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Shown in order from the Sun
Sun
and in true color. Sizes are not to scale.

A planet is an astronomical body orbiting a star or stellar remnant that

is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.[a][1][2]

The term planet is ancient, with ties to history, astrology, science, mythology, and religion. Several planets in the Solar System
Solar System
can be seen with the naked eye. These were regarded by many early cultures as divine, or as emissaries of deities. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects. In 2006, the International Astronomical Union
International Astronomical Union
(IAU) officially adopted a resolution defining planets within the Solar System. This definition is controversial because it excludes many objects of planetary mass based on where or what they orbit. Although eight of the planetary bodies discovered before 1950 remain "planets" under the modern definition, some celestial bodies, such as Ceres, Pallas, Juno and Vesta (each an object in the solar asteroid belt), and Pluto
Pluto
(the first trans-Neptunian object discovered), that were once considered planets by the scientific community, are no longer viewed as such. The planets were thought by Ptolemy
Ptolemy
to orbit Earth
Earth
in deferent and epicycle motions. Although the idea that the planets orbited the Sun had been suggested many times, it was not until the 17th century that this view was supported by evidence from the first telescopic astronomical observations, performed by Galileo Galilei. At about the same time, by careful analysis of pre-telescopic observation data collected by Tycho Brahe, Johannes Kepler
Johannes Kepler
found the planets' orbits were not circular but elliptical. As observational tools improved, astronomers saw that, like Earth, the planets rotated around tilted axes, and some shared such features as ice caps and seasons. Since the dawn of the Space
Space
Age, close observation by space probes has found that Earth
Earth
and the other planets share characteristics such as volcanism, hurricanes, tectonics, and even hydrology. Planets are generally divided into two main types: large low-density giant planets, and smaller rocky terrestrials. Under IAU definitions, there are eight planets in the Solar System. In order of increasing distance from the Sun, they are the four terrestrials, Mercury, Venus, Earth, and Mars, then the four giant planets, Jupiter, Saturn, Uranus, and Neptune. Six of the planets are orbited by one or more natural satellites. Several thousands of planets around other stars ("extrasolar planets" or "exoplanets") have been discovered in the Milky Way. As of 1 April 2018, 3,758 known extrasolar planets in 2,808 planetary systems (including 627 multiple planetary systems), ranging in size from just above the size of the Moon
Moon
to gas giants about twice as large as Jupiter
Jupiter
have been discovered, out of which more than 100 planets are the same size as Earth, nine of which are at the same relative distance from their star as Earth
Earth
from the Sun, i.e. in the habitable zone.[3][4] On December 20, 2011, the Kepler Space
Space
Telescope
Telescope
team reported the discovery of the first Earth-sized extrasolar planets, Kepler-20e[5] and Kepler-20f,[6] orbiting a Sun-like star, Kepler-20.[7][8][9] A 2012 study, analyzing gravitational microlensing data, estimates an average of at least 1.6 bound planets for every star in the Milky Way.[10] Around one in five Sun-like[b] stars is thought to have an Earth-sized[c] planet in its habitable[d] zone.

Contents

1 History

1.1 Babylon 1.2 Greco-Roman astronomy 1.3 India 1.4 Medieval Muslim astronomy 1.5 European Renaissance 1.6 19th century 1.7 20th century 1.8 21st century

1.8.1 Extrasolar planets 1.8.2 2006 IAU definition of planet

1.9 Objects formerly considered planets

2 Mythology
Mythology
and naming 3 Formation 4 Solar System

4.1 Planetary attributes

5 Exoplanets 6 Planetary-mass objects

6.1 Rogue planets 6.2 Sub-brown dwarfs 6.3 Former stars 6.4 Satellite planets and belt planets 6.5 Captured planets

7 Attributes

7.1 Dynamic characteristics

7.1.1 Orbit 7.1.2 Axial tilt 7.1.3 Rotation 7.1.4 Orbital clearing

7.2 Physical characteristics

7.2.1 Mass 7.2.2 Internal differentiation 7.2.3 Atmosphere 7.2.4 Magnetosphere

7.3 Secondary characteristics

8 See also 9 Notes 10 References 11 External links

History Further information: History of astronomy, Definition of planet, and Timeline of Solar System
Solar System
astronomy

Printed rendition of a geocentric cosmological model from Cosmographia, Antwerp, 1539

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 hundreds of other extrasolar systems. The ambiguities inherent in defining planets have led to much scientific controversy. The five classical planets, being visible to the naked eye, have been known since ancient times and have had a significant impact on mythology, religious cosmology, and ancient astronomy. In ancient times, astronomers noted how certain lights moved across the sky, as opposed to the "fixed stars", which maintained a constant relative position in the sky.[11] Ancient Greeks called these lights πλάνητες ἀστέρες (planētes asteres, "wandering stars") or simply πλανῆται (planētai, "wanderers"),[12] from which today's word "planet" was derived.[13][14][15] In ancient Greece, China, Babylon, and indeed all pre-modern civilizations,[16][17] it was almost universally believed that Earth was the center of the Universe
Universe
and that all the "planets" circled Earth. The reasons for this perception were that stars and planets appeared to revolve around Earth
Earth
each day[18] and the apparently common-sense perceptions that Earth
Earth
was solid and stable and that it was not moving but at rest. Babylon Main article: Babylonian astronomy The first civilization known to have a functional theory of the planets were the Babylonians, who lived in Mesopotamia
Mesopotamia
in the first and second millennia BC. The oldest surviving planetary astronomical text is the Babylonian Venus
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.[19] The MUL.APIN is a pair of cuneiform tablets dating from the 7th century BC that lays out the motions of the Sun, Moon, and planets over the course of the year.[20] The Babylonian astrologers also laid the foundations of what would eventually become Western astrology.[21] The Enuma anu enlil, written during the Neo-Assyrian
Neo-Assyrian
period in the 7th century BC,[22] comprises a list of omens and their relationships with various celestial phenomena including the motions of the planets.[23][24] Venus, Mercury, and the outer planets Mars, Jupiter, and Saturn
Saturn
were all identified by Babylonian astronomers. These would remain the only known planets until the invention of the telescope in early modern times.[25] Greco-Roman astronomy See also: Greek astronomy

Ptolemy's 7 planetary spheres

1 Moon

2 Mercury

3 Venus

4 Sun

5 Mars

6 Jupiter

7 Saturn

The ancient Greeks initially did not attach as much significance to the planets as the Babylonians. The Pythagoreans, 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
or Parmenides
Parmenides
is said to have been the first to identify the evening star (Hesperos) and morning star (Phosphoros) as one and the same (Aphrodite, Greek corresponding to Latin Venus).[26] In the 3rd century BC, Aristarchus of Samos
Aristarchus of Samos
proposed a heliocentric system, according to which Earth
Earth
and the planets revolved around the Sun. The geocentric system remained dominant until the Scientific Revolution. By the 1st century BC, during the Hellenistic period, 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
Almagest
written by Ptolemy
Ptolemy
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.[19][27] To the Greeks and Romans there were seven known planets, each presumed to be circling Earth
Earth
according to the complex laws laid out by Ptolemy. They were, in increasing order from Earth
Earth
(in Ptolemy's order): the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn.[15][27][28] India Main articles: Indian astronomy
Indian astronomy
and Hindu cosmology In 499 CE, the Indian astronomer Aryabhata
Aryabhata
propounded a planetary model that explicitly incorporated Earth's rotation
Earth's rotation
about its axis, which he explains as the cause of what appears to be an apparent westward motion of the stars. He also believed that the orbits of planets are elliptical.[29] Aryabhata's followers were particularly strong in South India, where his principles of the diurnal rotation of Earth, among others, were followed and a number of secondary works were based on them.[30] In 1500, Nilakantha Somayaji of the Kerala school of astronomy and mathematics, in his Tantrasangraha, revised Aryabhata's model.[31] In his Aryabhatiyabhasya, a commentary on Aryabhata's Aryabhatiya, he developed a planetary model where Mercury, Venus, Mars, Jupiter
Jupiter
and Saturn
Saturn
orbit the Sun, which in turn orbits Earth, similar to the Tychonic system
Tychonic system
later proposed by Tycho Brahe
Tycho Brahe
in the late 16th century. Most astronomers of the Kerala school who followed him accepted his planetary model.[31][32] Medieval Muslim astronomy Main articles: Astronomy
Astronomy
in the medieval Islamic world and Cosmology in medieval Islam In the 11th century, the transit of Venus
Venus
was observed by Avicenna, who established that Venus
Venus
was, at least sometimes, below the Sun.[33] In the 12th century, Ibn Bajjah observed "two planets as black spots on the face of the Sun", which was later identified as a transit of Mercury and Venus
Venus
by the Maragha astronomer Qotb al-Din Shirazi
Qotb al-Din Shirazi
in the 13th century.[34] Ibn Bajjah could not have observed a transit of Venus, because none occurred in his lifetime.[35] European Renaissance

Renaissance planets, c. 1543 to 1610 and c. 1680 to 1781

1 Mercury

2 Venus

3 Earth

4 Mars

5 Jupiter

6 Saturn

See also: Heliocentrism With the advent of the Scientific Revolution, use of the term "planet" changed from something that moved across the sky (in relation to the star field); to a body that orbited Earth
Earth
(or that was believed to do so at the time); and by the 18th century to something that directly orbited the Sun
Sun
when the heliocentric model of Copernicus, Galileo and Kepler gained sway. Thus, Earth
Earth
became included in the list of planets,[36] whereas the Sun
Sun
and Moon
Moon
were excluded. At first, when the first satellites of Jupiter
Jupiter
and Saturn
Saturn
were discovered in the 17th century, the terms "planet" and "satellite" were used interchangeably – although the latter would gradually become more prevalent in the following century.[37] Until the mid-19th century, the number of "planets" rose rapidly because any newly discovered object directly orbiting the Sun was listed as a planet by the scientific community. 19th century

Eleven planets, 1807–1845

1 Mercury

2 Venus

3 Earth

4 Mars

5 Vesta

6 Juno

7 Ceres

8 Pallas

9 Jupiter

10 Saturn

11 Uranus

In the 19th century astronomers began to realize that recently discovered bodies that had been classified as planets for almost half a century (such as Ceres, Pallas, Juno, and Vesta) were very different from the traditional ones. These bodies shared the same region of space between Mars
Mars
and Jupiter
Jupiter
(the asteroid belt), and had a much smaller mass; as a result they were reclassified as "asteroids". In the absence of any formal definition, a "planet" came to be understood as any "large" body that orbited the Sun. Because there was a dramatic size gap between the asteroids and the planets, and the spate of new discoveries seemed to have ended after the discovery of Neptune
Neptune
in 1846, there was no apparent need to have a formal definition.[38] 20th century

Planets 1854–1930, Solar planets 2006–present

1 Mercury

2 Venus

3 Earth

4 Mars

5 Jupiter

6 Saturn

7 Uranus

8 Neptune

In the 20th century, Pluto
Pluto
was discovered. After initial observations led to the belief that it was larger than Earth,[39] the object was immediately accepted as the ninth planet. Further monitoring found the body was actually much smaller: in 1936, Ray Lyttleton suggested that Pluto
Pluto
may be an escaped satellite of Neptune,[40] and Fred Whipple suggested in 1964 that Pluto
Pluto
may be a comet.[41] As it was still larger than all known asteroids and seemingly did not exist within a larger population,[42] it kept its status until 2006.

(Solar) planets 1930–2006

1 Mercury

2 Venus

3 Earth

4 Mars

5 Jupiter

6 Saturn

7 Uranus

8 Neptune

9 Pluto

In 1992, astronomers Aleksander Wolszczan
Aleksander Wolszczan
and Dale Frail announced the discovery of planets around a pulsar, PSR B1257+12.[43] This discovery is generally considered to be the first definitive detection of a planetary system around another star. Then, on October 6, 1995, Michel Mayor and Didier Queloz
Didier Queloz
of the Geneva Observatory
Geneva Observatory
announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[44] 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 dwarfs. Brown dwarfs are generally considered stars due to their ability to fuse deuterium, a heavier isotope of hydrogen. Although objects more massive than 75 times that of Jupiter
Jupiter
fuse hydrogen, objects of only 13 Jupiter
Jupiter
masses can fuse deuterium. Deuterium
Deuterium
is quite rare, and most brown dwarfs would have ceased fusing deuterium long before their discovery, making them effectively indistinguishable from supermassive planets.[45] 21st century With the discovery during the latter half of the 20th century of more objects within the Solar System
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 of deuterium. A growing number of astronomers argued for Pluto
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
Solar System
(the Kuiper belt) during the 1990s and early 2000s. Pluto
Pluto
was found to be just one small body in a population of thousands. Some of them, such as Quaoar, Sedna, and Eris, were heralded in the popular press as the tenth planet, failing to receive widespread scientific recognition. The announcement of Eris in 2005, an object then thought of as 27% more massive than Pluto, created the necessity and public desire for an official definition of a planet. Acknowledging the problem, the IAU set about creating the definition of planet, and produced one in August 2006. The number of planets 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 planets was created, initially containing three objects (Ceres, Pluto
Pluto
and Eris).[46] Extrasolar planets There is no official definition of extrasolar planets. In 2003, the International Astronomical Union
International Astronomical Union
(IAU) Working Group on Extrasolar Planets issued a position statement, but this position statement was never proposed as an official IAU resolution and was never voted on by IAU members. The positions statement incorporates the following guidelines, mostly focused upon the boundary between planets and brown dwarfs:[2]

Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 times the mass of Jupiter
Jupiter
for objects with the same isotopic abundance as the Sun[47]) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass and size required for an extrasolar object to be considered a planet should be the same as that used in the Solar System. Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed or where they are located. Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).

This working definition has since been widely used by astronomers when publishing discoveries of exoplanets in academic journals.[48] Although temporary, it remains an effective working definition until a more permanent one is formally adopted. It does not address the dispute over the lower mass limit,[49] and so it steered clear of the controversy regarding objects within the Solar System. This definition also makes no comment on the planetary status of objects orbiting brown dwarfs, such as 2M1207b. One definition of a sub-brown dwarf is a planet-mass object that formed through cloud collapse rather than accretion. This formation distinction between a sub-brown dwarf and a planet is not universally agreed upon; astronomers are divided into two camps as whether to consider the formation process of a planet as part of its division in classification.[50] One reason for the dissent is that often it may not be possible to determine the formation process. For example, a planet formed by accretion around a star may get ejected from the system to become free-floating, and likewise a sub-brown dwarf that formed on its own in a star cluster through cloud collapse may get captured into orbit around a star. The 13 Jupiter-mass cutoff represents an average mass rather than a precise threshold value. Large objects will fuse most of their deuterium and smaller ones will fuse only a little, and the 13 MJ value is somewhere in between. In fact, calculations show that an object fuses 50% of its initial deuterium content when the total mass ranges between 12 and 14 MJ.[51] The amount of deuterium fused depends not only on mass but also on the composition of the object, on the amount of helium and deuterium present.[52] The Extrasolar Planets Encyclopaedia includes objects up to 25 Jupiter
Jupiter
masses, saying, "The fact that there is no special feature around 13 MJ in the observed mass spectrum reinforces the choice to forget this mass limit."[53] The Exoplanet Data Explorer includes objects up to 24 Jupiter
Jupiter
masses with the advisory: "The 13 Jupiter-mass distinction by the IAU Working Group is physically unmotivated for planets with rocky cores, and observationally problematic due to the sin i ambiguity."[54] The NASA Exoplanet
Exoplanet
Archive includes objects with a mass (or minimum mass) equal to or less than 30 Jupiter
Jupiter
masses.[55] Another criterion for separating planets and brown dwarfs, rather than deuterium fusion, formation process or location, is whether the core pressure is dominated by coulomb pressure or electron degeneracy pressure.[56][57] 2006 IAU definition of planet Main article: IAU definition of planet

Euler diagram
Euler diagram
showing the types of bodies in the Solar System.

The matter of the lower limit was addressed during the 2006 meeting of the IAU's General Assembly. After much debate and one failed proposal, 232 members of the 10,000 member assembly, who nevertheless constituted a large majority of those remaining at the meeting, voted to pass a resolution. The 2006 resolution defines planets within the Solar System
Solar System
as follows:[1]

A "planet" [1] is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. [1] The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Under this definition, the Solar System
Solar System
is considered to have eight planets. Bodies that fulfill the first two conditions but not the third (such as Ceres, Pluto, and Eris) are classified as dwarf planets, provided they are not also natural satellites of other planets. Originally an IAU committee had proposed a definition that would have included a much larger number of planets as it did not include (c) as a criterion.[58] After much discussion, it was decided via a vote that those bodies should instead be classified as dwarf planets.[59] This definition is based in theories of planetary formation, in which planetary embryos initially clear their orbital neighborhood of other smaller objects. As described by astronomer Steven Soter:[60]

"The end product of secondary disk accretion is a small number of relatively large bodies (planets) in either non-intersecting or resonant orbits, which prevent collisions between them. Minor planets and comets, including KBOs [ Kuiper belt
Kuiper belt
objects], differ from planets in that they can collide with each other and with planets."

The 2006 IAU definition presents some challenges for exoplanets because the language is specific to the Solar System
Solar System
and because the criteria of roundness and orbital zone clearance are not presently observable. Astronomer
Astronomer
Jean-Luc Margot 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.[61][62] This formula produces a value π that is greater than 1 for planets. The eight known planets and all known exoplanets have π values above 100, while Ceres, Pluto, and Eris have π values of 0.1 or less. Objects with π values of 1 or more are also expected to be approximately spherical, so that objects that fulfill the orbital zone clearance requirement automatically fulfill the roundness requirement.[63] Objects formerly considered planets

The table below lists Solar System
Solar System
bodies once considered to be planets.

Body Current classification Notes

Sun Star Classified as classical planets (Ancient Greek πλανῆται, wanderers) in classical antiquity and medieval Europe, in accordance with the now-disproved geocentric model.[64]

Moon Natural satellite

Io, Europa, Ganymede, and Callisto Natural satellites The four largest moons of Jupiter, known as the Galilean moons
Galilean moons
after their discoverer Galileo Galilei. He referred to them as the "Medicean Planets" in honor of his patron, the Medici
Medici
family. They were known as secondary planets.[65]

Titan,[e] Iapetus,[f] Rhea,[f] Tethys,[g] and Dione[g] Natural satellites Five of Saturn's larger moons, discovered by Christiaan Huygens
Christiaan Huygens
and Giovanni Domenico Cassini. As with Jupiter's major moons, they were known as secondary planets.[65]

Pallas, Juno, and Vesta Asteroids Regarded as planets from their discoveries between 1801 and 1807 until they were reclassified as asteroids during the 1850s.[67] Ceres was subsequently classified as a dwarf planet in 2006.

Ceres Dwarf planet
Dwarf planet
and asteroid

Astraea, Hebe, Iris, Flora, Metis, Hygiea, Parthenope, Victoria, Egeria, Irene, Eunomia Asteroids More asteroids, discovered between 1845 and 1851. The rapidly expanding list of bodies between Mars
Mars
and Jupiter
Jupiter
prompted their reclassification as asteroids, which was widely accepted by 1854.[68]

Pluto Dwarf planet
Dwarf planet
and Kuiper belt
Kuiper belt
object The first known trans-Neptunian object (i.e. minor planet with a semi-major axis beyond Neptune). Regarded as a planet from its discovery in 1930 until it was reclassified as a dwarf planet in 2006.

Beyond the scientific community, Pluto
Pluto
still holds cultural significance for many in the general public due to its historical classification as a planet from 1930 to 2006.[69] A few astronomers, such as Alan Stern, consider dwarf planets and the larger moons to be planets, based on a purely geophysical definition of planet.[70] Mythology
Mythology
and naming See also: Weekday names
Weekday names
and Naked-eye planet

The Greek gods of Olympus, after whom the Solar System's Roman names of the planets are derived

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, the two great luminaries the Sun
Sun
and the Moon
Moon
were called Helios
Helios
and Selene; the farthest planet (Saturn) was called Phainon, the shiner; followed by Phaethon
Phaethon
(Jupiter), "bright"; the red planet (Mars) was known as Pyroeis, the "fiery"; the brightest (Venus) was known as Phosphoros, the light bringer; and the fleeting final planet (Mercury) was called Stilbon, the gleamer. The Greeks also made each planet sacred to one among their pantheon of gods, the Olympians: Helios
Helios
and Selene
Selene
were the names of both planets and gods; Phainon was sacred to Cronus, the Titan who fathered the Olympians; Phaethon
Phaethon
was sacred to Zeus, Cronus's son who deposed him as king; Pyroeis was given to Ares, son of Zeus
Zeus
and god of war; Phosphoros
Phosphoros
was ruled by Aphrodite, the goddess of love; and Hermes, messenger of the gods and god of learning and wit, ruled over Stilbon.[19] The Greek practice of grafting of their gods' names onto the planets was almost certainly borrowed from the Babylonians. The Babylonians named Phosphoros
Phosphoros
after their goddess of love, Ishtar; Pyroeis after their god of war, Nergal, Stilbon after their god of wisdom Nabu, and Phaethon
Phaethon
after their chief god, Marduk.[71] There are too many concordances between Greek and Babylonian naming conventions for them to have arisen separately.[19] The translation was not perfect. For instance, the Babylonian Nergal
Nergal
was a god of war, and thus the Greeks identified him with Ares. Unlike Ares, Nergal
Nergal
was also god of pestilence and the underworld.[72] 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
Roman Empire
and, later, the Catholic Church, use the Roman (Latin) names rather than the Greek ones. The Romans, who, like the Greeks, were Indo-Europeans, shared with them a common pantheon under different names but lacked the rich narrative traditions that Greek poetic culture had given their gods. During the later period of the Roman Republic, Roman writers borrowed much of the Greek narratives and applied them to their own pantheon, to the point where they became virtually indistinguishable.[73] When the Romans studied Greek astronomy, they gave the planets their own gods' names: Mercurius (for Hermes), Venus
Venus
(Aphrodite), Mars
Mars
(Ares), Iuppiter (Zeus) and Saturnus (Cronus). When subsequent planets were discovered in the 18th and 19th centuries, the naming practice was retained with Neptūnus (Poseidon). Uranus
Uranus
is unique in that it is named for a Greek deity rather than his Roman counterpart. Some Romans, following a belief possibly originating in 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
Moon
(from the farthest to the closest planet).[74] Therefore, the first day was started by Saturn
Saturn
(1st hour), second day by Sun
Sun
(25th hour), followed by Moon
Moon
(49th hour), Mars, Mercury, Jupiter
Jupiter
and Venus. Because each day was named by the god that started it, this is also the order of the days of the week in the Roman calendar after the Nundinal cycle was rejected – and still preserved in many modern languages.[75] In English, Saturday, Sunday, and Monday are straightforward translations of these Roman names. The other days were renamed after Tiw (Tuesday), Wóden (Wednesday), Thunor (Thursday), and Fríge (Friday), the Anglo-Saxon gods considered similar or equivalent to Mars, Mercury, Jupiter, and Venus, respectively. Earth
Earth
is the only planet whose name in English is not derived from Greco-Roman mythology. Because it was only generally accepted as a planet in the 17th century,[36] there is no tradition of naming it after a god. (The same is true, in English at least, of the Sun
Sun
and the Moon, though they are no longer generally considered planets.) The name originates from the 8th century Anglo-Saxon word erda, which means ground or soil and was first used in writing as the name of the sphere of Earth
Earth
perhaps around 1300.[76][77] As with its equivalents in the other Germanic languages, it derives ultimately from the Proto-Germanic
Proto-Germanic
word ertho, "ground",[77] as can be seen in the English earth, the German Erde, the Dutch aarde, and the Scandinavian jord. Many of the Romance languages
Romance languages
retain the old Roman word terra (or some variation of it) that was used with the meaning of "dry land" as opposed to "sea".[78] The non- Romance languages
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
uses a system based on the Navagraha, which incorporates the seven traditional planets ( Surya
Surya
for the Sun, Chandra
Chandra
for the Moon, and Budha, Shukra, Mangala, Bṛhaspati
Bṛhaspati
and Shani
Shani
for Mercury, Venus, Mars, Jupiter
Jupiter
and Saturn) and the ascending and descending lunar nodes Rahu
Rahu
and Ketu. China and the countries of eastern Asia historically subject to Chinese cultural influence (such as Japan, Korea
Korea
and Vietnam) use a naming system based on the five Chinese elements: water (Mercury), metal (Venus), fire (Mars), wood (Jupiter) and earth (Saturn).[75] In traditional Hebrew astronomy, the seven traditional planets have (for the most part) descriptive names - the Sun
Sun
is חמה Ḥammah or "the hot one," the Moon
Moon
is לבנה Levanah or "the white one," Venus
Venus
is כוכב נוגה Kokhav Nogah or "the bright planet," Mercury is כוכב Kokhav or "the planet" (given its lack of distinguishing features), Mars
Mars
is מאדים Ma'adim or "the red one," and Saturn
Saturn
is שבתאי Shabbatai or "the resting one" (in reference to its slow movement compared to the other visible planets).[79] The odd one out is Jupiter, called צדק Tzedeq or "justice." Steiglitz suggests that this may be a euphemism for the original name of כוכב בעל Kokhav Ba'al or "Baal's planet," seen as idolatrous and euphemized in a similar manner to Ishbosheth
Ishbosheth
from II Samuel.[79] Formation Main article: Nebular hypothesis

An artist's impression of protoplanetary disk

It is not known with certainty how planets are formed. The prevailing theory is that they are formed during the collapse of a nebula into a thin disk of gas and dust. A protostar forms at the core, surrounded by a rotating protoplanetary disk. Through accretion (a process of sticky collision) dust particles in the disk steadily accumulate mass to form ever-larger bodies. Local concentrations of mass known as planetesimals 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 protoplanets.[80] After a planet reaches a mass somewhat larger than Mars' mass, it begins to accumulate an extended atmosphere,[81] greatly increasing the capture rate of the planetesimals by means of atmospheric drag.[82][83] Depending on the accretion history of solids and gas, a giant planet, an ice giant, or a terrestrial planet may result.[84][85][86]

Asteroid
Asteroid
collision - building planets (artist concept).

When the protostar has grown such that it ignites to form a star, the surviving disk is removed from the inside outward by photoevaporation, the solar wind, Poynting–Robertson drag and other effects.[87][88] Thereafter there still may be many protoplanets orbiting the star or each other, but over time many will collide, either to form a single larger planet or release material for other larger protoplanets or planets to absorb.[89] 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 satellites 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) will heat up the growing planet, causing it to at least partially melt. The interior of the planet begins to differentiate by mass, developing a denser core.[90] 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 comets.[91] (Smaller planets will lose any atmosphere they gain through various escape mechanisms.) With the discovery and observation of planetary systems around stars other than the Sun, it is becoming possible to elaborate, revise or even replace this account. The level of metallicity—an astronomical term describing the abundance of chemical elements with an atomic number greater than 2 (helium)—is now thought to determine the likelihood that a star will have planets.[92] Hence, it is thought that a metal-rich population I star will likely have a more substantial planetary system than a metal-poor, population II star.

Supernova remnant
Supernova remnant
ejecta producing planet-forming material.

Solar System

Solar System
Solar System
– sizes but not distances are to scale

The Sun
Sun
and the eight planets of the Solar System

The inner planets, Mercury, Venus, Earth, and Mars

The four giant planets Jupiter, Saturn, Uranus, and Neptune
Neptune
against the Sun
Sun
and some sunspots

Main article: Solar System See also: List of gravitationally rounded objects of the Solar System There are eight planets in the Solar System, which are in increasing distance from the Sun:

Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune

Jupiter
Jupiter
is the largest, at 318 Earth
Earth
masses, whereas Mercury is the smallest, at 0.055 Earth
Earth
masses. The planets of the Solar System
Solar System
can be divided into categories based on their composition:

Terrestrials: Planets that are similar to Earth, with bodies largely composed of rock: Mercury, Venus, Earth
Earth
and Mars. At 0.055 Earth masses, Mercury is the smallest terrestrial planet (and smallest planet) in the Solar System. Earth
Earth
is the largest terrestrial planet. Giant planets (Jovians): Massive planets significantly more massive than the terrestrials: Jupiter, Saturn, Uranus, Neptune.

Gas giants, Jupiter
Jupiter
and Saturn, are giant planets primarily composed of hydrogen and helium and are the most massive planets in the Solar System. Jupiter, at 318 Earth
Earth
masses, is the largest planet in the Solar System, and Saturn
Saturn
is one third as massive, at 95 Earth
Earth
masses. Ice giants, Uranus
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
Earth
masses).

Planetary attributes

Name Equatorial diameter [h] Mass [h] Semi-major axis
Semi-major axis
(AU) Orbital period (years) [h] Inclination to Sun's equator (°) Orbital eccentricity Rotation period (days) Confirmed moons [i] Axial tilt
Axial tilt
(°) Rings Atmosphere

1. Mercury 0.382 0.06 0.39 0.24 3.38 0.206 58.64 0 0.04 no minimal

2. Venus 0.949 0.82 0.72 0.62 3.86 0.007 −243.02 0 177.36 no CO2, N2

3. Earth (a) 1.00 1.00 1.00 1.00 7.25 0.017 1.00 1 23.44 no N2, O2, Ar

4. Mars 0.532 0.11 1.52 1.88 5.65 0.093 1.03 2 25.19 no CO2, N2, Ar

5. Jupiter 11.209 317.8 5.20 11.86 6.09 0.048 0.41 69 3.13 yes H2, He

6. Saturn 9.449 95.2 9.54 29.46 5.51 0.054 0.43 62 26.73 yes H2, He

7. Uranus 4.007 14.6 19.22 84.01 6.48 0.047 −0.72 27 97.77 yes H2, He, CH4

8. Neptune 3.883 17.2 30.06 164.8 6.43 0.009 0.67 14 28.32 yes H2, He, CH4

Color legend:   terrestrial planets   gas giants   ice giants (both are giant planets). (a) Find absolute values in article Earth

Exoplanets Main article: Exoplanet

Exoplanets, by year of discovery, through September 2014.

An exoplanet (extrasolar planet) is a planet outside the Solar System. As of 1 April 2018, there are 3,758 confirmed planets in 2,808 systems, with 627 systems having more than one planet.[94][95][96][97] In early 1992, radio astronomers Aleksander Wolszczan
Aleksander Wolszczan
and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12.[43] This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of giant planets that survived the supernova and then decayed into their current orbits.

Sizes of Kepler Planet
Planet
Candidates – based on 2,740 candidates orbiting 2,036 stars as of 4 November 2013[update] (NASA).

The first confirmed discovery of an extrasolar planet orbiting an ordinary main-sequence star occurred on 6 October 1995, when Michel Mayor and Didier Queloz
Didier Queloz
of the University of Geneva
University of Geneva
announced the detection of an exoplanet around 51 Pegasi. From then until the Kepler mission most known extrasolar planets were gas giants comparable in mass to Jupiter
Jupiter
or larger as they were more easily detected. The catalog of Kepler candidate planets consists mostly of planets the size of Neptune
Neptune
and smaller, down to smaller than Mercury. There are types of planets that do not exist in the Solar System: super-Earths and mini-Neptunes, which could be rocky like Earth
Earth
or a mixture of volatiles and gas like Neptune—a radius of 1.75 times that of Earth
Earth
is a possible dividing line between the two types of planet.[98] There are hot Jupiters that orbit very close to their star and may evaporate to become chthonian planets, which are the leftover cores. Another possible type of planet is carbon planets, which form in systems with a higher proportion of carbon than in the Solar System. A 2012 study, analyzing gravitational microlensing data, estimates an average of at least 1.6 bound planets for every star in the Milky Way.[10] On December 20, 2011, the Kepler Space
Space
Telescope
Telescope
team reported the discovery of the first Earth-size exoplanets, Kepler-20e[5] and Kepler-20f,[6] orbiting a Sun-like star, Kepler-20.[7][8][9] Around 1 in 5 Sun-like[b] stars have an "Earth-sized"[c] planet in the habitable[d] zone, so the nearest would be expected to be within 12 light-years distance from Earth.[99][100] The frequency of occurrence of such terrestrial planets is one of the variables in the Drake equation, which estimates the number of intelligent, communicating civilizations that exist in the Milky Way.[101] There are exoplanets that are much closer to their parent star than any planet in the Solar System
Solar System
is to the Sun, and there are also exoplanets that are much farther from their star. Mercury, the closest planet to the Sun
Sun
at 0.4 AU, takes 88 days for an orbit, but the shortest known orbits for exoplanets take only a few hours, e.g. Kepler-70b. The Kepler-11
Kepler-11
system has five of its planets in shorter orbits than Mercury's, all of them much more massive than Mercury. Neptune
Neptune
is 30 AU from the Sun
Sun
and takes 165 years to orbit, but there are exoplanets that are hundreds of AU from their star and take more than a thousand years to orbit, e.g. 1RXS1609 b. The next few space telescopes to study exoplanets are expected to be Gaia launched in December 2013, CHEOPS in 2018, TESS in 2018, and the James Webb Space
Space
Telescope
Telescope
in 2019.

Planetary-mass objects

Artist's impression of a super- Jupiter
Jupiter
around the brown dwarf 2M1207.[102]

See also: List of gravitationally rounded objects of the Solar System A planetary-mass object (PMO), planemo,[103] or planetary body is a celestial object with a mass that falls within the range of the definition of a planet: massive enough to achieve hydrostatic equilibrium (to be rounded under its own gravity), but not enough to sustain core fusion like a star.[104][105] By definition, all planets are planetary-mass objects, but the purpose of this term is to refer to objects that do not conform to typical expectations for a planet. These include dwarf planets, which are rounded by their own gravity but not massive enough to clear their own orbit, the larger moons, and free-floating planemos, which may have been ejected from a system (rogue planets) or formed through cloud-collapse rather than accretion (sometimes called sub-brown dwarfs). Rogue planets Main article: Rogue planet See also: Five-planet Nice model Several computer simulations of stellar and planetary system formation have suggested that some objects of planetary mass would be ejected into interstellar space.[106] Some scientists have argued that such objects found roaming in deep space should be classed as "planets", although others have suggested that they should be called low-mass brown dwarfs.[107][108] Sub-brown dwarfs Main article: Sub-brown dwarf Stars form via the gravitational collapse of gas clouds, but smaller objects can also form via cloud-collapse. Planetary-mass objects formed this way are sometimes called sub-brown dwarfs. Sub-brown dwarfs may be free-floating such as Cha 110913-773444[107] and OTS 44,[109] or orbiting a larger object such as 2MASS J04414489+2301513. Binary systems of sub-brown dwarfs are theoretically possible; Oph 162225-240515 was initially thought to be a binary system of a brown dwarf of 14 Jupiter
Jupiter
masses and a sub-brown dwarf of 7 Jupiter
Jupiter
masses, but further observations revised the estimated masses upwards to greater than 13 Jupiter
Jupiter
masses, making them brown dwarfs according to the IAU working definitions.[110][111][112] Former stars In close binary star systems one of the stars can lose mass to a heavier companion. Accretion-powered pulsars may drive mass loss. The shrinking star can then become a planetary-mass object. An example is a Jupiter-mass object orbiting the pulsar PSR J1719-1438.[113] These shrunken white dwarfs may become a helium planet or carbon planet. Satellite planets and belt planets Some large satellites are of similar size or larger than the planet Mercury, e.g. Jupiter's Galilean moons
Galilean moons
and Titan. Alan Stern
Alan Stern
has argued that location should not matter and that only geophysical attributes should be taken into account in the definition of a planet, and proposes the term satellite planet for a planet-sized satellite. Likewise, dwarf planets in the asteroid belt and Kuiper belt
Kuiper belt
should be considered planets according to Stern.[70] Captured planets Free-floating planets in stellar clusters have similar velocities to the stars and so can be recaptured. They are typically captured into wide orbits between 100 and 105 AU. The capture efficiency decreases with increasing cluster volume, and for a given cluster size it increases with the host/primary mass. It is almost independent of the planetary mass. Single and multiple planets could be captured into arbitrary unaligned orbits, non-coplanar with each other or with the stellar host spin, or pre-existing planetary system.[114] 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 also commonly observed in extrasolar planets. Dynamic characteristics Orbit Main articles: Orbit
Orbit
and Orbital elements See also: Kepler's laws of planetary motion

The orbit of the planet Neptune
Neptune
compared to that of Pluto. Note the elongation of Pluto's orbit in relation to Neptune's (eccentricity), as well as its large angle to the ecliptic (inclination).

According to current definitions, all planets must revolve around stars; thus, any potential "rogue planets" are excluded. In the Solar System, all the planets orbit the Sun
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.[115] The period of one revolution of a planet's orbit is known as its sidereal period or year.[116] A planet's year depends on its distance from its star; the farther a planet is from its star, not only the longer the distance it must travel, but also the slower its speed, because it is less affected by its star's gravity. No planet's orbit is perfectly circular, and hence the distance of each varies over the course of its year. The closest approach to its star is called its periastron (perihelion in the Solar System), whereas its farthest separation from the star is called its apastron (aphelion). As a planet approaches periastron, its speed increases as it trades gravitational potential energy for kinetic energy, just as a falling object on Earth accelerates as it falls; as the planet reaches apastron, its speed decreases, just as an object thrown upwards on Earth
Earth
slows down as it reaches the apex of its trajectory.[117] Each planet's orbit is delineated by a set of elements:

The eccentricity of an orbit describes how elongated a planet's orbit is. Planets with low eccentricities have more circular orbits, whereas planets with high eccentricities have more elliptical orbits. The planets in the Solar System
Solar System
have very low eccentricities, and thus nearly circular orbits.[116] Comets and Kuiper belt
Kuiper belt
objects (as well as several extrasolar planets) have very high eccentricities, and thus exceedingly elliptical orbits.[118][119]

Illustration of the semi-major axis

The semi-major axis is the distance from a planet to the half-way point along the longest diameter of its elliptical orbit (see image). This distance is not the same as its apastron, because no planet's orbit has its star at its exact centre.[116] The inclination of a planet tells how far above or below an established reference plane its orbit lies. In the Solar System, the reference plane is the plane of Earth's orbit, called the ecliptic. 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.[120] The eight planets of the Solar System
Solar System
all lie very close to the ecliptic; comets and Kuiper belt
Kuiper belt
objects like Pluto
Pluto
are at far more extreme angles to it.[121] The points at which a planet crosses above and below its reference plane are called its ascending and descending nodes.[116] The longitude of the ascending node is the angle between the reference plane's 0 longitude and the planet's ascending node. The argument of periapsis (or perihelion in the Solar System) is the angle between a planet's ascending node and its closest approach to its star.[116]

Axial tilt Main article: Axial tilt

Earth's axial tilt is about 23.4°. It oscillates between 22.1° and 24.5° on a 41,000-year cycle and is currently decreasing.

Planets also have varying degrees of axial tilt; they lie at an angle to the plane 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, 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. Each planet has two in the course of its orbit; when one hemisphere has its summer solstice, when its day is 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 perpetually in sunlight or perpetually in darkness around the time of its solstices.[122] Among extrasolar planets, axial tilts are not known for certain, though most hot Jupiters are believed to have negligible to no axial tilt as a result of their proximity to their stars.[123] Rotation The planets rotate around invisible axes through their centres. A planet's rotation period is known as a stellar day. Most of the planets in the Solar System
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 exceptions being Venus[124] and Uranus,[125] 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.[126] Regardless of which convention is used, Uranus
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 can be induced by the individual angular momentum contributions of accreted objects. The accretion of gas by the giant planets can also contribute to the angular momentum. Finally, during the last stages of planet building, a stochastic process of protoplanetary accretion can randomly alter the spin axis of the planet.[127] There is great variation in the length of day between the planets, with Venus
Venus
taking 243 days to rotate, and the giant planets only a few hours.[128] The rotational periods of extrasolar planets are not known. However, for "hot" Jupiters, their proximity to their stars means that they are tidally locked (i.e., 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.[129] Orbital clearing Main article: Clearing the neighbourhood The defining dynamic characteristic of a planet 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 planetesimals in its orbit. In effect, it orbits its star in isolation, as opposed to sharing its orbit with a multitude of similar-sized objects. This characteristic was mandated as part of the IAU's official definition of a planet in August, 2006. This criterion excludes such planetary bodies as Pluto, Eris and Ceres from full-fledged planethood, making them instead dwarf planets.[1] 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.[130] Physical characteristics Mass Main article: Planetary 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 forces binding its physical structure, leading to a state of hydrostatic equilibrium. 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.[131] Mass is also the prime attribute by which planets are distinguished from stars. The upper mass limit for planethood is roughly 13 times Jupiter's mass for objects with solar-type isotopic abundance, beyond which it achieves conditions suitable for nuclear fusion. Other than the Sun, no objects of such mass exist in the Solar System; but there are exoplanets of this size. The 13-Jupiter-mass limit is not universally agreed upon and the Extrasolar Planets Encyclopaedia includes objects up to 20 Jupiter
Jupiter
masses,[132] and the Exoplanet
Exoplanet
Data Explorer up to 24 Jupiter
Jupiter
masses.[133] The smallest known planet is PSR B1257+12A, one of the first extrasolar planets discovered, which was found in 1992 in orbit around a pulsar. Its mass is roughly half that of the planet Mercury.[4] The smallest known planet orbiting a main-sequence star other than the Sun is Kepler-37b, with a mass (and radius) slightly higher than that of the Moon. Internal differentiation Main article: Planetary differentiation

Illustration of the interior of Jupiter, with a rocky core overlaid by a deep layer of metallic hydrogen

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 surrounded by a mantle that either is or was a fluid. The terrestrial planets are sealed within hard crusts,[134] but in the giant planets the mantle simply blends into the upper cloud layers. The terrestrial planets have cores of elements such as iron and nickel, and mantles of silicates. Jupiter
Jupiter
and Saturn
Saturn
are believed to have cores of rock and metal surrounded by mantles of metallic hydrogen.[135] Uranus
Uranus
and Neptune, which are smaller, have rocky cores surrounded by mantles of water, ammonia, methane and other ices.[136] The fluid action within these planets' cores creates a geodynamo that generates a magnetic field.[134] Atmosphere Main articles: Atmosphere
Atmosphere
and Extraterrestrial atmospheres See also: Extraterrestrial skies

Earth's atmosphere

All of the Solar System
Solar System
planets except Mercury[137] have substantial atmospheres because their gravity is strong enough to keep gases close to the surface. 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.[138] 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.[139] Planetary atmospheres are affected by the varying insolation or internal energy, leading to the formation of dynamic weather systems such as hurricanes, (on Earth), planet-wide dust storms (on Mars), a greater-than-Earth-sized anticyclone on Jupiter
Jupiter
(called the Great Red Spot), and holes in the atmosphere (on Neptune).[122] At least one extrasolar planet, HD 189733 b, has been claimed to have such a weather system, similar to the Great Red Spot
Great Red Spot
but twice as large.[140] 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.[141][142] These planets may have vast differences in temperature between their day and night sides that produce supersonic winds,[143] although the day and night sides of HD 189733 b
HD 189733 b
appear to have very similar temperatures, indicating that that planet's atmosphere effectively redistributes the star's energy around the planet.[140] Magnetosphere Main article: Magnetosphere

Earth's magnetosphere (diagram)

One important characteristic of the planets is their intrinsic magnetic moments, 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 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 with the solar wind, which cannot effectively protect the planet.[144] Of the eight planets in the Solar System, only Venus
Venus
and Mars
Mars
lack such a magnetic field.[144] In addition, the moon of Jupiter
Jupiter
Ganymede also has one. Of the magnetized planets the magnetic field of Mercury is the weakest, and is barely able to deflect the solar wind. Ganymede's magnetic field is several times larger, and Jupiter's is the strongest in the Solar System
Solar System
(so strong in fact that it poses a serious health risk to future manned missions to its moons). The magnetic fields of the other giant planets are roughly similar in strength to that of Earth, but their magnetic moments are significantly larger. The magnetic fields of Uranus
Uranus
and Neptune
Neptune
are strongly tilted relative the rotational axis and displaced from the centre of the planet.[144] In 2004, a team of astronomers in Hawaii observed an extrasolar planet around the star HD 179949, which appeared to be creating a sunspot on the surface of its parent star. The team hypothesized that the planet's magnetosphere was transferring energy onto the star's surface, increasing its already high 7,760 °C temperature by an additional 400 °C.[145] Secondary characteristics Main articles: Natural satellite
Natural satellite
and Planetary ring

The rings of Saturn

Several planets or dwarf planets in the Solar System
Solar System
(such as Neptune and Pluto) have orbital periods that are in resonance with each other or with smaller bodies (this is also common in satellite systems). All except Mercury and Venus
Venus
have natural satellites, often called "moons". Earth
Earth
has one, Mars
Mars
has two, and the giant planets have numerous moons in complex planetary-type systems. 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).[146][147][148] The four giant planets are also orbited by planetary rings 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
Roche limit
and were torn apart by tidal forces.[149][150] No secondary characteristics have been observed around extrasolar planets. The sub-brown dwarf Cha 110913-773444, which has been described as a rogue planet, is believed to be orbited by a tiny protoplanetary disc[107] and the sub-brown dwarf OTS 44
OTS 44
was shown to be surrounded by a substantial protoplanetary disk of at least 10 Earth
Earth
masses.[109] See also

Astronomy
Astronomy
portal Solar System
Solar System
portal Space
Space
portal

Double planet
Double planet
– Two planetary mass objects orbiting each other List of exoplanets List of hypothetical Solar System
Solar System
objects List of landings on extraterrestrial bodies Lists of planets
Lists of planets
– A list of lists of planets sorted by diverse attributes Mesoplanet – A celestial body smaller than Mercury but larger than Ceres Minor planet
Minor planet
– A celestial body smaller than a planet Planetary habitability
Planetary habitability
– The measure of a planet's ability to sustain life Planetary mnemonic
Planetary mnemonic
– A phrase used to remember the names of the planets Planetary science
Planetary science
– The scientific study of planets Planets in astrology Planets in science fiction Theoretical planetology

Notes

^ This definition is drawn from two separate IAU declarations; a formal definition agreed by the IAU in 2006, and an informal working definition established by the IAU in 2001/2003 for objects outside of the Solar System. The official 2006 definition applies only to the Solar System, whereas the 2003 definition applies to planets around other stars. The extrasolar planet issue was deemed too complex to resolve at the 2006 IAU conference. ^ a b For the purpose of this 1 in 5 statistic, "Sun-like" means G-type star. Data for Sun-like stars wasn't available so this statistic is an extrapolation from data about K-type stars ^ a b For the purpose of this 1 in 5 statistic, Earth-sized means 1–2 Earth
Earth
radii ^ a b For the purpose of this 1 in 5 statistic, "habitable zone" means the region with 0.25 to 4 times Earth's stellar flux (corresponding to 0.5–2 AU for the Sun). ^ Referred to by Huygens as a Planetes novus ("new planet") in his Systema Saturnium ^ a b Both labelled nouvelles planètes (new planets) by Cassini in his Découverte de deux nouvelles planetes autour de Saturne[66] ^ a b Both once referred to as "planets" by Cassini in his An Extract of the Journal Des Scavans.... The term "satellite" had already begun to be used to distinguish such bodies from those around which they orbited ("primary planets"). ^ a b c Measured relative to Earth. ^ Jupiter
Jupiter
has the most verified satellites (69) in the Solar System.[93]

References

^ a b c "IAU 2006 General Assembly: Result of the IAU Resolution votes". International Astronomical Union. 2006. Retrieved 2009-12-30.  ^ a b "Working Group on Extrasolar Planets (WGESP) of the International Astronomical Union". IAU. 2001. Archived from the original on 2006-09-16. Retrieved 2008-08-23.  ^ " NASA
NASA
discovery doubles the number of known planets". USA TODAY. 10 May 2016. Retrieved 10 May 2016.  ^ a b Schneider, Jean (16 January 2013). "Interactive Extra-solar Planets Catalog". The Extrasolar Planets Encyclopaedia. Retrieved 2013-01-15.  ^ a b NASA
NASA
Staff (20 December 2011). "Kepler: A Search For Habitable Planets – Kepler-20e". NASA. Retrieved 2011-12-23.  ^ a b NASA
NASA
Staff (20 December 2011). "Kepler: A Search For Habitable Planets – Kepler-20f". NASA. Retrieved 2011-12-23.  ^ a b Johnson, Michele (20 December 2011). " NASA
NASA
Discovers First Earth-size Planets Beyond Our Solar System". NASA. Retrieved 2011-12-20.  ^ a b Hand, Eric (20 December 2011). "Kepler discovers first Earth-sized exoplanets". Nature. doi:10.1038/nature.2011.9688.  ^ a b Overbye, Dennis (20 December 2011). "Two Earth-Size Planets Are Discovered". New York Times. Retrieved 2011-12-21.  ^ a b Cassan, Arnaud; D. Kubas; J.-P. Beaulieu; M. Dominik; et al. (12 January 2012). "One or more bound planets per Milky Way
Milky Way
star from microlensing observations". Nature. 481 (7380): 167–169. arXiv:1202.0903 . Bibcode:2012Natur.481..167C. doi:10.1038/nature10684. PMID 22237108. Retrieved 11 January 2012. CS1 maint: Explicit use of et al. (link) ^ "Ancient Greek Astronomy
Astronomy
and Cosmology". The Library of Congress. Retrieved 2016-05-19.  ^ πλανήτης, H. G. Liddell and R. Scott, A Greek–English Lexicon, ninth edition, (Oxford: Clarendon Press, 1940). ^ "Definition of planet". Merriam-Webster OnLine. Retrieved 2007-07-23.  ^ " Planet
Planet
Etymology". dictionary.com. Retrieved 29 June 2015.  ^ a b "planet, n". Oxford English Dictionary. 2007. Retrieved 2008-02-07.  Note: select the Etymology tab ^ Neugebauer, Otto E. (1945). "The History of Ancient Astronomy Problems and Methods". Journal of Near Eastern Studies. 4 (1): 1–38. doi:10.1086/370729.  ^ Ronan, Colin. " Astronomy
Astronomy
Before the Telescope". Astronomy
Astronomy
in China, Korea
Korea
and Japan (Walker ed.). pp. 264–265.  ^ Kuhn, Thomas S. (1957). The Copernican Revolution. Harvard University Press. pp. 5–20. ISBN 0-674-17103-9.  ^ a b c d Evans, James (1998). The History and Practice of Ancient Astronomy. Oxford University Press. pp. 296–7. ISBN 978-0-19-509539-5. Retrieved 2008-02-04.  ^ Francesca Rochberg (2000). " Astronomy
Astronomy
and Calendars in Ancient Mesopotamia". In Jack Sasson. Civilizations of the Ancient Near East. III. p. 1930.  ^ Holden, James Herschel (1996). A History of Horoscopic Astrology. AFA. p. 1. ISBN 978-0-86690-463-6.  ^ Hermann Hunger, ed. (1992). Astrological reports to Assyrian kings. State Archives of Assyria. 8. Helsinki University Press. ISBN 951-570-130-9.  ^ Lambert, W. G.; Reiner, Erica (1987). "Babylonian Planetary Omens. Part One. Enuma Anu Enlil, Tablet 63: The Venus
Venus
Tablet of Ammisaduqa". Journal of the American Oriental Society. 107 (1): 93–96. doi:10.2307/602955. JSTOR 602955.  ^ Kasak, Enn; Veede, Raul (2001). Mare Kõiva; Andres Kuperjanov, eds. "Understanding Planets in Ancient Mesopotamia" (PDF). Electronic Journal of Folklore. Estonian Literary Museum. 16: 7–35. doi:10.7592/fejf2001.16.planets. Retrieved 2008-02-06.  ^ A. Sachs (May 2, 1974). "Babylonian Observational Astronomy". Philosophical Transactions of the Royal Society. Royal Society of London. 276 (1257): 43–50 [45 & 48–9]. Bibcode:1974RSPTA.276...43S. doi:10.1098/rsta.1974.0008. JSTOR 74273.  ^ Burnet, John (1950). Greek philosophy: Thales to Plato. Macmillan and Co. pp. 7–11. ISBN 978-1-4067-6601-1. Retrieved 2008-02-07.  ^ a b Goldstein, Bernard R. (1997). "Saving the phenomena: the background to Ptolemy's planetary theory". Journal for the History of Astronomy. Cambridge (UK). 28 (1): 1–12. Bibcode:1997JHA....28....1G.  ^ Ptolemy; Toomer, G. J. (1998). Ptolemy's Almagest. Princeton University Press. ISBN 978-0-691-00260-6.  ^ J. J. O'Connor and E. F. Robertson, Aryabhata
Aryabhata
the Elder, MacTutor History of Mathematics archive ^ Sarma, K. V. (1997) " Astronomy
Astronomy
in India" in Selin, Helaine (editor) Encyclopaedia of the History of Science, Technology, and Medicine in Non-Western Cultures, Kluwer Academic Publishers, ISBN 0-7923-4066-3, p. 116 ^ a b Ramasubramanian, K. (1998). "Model of planetary motion in the works of Kerala astronomers". Bulletin of the Astronomical Society of India. 26: 11–31 [23–4]. Bibcode:1998BASI...26...11R.  ^ Ramasubramanian etc. (1994) ^ Sally P. Ragep (2007). "Ibn Sina, Abu Ali [known as Avicenna] (980?1037)". In Thomas Hockey. Ibn Sīnā: Abū ʿAlī al‐Ḥusayn ibn ʿAbdallāh ibn Sīnā. The Biographical Encyclopedia of Astronomers. Springer Science+Business Media. pp. 570–572. Bibcode:2000eaa..bookE3736.. doi:10.1888/0333750888/3736. ISBN 0-333-75088-8.  ^ S. M. Razaullah Ansari (2002). History of oriental astronomy: proceedings of the joint discussion-17 at the 23rd General Assembly of the International Astronomical Union, organised by the Commission 41 (History of Astronomy), held in Kyoto, August 25–26, 1997. Springer. p. 137. ISBN 1-4020-0657-8.  ^ Fred Espenak. "Six millennium catalog of Venus
Venus
transits: 2000 BCE to 4000 CE". NASA/GSFC. Retrieved 11 February 2012.  ^ a b Van Helden, Al (1995). "Copernican System". The Galileo Project. Retrieved 2008-01-28.  ^ See primary citations in Timeline of discovery of Solar System planets and their moons ^ Hilton, James L. (2001-09-17). "When Did the Asteroids Become Minor Planets?". U.S. Naval Observatory. Archived from the original on 2007-09-21. Retrieved 2007-04-08.  ^ Croswell, K. (1997). Planet
Planet
Quest: The Epic Discovery of Alien Solar Systems. The Free Press. p. 57. ISBN 978-0-684-83252-4.  ^ Lyttleton, Raymond A. (1936). "On the possible results of an encounter of Pluto
Pluto
with the Neptunian system". Monthly Notices of the Royal Astronomical Society. 97 (2): 108–115. Bibcode:1936MNRAS..97..108L. doi:10.1093/mnras/97.2.108.  ^ Whipple, Fred (1964). "The History of the Solar System". Proceedings of the National Academy of Sciences of the United States of America. 52 (2): 565–594. Bibcode:1964PNAS...52..565W. doi:10.1073/pnas.52.2.565. PMC 300311 . PMID 16591209.  ^ Luu, Jane X.; Jewitt, David C. (1996). "The Kuiper Belt". Scientific American. 274 (5): 46–52. Bibcode:1996SciAm.274e..46L. doi:10.1038/scientificamerican0596-46.  ^ a b Wolszczan, A.; Frail, D. A. (1992). "A planetary system around the millisecond pulsar PSR1257 + 12". Nature. 355 (6356): 145–147. Bibcode:1992Natur.355..145W. doi:10.1038/355145a0.  ^ Mayor, Michel; Queloz, Didier (1995). "A Jupiter-mass companion to a solar-type star". Nature. 378 (6356): 355–359. Bibcode:1995Natur.378..355M. doi:10.1038/378355a0.  ^ Basri, Gibor (2000). "Observations of Brown Dwarfs". Annual Review of Astronomy
Astronomy
and Astrophysics. 38 (1): 485–519. Bibcode:2000ARA&A..38..485B. doi:10.1146/annurev.astro.38.1.485.  ^ Green, D. W. E. (2006-09-13). "(134340) Pluto, (136199) Eris, and (136199) Eris I (Dysnomia)" (PDF). Central Bureau for Astronomical Telegrams, International Astronomical Union. Circular No. 8747. Archived from the original on June 24, 2008. Retrieved 2011-07-05.  ^ Saumon, D.; Hubbard, W. B.; Burrows, A.; Guillot, T.; et al. (1996). "A Theory of Extrasolar Giant Planets". Astrophysical Journal. 460: 993–1018. arXiv:astro-ph/9510046 . Bibcode:1996ApJ...460..993S. doi:10.1086/177027.  ^ See for example the list of references for: Butler, R. P.; et al. (2006). "Catalog of Nearby Exoplanets". University of California and the Carnegie Institution. Retrieved 2008-08-23.  ^ Stern, S. Alan (2004-03-22). " Gravity
Gravity
Rules: The Nature and Meaning of Planethood". SpaceDaily. Retrieved 2008-08-23.  ^ Whitney Clavin (2005-11-29). "A Planet
Planet
With Planets? Spitzer Finds Cosmic Oddball". NASA. Retrieved 2006-03-26.  ^ Bodenheimer, Peter; D'Angelo, Gennaro; Lissauer, Jack J.; Fortney, Jonathan J.; Saumon, Didier (20 June 2013). " Deuterium
Deuterium
Burning in Massive Giant Planets and Low-mass Brown Dwarfs Formed by Core-nucleated Accretion". The Astrophysical Journal. 770 (2): 120. arXiv:1305.0980 . Bibcode:2013ApJ...770..120B. doi:10.1088/0004-637X/770/2/120.  ^ Spiegel; Adam Burrows; Milsom (2010). "The Deuterium-Burning Mass Limit for Brown Dwarfs and Giant Planets". The Astrophysical Journal. 727: 57. arXiv:1008.5150 . doi:10.1088/0004-637X/727/1/57.  ^ Schneider, J.; Dedieu, C.; Le Sidaner, P.; Savalle, R.; et al. (2011). "Defining and cataloging exoplanets: The exoplanet.eu database". Astronomy
Astronomy
& Astrophysics. 532 (79): A79. arXiv:1106.0586 . Bibcode:2011A&A...532A..79S. doi:10.1051/0004-6361/201116713.  ^ Wright, J. T.; et al. (2010). "The Exoplanet
Exoplanet
Orbit
Orbit
Database". arXiv:1012.5676v1  [astro-ph.SR].  ^ Exoplanet
Exoplanet
Criteria for Inclusion in the Archive, NASA
NASA
Exoplanet Archive ^ Basri, Gibor; Brown, Michael E (2006). "Planetesimals To Brown Dwarfs: What is a Planet?". Annu. Rev. Earth
Earth
Planet. Sci. 34: 193–216. arXiv:astro-ph/0608417 . Bibcode:2006AREPS..34..193B. doi:10.1146/annurev.earth.34.031405.125058.  ^ Boss, Alan P.; Basri, Gibor; Kumar, Shiv S.; Liebert, James; et al. (2003). "Nomenclature: Brown Dwarfs, Gas Giant Planets, and ?". Brown Dwarfs. 211: 529. Bibcode:2003IAUS..211..529B.  ^ Rincon, Paul (2006-08-16). "Planets plan boosts tally 12". BBC. Retrieved 2008-08-23.  ^ " Pluto
Pluto
loses status as a planet". BBC. 2006-08-24. Retrieved 2008-08-23.  ^ Soter, Steven (2006). "What is a Planet". Astronomical Journal. 132 (6): 2513–19. arXiv:astro-ph/0608359 . Bibcode:2006AJ....132.2513S. doi:10.1086/508861.  ^ "Simpler way to define what makes a planet". Science Daily. 2015-11-10.  ^ "Why we need a new definition of the word 'planet'". Los Angeles Times.  ^ Jean-Luc Margot (2015). "A Quantitative Criterion For Defining Planets". The Astronomical Journal. 150 (6): 185. arXiv:1507.06300 . Bibcode:2015AJ....150..185M. doi:10.1088/0004-6256/150/6/185.  ^ Lindberg, David C. (2007). The Beginnings of Western Science (2nd ed.). Chicago: The University of Chicago Press. p. 257. ISBN 978-0-226-48205-7.  ^ a b "The New Universal Geographical Grammar".  ^ Giovanni Cassini (1673). Decouverte de deux Nouvelles Planetes autour de Saturne. Sabastien Mabre-Craniusy. pp. 6–14. ^ Hilton, James L. "When did the asteroids become minor planets?". U.S. Naval Observatory. Archived from the original on 2008-03-24. Retrieved 2008-05-08.  ^ "The Planet
Planet
Hygea". spaceweather.com. 1849. Retrieved 2008-04-18.  ^ Moskowitz, Clara (2006-10-18). "Scientist who found '10th planet' discusses downgrading of Pluto". Stanford news. Retrieved 2008-08-23.  ^ a b "Should Large Moons Be Called 'Satellite Planets'?". News.discovery.com. 2010-05-14. Retrieved 2011-11-04.  ^ Ross, Kelley L. (2005). "The Days of the Week". The Friesian School. Retrieved 2008-08-23.  ^ Cochrane, Ev (1997). Martian Metamorphoses: The Planet
Planet
Mars
Mars
in Ancient Myth and Tradition. Aeon Press. ISBN 0-9656229-0-8. Retrieved 2008-02-07.  ^ Cameron, Alan (2005). Greek Mythography in the Roman World. Oxford University Press. ISBN 0-19-517121-7.  ^ Zerubavel, Eviatar (1989). The Seven Day
Day
Circle: The History and Meaning of the Week. University of Chicago Press. p. 14. ISBN 0-226-98165-7. Retrieved 2008-02-07.  ^ a b Falk, Michael; Koresko, Christopher (1999). "Astronomical Names for the Days of the Week". Journal of the Royal Astronomical Society of Canada. 93: 122–133. Bibcode:1999JRASC..93..122F. doi:10.1016/j.newast.2003.07.002.  ^ "earth, n". Oxford English Dictionary. 1989. Retrieved 2008-02-06.  ^ a b Harper, Douglas (September 2001). "Earth". Online Etymology Dictionary. Retrieved 2008-08-23.  ^ Harper, Douglas (September 2001). "Etymology of "terrain"". Online Etymology Dictionary. Retrieved 2008-01-30.  ^ a b Stieglitz, Robert (Apr 1981). "The Hebrew Names of the Seven Planets". Journal of Near Eastern Studies. 40 (2): 135–137. doi:10.1086/372867. JSTOR 545038.  ^ Wetherill, G. W. (1980). "Formation of the Terrestrial Planets". Annual Review of Astronomy
Astronomy
and Astrophysics. 18 (1): 77–113. Bibcode:1980ARA&A..18...77W. doi:10.1146/annurev.aa.18.090180.000453.  ^ D'Angelo, G.; Bodenheimer, P. (2013). "Three-dimensional Radiation-hydrodynamics Calculations of the Envelopes of Young Planets Embedded in Protoplanetary Disks". The Astrophysical Journal. 778 (1): 77 (29 pp.). arXiv:1310.2211 . Bibcode:2013ApJ...778...77D. doi:10.1088/0004-637X/778/1/77.  ^ Inaba, S.; Ikoma, M. (2003). "Enhanced Collisional Growth of a Protoplanet
Protoplanet
that has an Atmosphere". Astronomy
Astronomy
and Astrophysics. 410 (2): 711–723. Bibcode:2003A&A...410..711I. doi:10.1051/0004-6361:20031248.  ^ D'Angelo, G.; Weidenschilling, S. J.; Lissauer, J. J.; Bodenheimer, P. (2014). "Growth of Jupiter: Enhancement of core accretion by a voluminous low-mass envelope". Icarus. 241: 298–312. arXiv:1405.7305 . Bibcode:2014Icar..241..298D. doi:10.1016/j.icarus.2014.06.029.  ^ Lissauer, J. J.; Hubickyj, O.; D'Angelo, G.; Bodenheimer, P. (2009). "Models of Jupiter's growth incorporating thermal and hydrodynamic constraints". Icarus. 199 (2): 338–350. arXiv:0810.5186 . Bibcode:2009Icar..199..338L. doi:10.1016/j.icarus.2008.10.004.  ^ D'Angelo, G.; Durisen, R. H.; Lissauer, J. J. (2011). "Giant Planet Formation". In S. Seager. Exoplanets. University of Arizona Press, Tucson, AZ. pp. 319–346. arXiv:1006.5486 . Bibcode:2010exop.book..319D.  ^ Chambers, J. (2011). "Terrestrial Planet
Planet
Formation". In S. Seager. Exoplanets. University of Arizona Press, Tucson, AZ. pp. 297–317. Bibcode:2010exop.book..297C.  ^ Dutkevitch, Diane (1995). "The Evolution
Evolution
of Dust in the Terrestrial Planet
Planet
Region of Circumstellar Disks Around Young Stars". PhD thesis, University of Massachusetts Amherst. Bibcode:1995PhDT..........D. Archived from the original on 2007-11-25. Retrieved 2008-08-23.  ^ Matsuyama, I.; Johnstone, D.; Murray, N. (2005). "Halting Planet Migration by Photoevaporation
Photoevaporation
from the Central Source". The Astrophysical Journal. 585 (2): L143–L146. arXiv:astro-ph/0302042 . Bibcode:2003astro.ph..2042M. doi:10.1086/374406.  ^ Kenyon, Scott J.; Bromley, Benjamin C. (2006). "Terrestrial Planet Formation. I. The Transition from Oligarchic Growth to Chaotic Growth". Astronomical Journal. 131 (3): 1837–1850. arXiv:astro-ph/0503568 . Bibcode:2006AJ....131.1837K. doi:10.1086/499807. Lay summary – Kenyon, Scott J. Personal web page.  ^ Ida, Shigeru; Nakagawa, Yoshitsugu; Nakazawa, Kiyoshi (1987). "The Earth's core formation due to the Rayleigh-Taylor instability". Icarus. 69 (2): 239–248. Bibcode:1987Icar...69..239I. doi:10.1016/0019-1035(87)90103-5.  ^ Kasting, James F. (1993). "Earth's early atmosphere". Science. 259 (5097): 920–6. Bibcode:1993Sci...259..920K. doi:10.1126/science.11536547. PMID 11536547.  ^ Aguilar, David; Pulliam, Christine (2004-01-06). "Lifeless Suns Dominated The Early Universe" (Press release). Harvard-Smithsonian Center for Astrophysics. Retrieved 2011-10-23.  ^ Scott S. Sheppard (2013-01-04). "The Jupiter
Jupiter
Satellite Page (Now Also The Giant Planet
Planet
Satellite and Moon
Moon
Page)". Carnegie Institution for Science. Retrieved 2013-04-12.  ^ Schneider, J. "Interactive Extra-solar Planets Catalog". The Extrasolar Planets Encyclopedia. Retrieved 1 April 2018.  ^ " Exoplanet
Exoplanet
Archive Planet
Planet
Counts".  ^ Johnson, Michele; Harrington, J.D. (February 26, 2014). "NASA's Kepler Mission Announces a Planet
Planet
Bonanza, 715 New Worlds". NASA. Retrieved February 26, 2014.  ^ "The Habitable Exoplanets Catalog - Planetary Habitability Laboratory @ UPR Arecibo".  ^ Lopez, E. D.; Fortney, J. J. (2013). "Understanding the Mass-Radius Relation for Sub-Neptunes: Radius as a Proxy for Composition". The Astrophysical Journal. 792: 1. arXiv:1311.0329  [astro-ph.EP]. Bibcode:2014ApJ...792....1L. doi:10.1088/0004-637X/792/1/1.  ^ Sanders, R. (4 November 2013). "Astronomers answer key question: How common are habitable planets?". newscenter.berkeley.edu.  ^ Petigura, E. A.; Howard, A. W.; Marcy, G. W. (2013). "Prevalence of Earth-size planets orbiting Sun-like stars". Proceedings of the National Academy of Sciences. 110 (48): 19273–19278. arXiv:1311.6806 . Bibcode:2013PNAS..11019273P. doi:10.1073/pnas.1319909110. PMC 3845182 . PMID 24191033.  ^ Drake, Frank (2003-09-29). "The Drake Equation Revisited". Astrobiology
Astrobiology
Magazine. Archived from the original on 2011-06-28. Retrieved 2008-08-23.  ^ "Artist's View of a Super- Jupiter
Jupiter
around a Brown Dwarf (2M1207)". Retrieved 22 February 2016.  ^ Weintraub, David A. (2014), Is Pluto
Pluto
a Planet?: A Historical Journey through the Solar System, Princeton University Press, p. 226, ISBN 1400852978  ^ Basri, G.; Brown, E. M. (May 2006), "Planetesimals to Brown Dwarfs: What is a Planet?", Annual Review of Earth
Earth
and Planetary Sciences, 34: 193–216, arXiv:astro-ph/0608417 , Bibcode:2006AREPS..34..193B, doi:10.1146/annurev.earth.34.031405.125058  ^ Stern, S. Alan; Levison, Harold F. (2002), Rickman, H., ed., "Regarding the criteria for planethood and proposed planetary classification schemes", Highlights of Astronomy, San Francisco, CA: Astronomical Society of the Pacific, 12, pp. 205–213, Bibcode:2002HiA....12..205S, ISBN 1-58381-086-2. See p. 208.  ^ Lissauer, J. J. (1987). "Timescales for Planetary Accretion and the Structure of the Protoplanetary disk". Icarus. 69 (2): 249–265. Bibcode:1987Icar...69..249L. doi:10.1016/0019-1035(87)90104-7.  ^ a b c Luhman, K. L.; Adame, Lucía; D'Alessio, Paola; Calvet, Nuria (2005). "Discovery of a Planetary-Mass Brown Dwarf with a Circumstellar Disk". Astrophysical Journal. 635 (1): L93. arXiv:astro-ph/0511807 . Bibcode:2005ApJ...635L..93L. doi:10.1086/498868. Lay summary – NASA
NASA
Press Release (2005-11-29).  ^ Clavin, Whitney (November 9, 2005). "A Planet
Planet
with Planets? Spitzer Finds Cosmic Oddball". Spitzer Space
Space
Telescope
Telescope
Newsroom. Archived from the original on July 11, 2007. Retrieved 2009-11-18.  ^ a b Joergens, V.; Bonnefoy, M.; Liu, Y.; Bayo, A.; et al. (2013). "OTS 44: Disk and accretion at the planetary border". Astronomy
Astronomy
& Astrophysics. 558 (7): L7. arXiv:1310.1936 . Bibcode:2013A&A...558L...7J. doi:10.1051/0004-6361/201322432.  ^ Close, Laird M.; Zuckerman, B.; Song, Inseok; Barman, Travis; et al. (2007). "The Wide Brown Dwarf Binary Oph 1622–2405 and Discovery of A Wide, Low Mass Binary in Ophiuchus (Oph 1623–2402): A New Class of Young Evaporating Wide Binaries?". Astrophysical Journal. 660 (2): 1492–1506. arXiv:astro-ph/0608574 . Bibcode:2007ApJ...660.1492C. doi:10.1086/513417.  ^ Luhman, K. L.; Allers, K. N.; Jaffe, D. T.; Cushing, M. C.; et al. (2007). "Ophiuchus 1622–2405: Not a Planetary-Mass Binary". The Astrophysical Journal. 659 (2): 1629–36. arXiv:astro-ph/0701242 . Bibcode:2007ApJ...659.1629L. doi:10.1086/512539.  ^ Britt, Robert Roy (2004-09-10). "Likely First Photo of Planet
Planet
Beyond the Solar System". Space.com. Retrieved 2008-08-23.  ^ Bailes, M.; Bates, S. D.; Bhalerao, V.; Bhat, N. D. R.; et al. (2011). "Transformation of a Star
Star
into a Planet
Planet
in a Millisecond Pulsar
Pulsar
Binary". Science. 333 (6050): 1717–20. arXiv:1108.5201 . Bibcode:2011Sci...333.1717B. doi:10.1126/science.1208890. PMID 21868629.  ^ On the origin of planets at very wide orbits from the re-capture of free floating planets, Hagai B. Perets, M. B. N. Kouwenhoven, 2012 ^ D. R. Anderson; Hellier, C.; Gillon, M.; Triaud, A. H. M. J.; Smalley, B.; Hebb, L.; Collier Cameron, A.; Maxted, P. F. L.; Queloz, D.; West, R. G.; Bentley, S. J.; Enoch, B.; Horne, K.; Lister, T. A.; Mayor, M.; Parley, N. R.; Pepe, F.; Pollacco, D.; Ségransan, D.; Udry, S.; Wilson, D. M. (2009). "WASP-17b: an ultra-low density planet in a probable retrograde orbit". The Astrophysical Journal. 709: 159–167. arXiv:0908.1553  [astro-ph.EP]. Bibcode:2010ApJ...709..159A. doi:10.1088/0004-637X/709/1/159.  ^ a b c d e Young, Charles Augustus (1902). Manual of Astronomy: A Text Book. Ginn & company. pp. 324–7.  ^ Dvorak, R.; Kurths, J.; Freistetter, F. (2005). Chaos And Stability in Planetary Systems. New York: Springer. ISBN 3-540-28208-4.  ^ Moorhead, Althea V.; Adams, Fred C. (2008). "Eccentricity evolution of giant planet orbits due to circumstellar disk torques". Icarus. 193 (2): 475–484. arXiv:0708.0335 . Bibcode:2008Icar..193..475M. doi:10.1016/j.icarus.2007.07.009.  ^ "Planets – Kuiper Belt Objects". The Astrophysics Spectator. 2004-12-15. Retrieved 2008-08-23.  ^ Tatum, J. B. (2007). "17. Visual binary stars". Celestial Mechanics. Personal web page. Retrieved 2008-02-02.  ^ Trujillo, Chadwick A.; Brown, Michael E. (2002). "A Correlation between Inclination
Inclination
and Color in the Classical Kuiper Belt". Astrophysical Journal. 566 (2): L125. arXiv:astro-ph/0201040 . Bibcode:2002ApJ...566L.125T. doi:10.1086/339437.  ^ a b Harvey, Samantha (2006-05-01). "Weather, Weather, Everywhere?". NASA. Retrieved 2008-08-23.  ^ Winn, Joshua N.; Holman, Matthew J. (2005). "Obliquity Tides on Hot Jupiters". The Astrophysical Journal. 628 (2): L159. arXiv:astro-ph/0506468 . Bibcode:2005ApJ...628L.159W. doi:10.1086/432834.  ^ Goldstein, R. M.; Carpenter, R. L. (1963). "Rotation of Venus: Period Estimated from Radar Measurements". Science. 139 (3558): 910–1. Bibcode:1963Sci...139..910G. doi:10.1126/science.139.3558.910. PMID 17743054.  ^ Belton, M. J. S.; Terrile, R. J. (1984). Bergstralh, J. T., ed. "Rotational properties of Uranus
Uranus
and Neptune". Uranus
Uranus
and Neptune. NASA. CP-2330: 327–347. Bibcode:1984urnp.nasa..327B.  ^ Borgia, Michael P. (2006). The Outer Worlds; Uranus, Neptune, Pluto, and Beyond. Springer New York. pp. 195–206.  ^ Lissauer, Jack J. (1993). " Planet
Planet
formation". Annual Review of Astronomy
Astronomy
and Astrophysics. 31. (A94-12726 02–90) (1): 129–174. Bibcode:1993ARA&A..31..129L. doi:10.1146/annurev.aa.31.090193.001021.  ^ Strobel, Nick. " Planet
Planet
tables". astronomynotes.com. Retrieved 2008-02-01.  ^ Zarka, Philippe; Treumann, Rudolf A.; Ryabov, Boris P.; Ryabov, Vladimir B. (2001). "Magnetically-Driven Planetary Radio Emissions and Application to Extrasolar Planets". Astrophysics & Space
Space
Science. 277 (1/2): 293–300. Bibcode:2001Ap&SS.277..293Z. doi:10.1023/A:1012221527425.  ^ Faber, Peter; Quillen, Alice C. (2007-07-12). "The Total Number of Giant Planets in Debris Disks with Central Clearings". arXiv:0706.1684  [astro-ph].  ^ Brown, Michael E. (2006). "The Dwarf Planets". California Institute of Technology. Retrieved 2008-02-01.  ^ How One Astronomer
Astronomer
Became the Unofficial Exoplanet
Exoplanet
Record-Keeper, www.scientificamerican.com ^ Jason T Wright; Onsi Fakhouri; Marcy; Eunkyu Han; Ying Feng; John Asher Johnson; Howard; Fischer; Valenti; Anderson, Jay; Piskunov, Nikolai (2010). "The Exoplanet
Exoplanet
Orbit
Orbit
Database". Publications of the Astronomical Society of the Pacific. 123 (902): 412–422. arXiv:1012.5676  [astro-ph.SR]. Bibcode:2011PASP..123..412W. doi:10.1086/659427.  ^ a b "Planetary Interiors". Department of Physics, University of Oregon. Retrieved 2008-08-23.  ^ Elkins-Tanton, Linda T. (2006). Jupiter
Jupiter
and Saturn. New York: Chelsea House. ISBN 0-8160-5196-8.  ^ Podolak, M.; Weizman, A.; Marley, M. (December 1995). "Comparative models of Uranus
Uranus
and Neptune". Planetary and Space
Space
Science. 43 (12): 1517–1522. Bibcode:1995P&SS...43.1517P. doi:10.1016/0032-0633(95)00061-5.  ^ Hunten D. M., Shemansky D. E., Morgan T. H. (1988), The Mercury atmosphere, In: Mercury (A89-43751 19–91). University of Arizona Press, pp. 562–612 ^ Sheppard, S. S.; Jewitt, D.; Kleyna, J. (2005). "An Ultradeep Survey for Irregular Satellites of Uranus: Limits to Completeness". The Astronomical Journal. 129: 518–525. arXiv:astro-ph/0410059 . Bibcode:2005AJ....129..518S. doi:10.1086/426329.  ^ Zeilik, Michael A.; Gregory, Stephan A. (1998). Introductory Astronomy
Astronomy
& Astrophysics (4th ed.). Saunders College Publishing. p. 67. ISBN 0-03-006228-4.  ^ a b Knutson, Heather A.; Charbonneau, David; Allen, Lori E.; Fortney, Jonathan J. (2007). "A map of the day-night contrast of the extrasolar planet HD 189733 b". Nature. 447 (7141): 183–6. arXiv:0705.0993 . Bibcode:2007Natur.447..183K. doi:10.1038/nature05782. PMID 17495920. Lay summary – Center for Astrophysics press release (2007-05-09).  ^ Weaver, Donna; Villard, Ray (2007-01-31). "Hubble Probes Layer-cake Structure of Alien World's Atmosphere" (Press release). Space Telescope
Telescope
Science Institute. Retrieved 2011-10-23.  ^ Ballester, Gilda E.; Sing, David K.; Herbert, Floyd (2007). "The signature of hot hydrogen in the atmosphere of the extrasolar planet HD 209458b". Nature. 445 (7127): 511–4. Bibcode:2007Natur.445..511B. doi:10.1038/nature05525. PMID 17268463.  ^ Harrington, Jason; Hansen, Brad M.; Luszcz, Statia H.; Seager, Sara (2006). "The phase-dependent infrared brightness of the extrasolar planet Andromeda b". Science. 314 (5799): 623–6. arXiv:astro-ph/0610491 . Bibcode:2006Sci...314..623H. doi:10.1126/science.1133904. PMID 17038587. Lay summary – NASA press release (2006-10-12).  ^ a b c Kivelson, Margaret Galland; Bagenal, Fran (2007). "Planetary Magnetospheres". In Lucyann Mcfadden; Paul Weissman; Torrence Johnson. Encyclopedia of the Solar System. Academic Press. p. 519. ISBN 978-0-12-088589-3.  ^ Gefter, Amanda (2004-01-17). "Magnetic planet". Astronomy. Retrieved 2008-01-29.  ^ Grasset, O.; Sotin C.; Deschamps F. (2000). "On the internal structure and dynamic of Titan". Planetary and Space
Space
Science. 48 (7–8): 617–636. Bibcode:2000P&SS...48..617G. doi:10.1016/S0032-0633(00)00039-8.  ^ Fortes, A. D. (2000). "Exobiological implications of a possible ammonia-water ocean inside Titan". Icarus. 146 (2): 444–452. Bibcode:2000Icar..146..444F. doi:10.1006/icar.2000.6400.  ^ Jones, Nicola (2001-12-11). "Bacterial explanation for Europa's rosy glow". New Scientist Print Edition. Retrieved 2008-08-23.  ^ Molnar, L. A.; Dunn, D. E. (1996). "On the Formation of Planetary Rings". Bulletin of the American Astronomical Society. 28: 77–115. Bibcode:1996DPS....28.1815M.  ^ Thérèse, Encrenaz (2004). The Solar System
Solar System
(Third ed.). Springer. pp. 388–390. ISBN 3-540-00241-3. 

External links

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Look up planet in Wiktionary, the free dictionary.

International Astronomical Union
International Astronomical Union
website Photojournal NASA NASA
NASA
Planet
Planet
Quest – Exoplanet
Exoplanet
Exploration Illustration comparing the sizes of the planets with each other, the Sun, and other stars "IAU Press Releases since 1999 "The status of Pluto: A Clarification"". Archived from the original on 2007-12-14.  "Regarding the criteria for planethood and proposed planetary classification schemes." article by Stern and Levinson Planetary Science Research Discoveries (educational site with illustrated articles)

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The Solar System

The Sun Mercury Venus Earth Mars Ceres Jupiter Saturn Uranus Neptune Pluto Haumea Makemake Eris

Planets

Terrestrial planets

Mercury Venus Earth Mars

Giant planets

Jupiter Saturn Uranus Neptune

Dwarf planets

Ceres Pluto Haumea Makemake Eris

Rings

Jovian Saturnian (Rhean) Charikloan Chironean Uranian Neptunian Haumean

Moons

Terrestrial

Moon other near- Earth
Earth
objects

Martian

Phobos Deimos

Jovian

Ganymede Callisto Io Europa all 69

Saturnian

Titan Rhea Iapetus Dione Tethys Enceladus Mimas Hyperion Phoebe all 62

Uranian

Titania Oberon Umbriel Ariel Miranda all 27

Neptunian

Triton Proteus Nereid all 14

Plutonian

Charon Nix Hydra Kerberos Styx

Haumean

Hiʻiaka Namaka

Makemakean

S/2015 (136472) 1

Eridian

Dysnomia

Lists

Solar System
Solar System
objects

By size By discovery date

Minor planets Gravitationally rounded objects Possible dwarf planets Natural satellites Comets

Small Solar System bodies

Meteoroids Minor planets

moons

Comets Damocloids Mercury-crossers Venus-crossers Venus
Venus
trojans Near- Earth
Earth
objects Earth-crossers Earth
Earth
trojans Mars-crossers Mars
Mars
trojans Asteroid
Asteroid
belt Asteroids

first discovered: Ceres Pallas Juno Vesta

Families Notable asteroids Kirkwood gap Main-belt comets Jupiter
Jupiter
trojans Jupiter-crossers Centaurs Saturn-crossers Uranus
Uranus
trojans Uranus-crossers Neptune
Neptune
trojans Cis-Neptunian objects Trans-Neptunian objects Neptune-crossers Plutoids Kuiper belt

Plutinos Cubewanos

Scattered disc Detached objects Sednoids Hills cloud Oort cloud

Hypothetical objects

Vulcan Vulcanoids Phaeton Planet
Planet
V Theia Fifth giant Planets beyond Neptune Tyche Nemesis Planet
Planet
Nine

Exploration (outline)

Discovery

Astronomy Timeline

Spaceflight Robotic spacecraft Human spaceflight List of crewed spacecraft Colonization List of probes Timeline

Mercury Venus Moon Mars Ceres Asteroids

Mining

Comets Jupiter Saturn Uranus Neptune Pluto Deep space

Outline of the Solar System Portals Solar System Astronomy Earth
Earth
sciences Mars Jupiter Uranus Cosmology

Solar System → Local Interstellar Cloud → Local Bubble → Gould Belt → Orion Arm → Milky Way → Milky Way
Milky Way
subgroup → Local Group → Virgo Supercluster → Laniakea Supercluster → Observable universe → Universe Each arrow (→) may be read as "within" or "part of".

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Exoplanetology

Planet

Definition

IAU

Planetary science

Main topics

Exoplanet Methods of detecting exoplanets Planetary system

Sizes and types

Terrestrial

Carbon planet Coreless planet Desert planet Dwarf planet Ice planet Iron
Iron
planet Lava planet Mega-Earth Ocean planet Sub-Earth Super-Earth

Gaseous

Chthonian planet Eccentric Jupiter Gas dwarf Helium
Helium
planet Hot Jupiter Hot Neptune Ice giant Mini-Neptune Super-Neptune Super-Jupiter

Other types

Brown dwarf Circumbinary planet Double planet Mesoplanet Planemo Planet/ Brown dwarf
Brown dwarf
boundary Planetesimal Protoplanet Pulsar
Pulsar
planet Sub-brown dwarf Ultra-cool dwarf Ultra-short period planets
Ultra-short period planets
(USP)

Formation and evolution

Accretion Merging stars Nebular hypothesis Planetary migration

Systems

Exocomet Exomoon Interstellar comet Mean-motion resonances Retrograde planet Rogue planet Titius–Bode laws Trojan planet

Host stars

A B Binary star Brown dwarfs Extragalactic planet F/Yellow-white dwarfs G/Yellow dwarfs Herbig Ae/Be K/Orange dwarfs M/Red dwarfs Planets in globular clusters Pulsar Red giant Subdwarf B Subgiant T Tauri White dwarfs Yellow giant

Detection

Astrometry Direct imaging

list

Microlensing

list

Polarimetry Pulsar
Pulsar
timing

list

Radial velocity

list

Transit method

list

Transit-timing variation

Habitability

Astrobiology Circumstellar habitable zone Earth
Earth
analog Extraterrestrial liquid water Habitability of natural satellites Superhabitable planet

Catalogues

Catalog of Nearby Habitable Systems Exoplanet
Exoplanet
Data Explorer Extrasolar Planets Encyclopaedia NASA
NASA
Exoplanet
Exoplanet
Archive NASA
NASA
Star
Star
and Exoplanet
Exoplanet
Database

Lists

Exoplanetary systems

Host stars Multiplanetary systems Stars with proplyds

Exoplanets

List of exoplanets Discoveries Extremes Firsts Nearest Largest Most massive Terrestrial candidates Kepler Potentially habitable

Discovered exoplanets by year

before 2000 2000–2009 2010 2011 2012 2013 2014 2015 2016 2017 2018

Other

Carl Sagan
Carl Sagan
Institute Exoplanet
Exoplanet
phase curves Nexus for Exoplanet
Exoplanet
System Science Planets in science fiction Sudarsky's gas giant classification

Discoveries of exoplanets Search projects

v t e

Big History

Themes and subjects

Chronology of the universe Cosmic evolution Deep time Time scales Goldilocks principle Modernity

Eight thresholds

1: Creation - Big Bang
Big Bang
and cosmogony 2: Stars - creation of stars 3: Elements - creation of chemical elements inside dying stars 4: Planets - formation of planets 5: Life
Life
- abiogenesis and evolution of life 6: Humans - development of Homo sapiens

Paleolithic
Paleolithic
era

7: Agriculture - Agricultural Revolution 8: Modernity
Modernity
- modern era

Web-based education

Big History
Big History
Project

Crash Course Big History

ChronoZoom

Notable people

Walter Alvarez Cynthia Stokes Brown Eric Chaisson David Christian Bill Gates Carl Sagan Graeme Snooks Jimmy Wales Bill Wurtz

Authority control

GND: 40462

.