orbit the Sun, which is counter-clockwise as seen from above the Sun's
north pole The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is the point in the Northern Hemisphere where the Earth's rotation, Earth's axis of rotation meets its surface. It is called the True North Pole to distingu ...
. The exceptions are Venus and Uranus, which rotate clockwise, though Uranus's extreme axial tilt means there are differing conventions on which of its poles is "north", and therefore whether it is rotating clockwise or anti-clockwise. Regardless of which convention is used, Uranus has a retrograde rotation relative to its orbit. The rotation of a planet can be induced by several factors during formation. A net
angular momentum In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of Momentum, linear momentum. It is an important physical quantity because it is a Conservation law, conserved quantity—the total angular ...
can be induced by the individual angular momentum contributions of accreted objects. The accretion of gas by the giant planets contributes to the angular momentum. Finally, during the last stages of planet building, a
stochastic process In probability theory and related fields, a stochastic () or random process is a mathematical object usually defined as a Indexed family, family of random variables. Stochastic processes are widely used as mathematical models of systems and phen ...
of protoplanetary accretion can randomly alter the spin axis of the planet. There is great variation in the length of day between the planets, with Venus taking 243
days A day is the time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past, through the present, into the future. It is a component ...
to rotate, and the giant planets only a few hours. The rotational periods of extrasolar planets are not known, but for
hot Jupiter Hot Jupiters (sometimes called hot Saturns) are a class of gas giant A gas giant is a giant planet composed mainly of hydrogen and helium. Gas giants are also called failed stars because they contain the same basic elements as a star. Jupiter ...
s, their proximity to their stars means that they are
tidally locked Tidal locking between a pair of co-orbit In celestial mechanics, an orbit is the curved trajectory of an physical body, object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an satellite, ...
(that is, their orbits are in sync with their rotations). This means, they always show one face to their stars, with one side in perpetual day, the other in perpetual night. Mercury and Venus, the closest planets to the Sun, similarly exhibit very slow rotation: Mercury is tidally locked into a 3:2 spin–orbit resonance (rotating three times for every two revolutions around the Sun), and Venus' rotation may be in equilibrium between tidal forces slowing it down and atmospheric tides created by solar heating speeding it up. All the large moons are tidally locked to their parent planets; Pluto and Charon are tidally locked to each other, as are Eris and Dysnomia. The other dwarf planets with known rotation periods rotate faster than Earth; Haumea rotates so fast that it has been distorted into a triaxial ellipsoid. The exoplanet Tau Boötis b and its parent star Tau Boötis appear to be mutually tidally locked.


Orbital clearing

The defining dynamic characteristic of a planet, according to the IAU definition, is that it has ''cleared its neighborhood''. A planet that has cleared its neighborhood has accumulated enough mass to gather up or sweep away all the
planetesimal Planetesimals are solid objects thought to exist in protoplanetary disks and debris disks. Per the Chamberlin–Moulton planetesimal hypothesis, they are believed to form out of cosmic dust grains. Believed to have formed in the Solar System ab ...
s in its orbit. In effect, it orbits its star in isolation, as opposed to sharing its orbit with a multitude of similar-sized objects. As described above, this characteristic was mandated as part of the IAU's official definition of a planet in August 2006. Although to date this criterion only applies to the Solar System, a number of young extrasolar systems have been found in which evidence suggests orbital clearing is taking place within their circumstellar discs.


Physical characteristics


Size and shape

Gravity causes planets to be pulled into a roughly spherical shape, so a planet's size can be expressed roughly by an average radius (for example,
Earth radius Earth radius (denoted as ''R''🜨 or R_E) is the distance from the center of Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large list of largest lakes and seas in the Solar ...
or
Jupiter radius The Jupiter radius or Jovian radius ( or ) has a value of , or 11.2 Earth radii () (one Earth radius equals ). The Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar Syste ...
). However, planets are not perfectly spherical; for example, the
Earth's rotation Earth's rotation or Earth's spin is the rotation of planet Earth around its own Rotation around a fixed axis, axis, as well as changes in the orientation (geometry), orientation of the rotation axis in space. Earth rotates eastward, in retrograd ...
causes it to be slightly flattened at the poles with a bulge around the equator. Therefore, a better approximation of Earth's shape is an
oblate spheroid A spheroid, also known as an ellipsoid of revolution or rotational ellipsoid, is a quadric surface (mathematics), surface obtained by Surface of revolution, rotating an ellipse about one of its principal axes; in other words, an ellipsoid with ...
, whose equatorial diameter is larger than the
pole Pole may refer to: Astronomy *Celestial pole, the projection of the planet Earth's axis of rotation onto the celestial sphere; also applies to the axis of rotation of other planets *Pole star, a visible star that is approximately aligned with the ...
-to-pole diameter. Generally, a planet's shape may be described by giving polar and equatorial radii of a
spheroid A spheroid, also known as an ellipsoid of revolution or rotational ellipsoid, is a quadric surface obtained by rotating an ellipse about one of its principal axes; in other words, an ellipsoid with two equal semi-diameters. A spheroid has ...
or specifying a
reference ellipsoid An Earth ellipsoid or Earth spheroid is a mathematical figure approximating the Earth's form, used as a reference frame for computations in geodesy Geodesy ( ) is the Earth science of accurately measuring and understanding Earth's fi ...
. From such a specification, the planet's flattening, surface area, and volume can be calculated; its normal gravity can be computed knowing its size, shape, rotation rate and mass.


Mass

A planet's defining physical characteristic is that it is massive enough for the force of its own gravity to dominate over the
electromagnetic force In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions of a ...
s binding its physical structure, leading to a state of
hydrostatic equilibrium In fluid mechanics Fluid mechanics is the branch of physics concerned with the mechanics of fluids (liquids, gases, and plasma (physics), plasmas) and the forces on them. It has applications in a wide range of disciplines, including mechanic ...
. This effectively means that all planets are spherical or spheroidal. Up to a certain mass, an object can be irregular in shape, but beyond that point, which varies depending on the chemical makeup of the object, gravity begins to pull an object towards its own centre of mass until the object collapses into a sphere. Mass is the prime attribute by which planets are distinguished from stars. While the lower
stellar mass Stellar mass is a phrase that is used by astronomers to describe the mass of a star. It is usually enumerated in terms of the Sun's mass as a proportion of a solar mass (). Hence, the bright star Sirius has around . A star's mass will vary over its ...
limit is estimated to be around 75 times that of Jupiter (), the upper planetary mass limit for planethood is only roughly 13 for objects with solar-type
isotopic abundance In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its motion and behavior through Spacetime, space and time, and the related entities of energy and force. "Physical science i ...
, beyond which it achieves conditions suitable for
nuclear fusion Nuclear fusion is a reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles ( neutrons or protons). The difference in mass between the reactants and products is mani ...
of
deuterium Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two Stable isotope ratio, stable isotopes of hydrogen (the other being Hydrogen atom, protium, or hydrogen-1). The atomic nucleus, nucleus of a deuterium ato ...
. Other than the Sun, no objects of such mass exist in the Solar System; but there are exoplanets of this size. The 13 limit is not universally agreed upon and the
Extrasolar Planets Encyclopaedia The Extrasolar Planets Encyclopaedia is an astronomy website, founded in Paris, France at the Meudon Observatory by Jean Schneider (Extrasolar Planets Encyclopaedia), Jean Schneider in February 1995, which maintains a database of all the currently ...
includes objects up to 60 , and the Exoplanet Data Explorer up to 24 . The smallest known exoplanet with an accurately known mass is PSR B1257+12A, one of the first extrasolar planets discovered, which was found in 1992 in orbit around a
pulsar A pulsar (from ''pulsating radio source'') is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its Poles of astronomical bodies#Magnetic poles, magnetic poles. This radiation can be observed only w ...
. Its mass is roughly half that of the planet Mercury. Even smaller is WD 1145+017 b, orbiting a white dwarf; its mass is roughly that of the dwarf planet Haumea, and it is typically termed a minor planet. The smallest known planet orbiting a main-sequence star other than the Sun is Kepler-37b, with a mass (and radius) that is probably slightly higher than that of the Moon.


Internal differentiation

Every planet began its existence in an entirely fluid state; in early formation, the denser, heavier materials sank to the centre, leaving the lighter materials near the surface. Each therefore has a differentiated interior consisting of a dense
planetary core A planetary core consists of the innermost layers of a planet. Cores may be entirely solid or entirely liquid, or a mixture of solid and liquid layers as is the case in the Earth. In the Solar System, core sizes range from about 20% (the Moon ...
surrounded by a mantle that either is or was a
fluid In physics, a fluid is a liquid, gas, or other material that continuously Deformation (physics), deforms (''flows'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are Matter, substances wh ...
. The terrestrial planets' mantles are sealed within hard crusts, but in the giant planets the mantle simply blends into the upper cloud layers. The terrestrial planets have cores of elements such as
iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundance ...
and
nickel Nickel is a chemical element with Chemical symbol, symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and Ductility, ductile transition metal. Pure nickel is chemically reactive bu ...
, and mantles of
silicate In chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, struc ...
s. Jupiter and Saturn are believed to have cores of rock and metal surrounded by mantles of
metallic hydrogen Metallic hydrogen is a phase (matter), phase of hydrogen in which it behaves like an electrical conductor. This phase was predicted in 1935 on theoretical grounds by Eugene Wigner and Hillard Bell Huntington. At high pressure and temperatures, met ...
. Uranus and Neptune, which are smaller, have rocky cores surrounded by mantles of
water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
,
ammonia Ammonia is an inorganic chemical compound, compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a dis ...
,
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...
and other ices. The fluid action within these planets' cores creates a geodynamo that generates a
magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to t ...

magnetic field
. Similar differentiation processes are believed to have occurred on some of the large moons and dwarf planets, though the process may not always have been completed: Ceres, Callisto, and Titan appear to be incompletely differentiated.


Atmosphere

All of the Solar System planets except Mercury have substantial
atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...

atmosphere
s because their gravity is strong enough to keep gases close to the surface. Saturn's largest moon Titan also has a substantial atmosphere thicker than that of Earth; Neptune's largest moon Triton and the dwarf planet
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the S ...

Pluto
have more tenuous atmospheres. The larger giant planets are massive enough to keep large amounts of the light gases hydrogen and helium, whereas the smaller planets lose these gases into
space Space is the boundless Three-dimensional space, three-dimensional extent in which Physical body, objects and events have relative position (geometry), position and direction (geometry), direction. In classical physics, physical space is often ...
. The composition of Earth's atmosphere is different from the other planets because the various life processes that have transpired on the planet have introduced free molecular
oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group (periodic table), group in the periodic table, a highly Chemical reaction, reactive nonmetal, and an oxidizing a ...
. Planetary atmospheres are affected by the varying
insolation Solar irradiance is the power (physics), power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per ...
or internal energy, leading to the formation of dynamic
weather system In meteorology, a low-pressure area, low area or low is a region where the atmospheric pressure is lower than that of surrounding locations. Low-pressure areas are commonly associated with inclement weather (such as cloudy, windy, with possible ...
s such as
hurricane A tropical cyclone is a rapidly rotating storm, storm system characterized by a Low-pressure area, low-pressure center, a closed low-level atmospheric circulation, Beaufort scale, strong winds, and a spiral arrangement of thunderstorms tha ...

hurricane
s (on Earth), planet-wide
dust storm A dust storm, also called a sandstorm, is a meteorological phenomenon common in arid and semi-arid regions. Dust storms arise when a Outflow boundary, gust front or other strong wind blows loose sand and dirt from a dry surface. Fine partic ...
s (on Mars), a greater-than-Earth-sized
anticyclone An anticyclone is a weather meteorological phenomenon, phenomenon defined as a large-scale circulation of winds around a central high-pressure system, region of high atmospheric pressure, clockwise in the Northern Hemisphere and counterclockwise ...
on Jupiter (called the
Great Red Spot The Great Red Spot is a persistent high-pressure area, high-pressure region in the atmosphere of Jupiter, producing an anticyclonic storm that is the largest in the Solar System. Located 22 degree (angle), degrees south of Jupiter's equator, it p ...
), and holes in the atmosphere (on Neptune). Weather patterns detected on exoplanets include a hot region on HD 189733 b twice the size of the Great Red Spot, * as well as
cloud In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid drop (liquid), droplets, ice crystals, frozen crystals, or other particulates, particles suspended in the atmosphere of a planetary body or similar space. ...
s on the hot Jupiter Kepler-7b, the super-Earth
Gliese 1214 b Gliese 1214 b (often shortened to GJ 1214 b) is an exoplanet that orbits the star Gliese 1214, and was discovered in December 2009. Its parent star is 48 light-years from the Sun, in the constellation Ophiuchus. As of 2017, GJ 1214 b is the most l ...
and others. Hot Jupiters, due to their extreme proximities to their host stars, have been shown to be losing their atmospheres into space due to stellar radiation, much like the tails of comets. These planets may have vast differences in temperature between their day and night sides that produce supersonic winds, although multiple factors are involved and the details of the atmospheric dynamics that affect the day-night temperature difference are complex.


Magnetosphere

One important characteristic of the planets is their intrinsic
magnetic moment In electromagnetism, the magnetic moment is the magnetic strength and orientation of a magnet or other object that produces a magnetic field. Examples of objects that have magnetic moments include loops of electric current (such as electromagnets ...
s, which in turn give rise to magnetospheres. The presence of a magnetic field indicates that the planet is still geologically alive. In other words, magnetized planets have flows of
electrically conducting Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allows ...
material in their interiors, which generate their magnetic fields. These fields significantly change the interaction of the planet and solar wind. A magnetized planet creates a cavity in the solar wind around itself called the magnetosphere, which the wind cannot penetrate. The magnetosphere can be much larger than the planet itself. In contrast, non-magnetized planets have only small magnetospheres induced by interaction of the
ionosphere The ionosphere () is the ionization, ionized part of the upper atmosphere of Earth, from about to height above sea level, above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionize ...
with the solar wind, which cannot effectively protect the planet. Of the eight planets in the Solar System, only Venus and Mars lack such a magnetic field. Of the magnetized planets the magnetic field of Mercury is the weakest, and is barely able to deflect the
solar wind The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the Stellar corona, corona. This Plasma (physics), plasma mostly consists of electrons, protons and alpha particles with kinetic energy betwee ...
. Jupiter's moon Ganymede has a magnetic field several times stronger, and Jupiter's is the strongest in the Solar System (so intense in fact that it poses a serious health risk to future crewed missions to all its moons inward of Callisto). The magnetic fields of the other giant planets, measured at their surfaces, are roughly similar in strength to that of Earth, but their magnetic moments are significantly larger. The magnetic fields of Uranus and Neptune are strongly tilted relative to the planets' rotational axes and displaced from the planets' centres. In 2003, a team of astronomers in Hawaii observing the star HD 179949 detected a bright spot on its surface, apparently created by the magnetosphere of an orbiting hot Jupiter.


Secondary characteristics

Several planets or dwarf planets in the Solar System (such as Neptune and Pluto) have orbital periods that are in
resonance Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied Periodic function, periodic force (or a Fourier analysis, Fourier component of it) is equal or close to a natural frequency of the system ...
with each other or with smaller bodies. This is common in satellite systems (e.g. the resonance between Io, Europa, and Ganymede around Jupiter, or between Enceladus and Dione around Saturn). All except Mercury and Venus have
natural satellite A natural satellite is, in the most common usage, an astronomical object, astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially ref ...

natural satellite
s, often called "moons". Earth has one, Mars has two, and the giant planets have numerous moons in complex planetary-type systems. Except for Ceres and Sedna, all the consensus dwarf planets are known to have at least one moon as well. Many moons of the giant planets have features similar to those on the terrestrial planets and dwarf planets, and some have been studied as possible abodes of life (especially Europa and Enceladus). The four giant planets are orbited by
planetary ring A ring system is a disc or ring, orbiting an astronomical object, that is composed of solid material such as cosmic dust, dust and moonlets, and is a common component of satellite system (astronomy), satellite systems around giant planets. A ri ...
s of varying size and complexity. The rings are composed primarily of dust or particulate matter, but can host tiny ' moonlets' whose gravity shapes and maintains their structure. Although the origins of planetary rings is not precisely known, they are believed to be the result of natural satellites that fell below their parent planet's
Roche limit In celestial mechanics Celestial mechanics is the branch of astronomy Astronomy () is a natural science that studies astronomical object, celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to expl ...
and were torn apart by
tidal force The tidal force is a gravitational effect that stretches a body along the line towards the center of mass of another body due to a gradient (difference in strength) in gravitational field from the other body; it is responsible for diverse pheno ...
s. The dwarf planet Haumea also has a ring. No secondary characteristics have been observed around extrasolar planets. The
sub-brown dwarf A sub-brown dwarf or planetary-mass brown dwarf is an astronomical object that formed in the same manner as stars and brown dwarfs (i.e. through the collapse of a Nebula, gas cloud) but that has a Planetary-mass object, planetary mass, therefore ...
Cha 110913-773444, which has been described as a rogue planet, is believed to be orbited by a tiny protoplanetary disc * and the sub-brown dwarf OTS 44 was shown to be surrounded by a substantial protoplanetary disk of at least 10 Earth masses.


See also

* *
List of landings on extraterrestrial bodies This is a list of all spacecraft landings on other planets and bodies in the Solar System, including Soft landing (rocketry), soft landings and both intended and unintended Hard landing, hard impacts. The list includes orbiters that were intentio ...
* Lists of planets – A list of lists of planets sorted by diverse attributes * * * *


Notes


References


External links


Photojournal NASA

''Planetary Science Research Discoveries''
(educational site with illustrated articles) {{Featured article Observational astronomy Planetary science Concepts in astronomy