Shown in order from the
Sun and in true color. Sizes are not to scale.
A planet is an astronomical body orbiting a star or stellar remnant
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]
The term planet is ancient, with ties to history, astrology, science,
mythology, and religion. Several planets in the
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),
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 to orbit
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 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 Age, close observation by space probes has found
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
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 to gas giants about twice as large as
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 from the Sun, i.e. in the habitable
zone. On December 20, 2011, the Kepler
reported the discovery of the first Earth-sized extrasolar planets,
Kepler-20e and Kepler-20f, orbiting a Sun-like star,
Kepler-20. A 2012 study, analyzing gravitational microlensing
data, estimates an average of at least 1.6 bound planets for every
star in the Milky Way. Around one in five Sun-like[b] stars is
thought to have an Earth-sized[c] planet in its habitable[d] zone.
1.2 Greco-Roman astronomy
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
Mythology and naming
4 Solar System
4.1 Planetary attributes
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
7.1 Dynamic characteristics
7.1.2 Axial tilt
7.1.4 Orbital clearing
7.2 Physical characteristics
7.2.2 Internal differentiation
7.3 Secondary characteristics
8 See also
11 External links
Further information: History of astronomy, Definition of planet, and
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. Ancient Greeks called these lights
πλάνητες ἀστέρες (planētes asteres, "wandering
stars") or simply πλανῆται (planētai, "wanderers"), from
which today's word "planet" was derived. In ancient
Greece, China, Babylon, and indeed all pre-modern
civilizations, it was almost universally believed that Earth
was the center of the
Universe and that all the "planets" circled
Earth. The reasons for this perception were that stars and planets
appeared to revolve around
Earth each day and the apparently
common-sense perceptions that
Earth was solid and stable and that it
was not moving but at rest.
Main article: Babylonian astronomy
The first civilization known to have a functional theory of the
planets were the Babylonians, who lived in
Mesopotamia in the first
and second millennia BC. The oldest surviving planetary astronomical
text is the Babylonian
Venus tablet of Ammisaduqa, a 7th-century BC
copy of a list of observations of the motions of the planet Venus,
that probably dates as early as the second millennium BC. The
MUL.APIN is a pair of cuneiform tablets dating from the 7th century BC
that lays out the motions of the Sun, Moon, and planets over the
course of the year. The Babylonian astrologers also laid the
foundations of what would eventually become Western astrology. The
Enuma anu enlil, written during the
Neo-Assyrian period in the 7th
century BC, comprises a list of omens and their relationships with
various celestial phenomena including the motions of the
planets. Venus, Mercury, and the outer planets Mars, Jupiter,
Saturn were all identified by Babylonian astronomers. These would
remain the only known planets until the invention of the telescope in
early modern times.
See also: Greek astronomy
Ptolemy's 7 planetary spheres
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.
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). In the 3rd
Aristarchus of Samos
Aristarchus of Samos proposed a heliocentric system,
according to which
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 written by
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. To
the Greeks and Romans there were seven known planets, each presumed to
Earth according to the complex laws laid out by Ptolemy.
They were, in increasing order from
Earth (in Ptolemy's order): the
Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn.
Indian astronomy and Hindu cosmology
In 499 CE, the Indian astronomer
Aryabhata propounded a planetary
model that explicitly incorporated
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. 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.
Nilakantha Somayaji of the Kerala school of astronomy and
mathematics, in his Tantrasangraha, revised Aryabhata's model. In
his Aryabhatiyabhasya, a commentary on Aryabhata's Aryabhatiya, he
developed a planetary model where Mercury, Venus, Mars,
Saturn orbit the Sun, which in turn orbits Earth, similar to the
Tychonic system later proposed by
Tycho Brahe in the late 16th
century. Most astronomers of the Kerala school who followed him
accepted his planetary model.
Medieval Muslim astronomy
Astronomy in the medieval Islamic world and Cosmology
in medieval Islam
In the 11th century, the transit of
Venus was observed by Avicenna,
who established that
Venus was, at least sometimes, below the Sun.
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
Venus by the Maragha astronomer
Qotb al-Din Shirazi
Qotb al-Din Shirazi in the
Ibn Bajjah could not have observed a transit of
Venus, because none occurred in his lifetime.
c. 1543 to 1610 and c. 1680 to 1781
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 (or that was believed to do
so at the time); and by the 18th century to something that directly
Sun when the heliocentric model of Copernicus, Galileo and
Kepler gained sway.
Earth became included in the list of planets, whereas the
Moon were excluded. At first, when the first satellites of
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. 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.
Eleven planets, 1807–1845
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
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
1846, there was no apparent need to have a formal definition.
Planets 1854–1930, Solar planets 2006–present
In the 20th century,
Pluto was discovered. After initial observations
led to the belief that it was larger than Earth, 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 may be an escaped satellite of Neptune, and Fred Whipple
suggested in 1964 that
Pluto may be a comet. As it was still
larger than all known asteroids and seemingly did not exist within a
larger population, it kept its status until 2006.
(Solar) planets 1930–2006
In 1992, astronomers
Aleksander Wolszczan and
Dale Frail announced the
discovery of planets around a pulsar, PSR B1257+12. This discovery
is generally considered to be the first definitive detection of a
planetary system around another star. Then, on October 6, 1995, Michel
Didier Queloz of the
Geneva Observatory announced the first
definitive detection of an exoplanet orbiting an ordinary
main-sequence star (51 Pegasi).
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 fuse hydrogen, objects of only
Jupiter masses can fuse deuterium.
Deuterium is quite rare, and
most brown dwarfs would have ceased fusing deuterium long before their
discovery, making them effectively indistinguishable from supermassive
With the discovery during the latter half of the 20th century of more
objects within the
Solar System and large objects around other stars,
disputes arose over what should constitute a planet. There were
particular disagreements over whether an object should be considered a
planet if it was part of a distinct population such as a belt, or if
it was large enough to generate energy by the thermonuclear fusion of
A growing number of astronomers argued for
Pluto to be declassified as
a planet, because many similar objects approaching its size had been
found in the same region of the
Solar System (the Kuiper belt) during
the 1990s and early 2000s.
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 and Eris).
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
Objects with true masses below the limiting mass for thermonuclear
fusion of deuterium (currently calculated to be 13 times the mass of
Jupiter for objects with the same isotopic abundance as the Sun)
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
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.
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, 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. 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. 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. The Extrasolar Planets
Encyclopaedia includes objects up to 25
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."
Exoplanet Data Explorer includes objects up to 24
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." The NASA
Exoplanet Archive includes objects with a mass (or minimum mass) equal
to or less than 30
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
2006 IAU definition of planet
Main article: IAU definition of planet
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 as follows:
A "planet"  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.
 The eight planets are: Mercury, Venus, Earth, Mars, Jupiter,
Saturn, Uranus, and Neptune.
Under this definition, the
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. After much discussion, it was decided
via a vote that those bodies should instead be classified as dwarf
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:
"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 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 and because the
criteria of roundness and orbital zone clearance are not presently
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. 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.
Objects formerly considered planets
The table below lists
Solar System bodies once considered to be
Classified as classical planets (Ancient Greek πλανῆται,
wanderers) in classical antiquity and medieval Europe, in accordance
with the now-disproved geocentric model.
Io, Europa, Ganymede, and Callisto
The four largest moons of Jupiter, known as the
Galilean moons after
their discoverer Galileo Galilei. He referred to them as the "Medicean
Planets" in honor of his patron, the
Medici family. They were known as
Titan,[e] Iapetus,[f] Rhea,[f] Tethys,[g] and Dione[g]
Five of Saturn's larger moons, discovered by
Christiaan Huygens and
Giovanni Domenico Cassini. As with Jupiter's major moons, they were
known as secondary planets.
Pallas, Juno, and Vesta
Regarded as planets from their discoveries between 1801 and 1807 until
they were reclassified as asteroids during the 1850s.
Ceres was subsequently classified as a dwarf planet in 2006.
Dwarf planet and asteroid
Astraea, Hebe, Iris, Flora, Metis, Hygiea, Parthenope, Victoria,
Egeria, Irene, Eunomia
More asteroids, discovered between 1845 and 1851. The rapidly
expanding list of bodies between
Jupiter prompted their
reclassification as asteroids, which was widely accepted by 1854.
Dwarf planet and
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 still holds cultural
significance for many in the general public due to its historical
classification as a planet from 1930 to 2006. 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.
Mythology and naming
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
Sun and the
Moon were called
Helios and Selene; the
farthest planet (Saturn) was called Phainon, the shiner; followed by
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:
the names of both planets and gods; Phainon was sacred to Cronus, the
Titan who fathered the Olympians;
Phaethon was sacred to Zeus,
Cronus's son who deposed him as king;
Pyroeis was given to Ares, son
Zeus and god of war;
Phosphoros was ruled by Aphrodite, the goddess
of love; and Hermes, messenger of the gods and god of learning and
wit, ruled over Stilbon.
The Greek practice of grafting of their gods' names onto the planets
was almost certainly borrowed from the Babylonians. The Babylonians
Phosphoros after their goddess of love, Ishtar;
their god of war, Nergal, Stilbon after their god of wisdom Nabu, and
Phaethon after their chief god, Marduk. There are too many
concordances between Greek and Babylonian naming conventions for them
to have arisen separately. The translation was not perfect. For
instance, the Babylonian
Nergal was a god of war, and thus the Greeks
identified him with Ares. Unlike Ares,
Nergal was also god of
pestilence and the underworld.
Today, most people in the western world know the planets by names
derived from the Olympian pantheon of gods. Although modern Greeks
still use their ancient names for the planets, other European
languages, because of the influence of the
Roman Empire 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. When
the Romans studied Greek astronomy, they gave the planets their own
gods' names: Mercurius (for Hermes),
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 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,
Moon (from the farthest to the closest planet).
Therefore, the first day was started by
Saturn (1st hour), second day
Sun (25th hour), followed by
Moon (49th hour), Mars, Mercury,
Jupiter and Venus. Because each day was named by the god that started
it, this 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. 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,
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, there is no tradition of naming it
after a god. (The same is true, in English at least, of the
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
Earth perhaps around 1300. As with its equivalents
in the other Germanic languages, it derives ultimately from the
Proto-Germanic word ertho, "ground", as can be seen in the English
earth, the German Erde, the Dutch aarde, and the Scandinavian jord.
Many of the
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". The non-
Romance languages use their own native
words. The Greeks retain their original name, Γή (Ge).
Non-European cultures use other planetary-naming systems.
India uses a
system based on the Navagraha, which incorporates the seven
traditional planets (
Surya for the Sun,
Chandra for the Moon, and
Budha, Shukra, Mangala,
Shani for Mercury, Venus,
Jupiter and Saturn) and the ascending and descending lunar nodes
Rahu and Ketu. China and the countries of eastern Asia historically
subject to Chinese cultural influence (such as Japan,
Vietnam) use a naming system based on the five Chinese elements: water
(Mercury), metal (Venus), fire (Mars), wood (Jupiter) and earth
(Saturn). In traditional Hebrew astronomy, the seven traditional
planets have (for the most part) descriptive names - the
Sun is חמה
Ḥammah or "the hot one," the
Moon is לבנה Levanah or "the white
Venus is כוכב נוגה Kokhav Nogah or "the bright planet,"
Mercury is כוכב Kokhav or "the planet" (given its lack of
Mars is מאדים Ma'adim or "the red one,"
Saturn is שבתאי Shabbatai or "the resting one" (in reference
to its slow movement compared to the other visible planets). The
odd one out is Jupiter, called צדק Tzedeq or "justice." 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 from II Samuel.
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. After a planet reaches a mass
somewhat larger than Mars' mass, it begins to accumulate an extended
atmosphere, greatly increasing the capture rate of the
planetesimals by means of atmospheric drag. Depending on the
accretion history of solids and gas, a giant planet, an ice giant, or
a terrestrial planet may result.
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.
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. 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
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. 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. (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. 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 ejecta producing planet-forming material.
Solar System – sizes but not distances are to scale
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
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:
Jupiter is the largest, at 318
Earth masses, whereas Mercury is the
smallest, at 0.055
The planets of the
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 and Mars. At 0.055 Earth
masses, Mercury is the smallest terrestrial planet (and smallest
planet) in the Solar System.
Earth is the largest terrestrial planet.
Giant planets (Jovians): Massive planets significantly more massive
than the terrestrials: Jupiter, Saturn, Uranus, Neptune.
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 masses, is the largest planet in the
Solar System, and
Saturn is one third as massive, at 95
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
Semi-major axis (AU)
to Sun's equator (°)
Axial tilt (°)
N2, O2, Ar
CO2, N2, Ar
H2, He, CH4
H2, He, CH4
Color legend: terrestrial planets gas
giants ice giants (both are giant planets).
(a) Find absolute values in article Earth
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.
In early 1992, radio astronomers
Aleksander Wolszczan and Dale Frail
announced the discovery of two planets orbiting the pulsar PSR
1257+12. 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
Sizes of Kepler
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
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
Jupiter or larger as they were more easily detected. The
catalog of Kepler candidate planets consists mostly of planets the
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 or a
mixture of volatiles and gas like Neptune—a radius of 1.75 times
Earth is a possible dividing line between the two types of
planet. 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
A 2012 study, analyzing gravitational microlensing data, estimates an
average of at least 1.6 bound planets for every star in the Milky
On December 20, 2011, the Kepler
Telescope team reported the
discovery of the first Earth-size exoplanets, Kepler-20e and
Kepler-20f, orbiting a Sun-like star, Kepler-20.
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. 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.
There are exoplanets that are much closer to their parent star than
any planet in the
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 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-11 system has five of its planets in shorter
orbits than Mercury's, all of them much more massive than Mercury.
Neptune is 30 AU from the
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
Telescope in 2019.
Artist's impression of a super-
Jupiter around the brown dwarf
See also: List of gravitationally rounded objects of the Solar System
A planetary-mass object (PMO), planemo, 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. 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).
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. 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
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 and OTS
44, 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 masses and a sub-brown dwarf of 7
but further observations revised the estimated masses upwards to
greater than 13
Jupiter masses, making them brown dwarfs according to
the IAU working definitions.
In close binary star systems one of the stars can lose mass to a
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. 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 and Titan.
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 should be
considered planets according to Stern.
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.
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.
Orbit and Orbital elements
See also: Kepler's laws of planetary motion
The orbit of the planet
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 in the same direction as the Sun
rotates (counter-clockwise as seen from above the Sun's north pole).
At least one extrasolar planet, WASP-17b, has been found to orbit in
the opposite direction to its star's rotation. The period of one
revolution of a planet's orbit is known as its sidereal period or
year. 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 slows down as it
reaches the apex of its trajectory.
Each planet's orbit is delineated by a set of elements:
The eccentricity of an orbit describes 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 have very low eccentricities, and thus
nearly circular orbits. Comets and
Kuiper belt objects (as well
as several extrasolar planets) have very high eccentricities, and thus
exceedingly elliptical orbits.
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.
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. The eight planets of the
Solar System all lie very
close to the ecliptic; comets and
Kuiper belt objects like
at far more extreme angles to it. The points at which a planet
crosses above and below its reference plane are called its ascending
and descending nodes. 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.
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. 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
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 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 and Uranus, which rotate
clockwise, though Uranus's extreme axial tilt means there are
differing conventions on which of its poles is "north", and therefore
whether it is rotating clockwise or anti-clockwise. Regardless of
which convention is used,
Uranus has a retrograde rotation relative to
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. There is great variation in the length of day
between the planets, with
Venus taking 243 days to rotate, and the
giant planets only a few hours. 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.
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.
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
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.
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 masses, and the
Explorer up to 24
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. 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
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, 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
Saturn are believed to have cores of rock and
metal surrounded by mantles of metallic hydrogen.
Neptune, which are smaller, have rocky cores surrounded by mantles of
water, ammonia, methane and other ices. The fluid action within
these planets' cores creates a geodynamo that generates a magnetic
Atmosphere and Extraterrestrial atmospheres
See also: Extraterrestrial skies
All of the
Solar System planets except Mercury 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. 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.
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 (called the Great Red
Spot), and holes in the atmosphere (on Neptune). 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.
Hot Jupiters, due to their extreme proximities to their host stars,
have been shown to be losing their atmospheres into space due to
stellar radiation, much like the tails of comets. These
planets may have vast differences in temperature between their day and
night sides that produce supersonic winds, although 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.
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.
Of the eight planets in the Solar System, only
such a magnetic field. In addition, the moon of
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 (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
strongly tilted relative the rotational axis and displaced from the
centre of the planet.
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.
Natural satellite and Planetary ring
The rings of Saturn
Several planets or dwarf planets in the
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 have natural satellites, often called
Earth has one,
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).
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 and were
torn apart by tidal forces.
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 and the sub-brown dwarf
OTS 44 was shown to
be surrounded by a substantial protoplanetary disk of at least 10
Solar System portal
Double planet – Two planetary mass objects orbiting each other
List of exoplanets
List of hypothetical
Solar System objects
List of landings on extraterrestrial bodies
Lists of planets
Lists of planets – A list of lists of planets sorted by diverse
Mesoplanet – A celestial body smaller than Mercury but larger than
Minor planet – A celestial body smaller than a planet
Planetary habitability – The measure of a planet's ability to
Planetary mnemonic – A phrase used to remember the names of the
Planetary science – The scientific study of planets
Planets in astrology
Planets in science fiction
^ 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
^ 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
^ a b Both labelled nouvelles planètes (new planets) by Cassini in
his Découverte de deux nouvelles planetes autour de Saturne
^ 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 has the most verified satellites (69) in the Solar
^ a b c "IAU 2006 General Assembly: Result of the IAU Resolution
votes". International Astronomical Union. 2006. Retrieved
^ 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 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
^ a b
NASA Staff (20 December 2011). "Kepler: A Search For Habitable
Planets – Kepler-20e". NASA. Retrieved 2011-12-23.
^ a b
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 Discovers First
Earth-size Planets Beyond Our Solar System". NASA. Retrieved
^ 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 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 and Cosmology". The Library of Congress.
^ πλανήτης, H. G. Liddell and R. Scott, A Greek–English
Lexicon, ninth edition, (Oxford: Clarendon Press, 1940).
^ "Definition of planet". Merriam-Webster OnLine. Retrieved
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.
^ Ronan, Colin. "
Astronomy Before the Telescope".
Astronomy in China,
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 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.
^ Lambert, W. G.; Reiner, Erica (1987). "Babylonian Planetary Omens.
Part One. Enuma Anu Enlil, Tablet 63: The
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].
^ Burnet, John (1950). Greek philosophy: Thales to Plato. Macmillan
and Co. pp. 7–11. ISBN 978-1-4067-6601-1. Retrieved
^ 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.
^ 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 the Elder, MacTutor
History of Mathematics archive
^ Sarma, K. V. (1997) "
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.
^ 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 transits: 2000 BCE to
4000 CE". NASA/GSFC. Retrieved 11 February 2012.
^ a b Van Helden, Al (1995). "Copernican System". The Galileo Project.
^ 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 Quest: The Epic Discovery of Alien Solar
Systems. The Free Press. p. 57.
^ Lyttleton, Raymond A. (1936). "On the possible results of an
Pluto with the Neptunian system". Monthly Notices of the
Royal Astronomical Society. 97 (2): 108–115.
^ 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 .
^ Luu, Jane X.; Jewitt, David C. (1996). "The Kuiper Belt". Scientific
American. 274 (5): 46–52. Bibcode:1996SciAm.274e..46L.
^ a b Wolszczan, A.; Frail, D. A. (1992). "A planetary system around
the millisecond pulsar PSR1257 + 12". Nature. 355 (6356): 145–147.
^ Mayor, Michel; Queloz, Didier (1995). "A Jupiter-mass companion to a
solar-type star". Nature. 378 (6356): 355–359.
^ Basri, Gibor (2000). "Observations of Brown Dwarfs". Annual Review
Astronomy and Astrophysics. 38 (1): 485–519.
^ 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
^ 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.
^ 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 Rules: The Nature and Meaning
of Planethood". SpaceDaily. Retrieved 2008-08-23.
^ Whitney Clavin (2005-11-29). "A
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 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.
^ 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
Astronomy & Astrophysics. 532 (79): A79.
arXiv:1106.0586 . Bibcode:2011A&A...532A..79S.
^ Wright, J. T.; et al. (2010). "The
Exoplanet Criteria for Inclusion in the Archive,
^ Basri, Gibor; Brown, Michael E (2006). "Planetesimals To Brown
Dwarfs: What is a Planet?". Annu. Rev.
Earth Planet. Sci. 34:
193–216. arXiv:astro-ph/0608417 . Bibcode:2006AREPS..34..193B.
^ 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.
Pluto loses status as a planet". BBC. 2006-08-24. Retrieved
^ Soter, Steven (2006). "What is a Planet". Astronomical Journal. 132
(6): 2513–19. arXiv:astro-ph/0608359 .
^ "Simpler way to define what makes a planet". Science Daily.
^ "Why we need a new definition of the word 'planet'". Los Angeles
Jean-Luc Margot (2015). "A Quantitative Criterion For Defining
Planets". The Astronomical Journal. 150 (6): 185. arXiv:1507.06300 .
^ Lindberg, David C. (2007). The Beginnings of Western Science (2nd
ed.). Chicago: The University of Chicago Press. p. 257.
^ 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.
Planet Hygea". spaceweather.com. 1849. Retrieved
^ Moskowitz, Clara (2006-10-18). "Scientist who found '10th planet'
discusses downgrading of Pluto". Stanford news. Retrieved
^ 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.
^ Cochrane, Ev (1997). Martian Metamorphoses: The
Ancient Myth and Tradition. Aeon Press. ISBN 0-9656229-0-8.
^ Cameron, Alan (2005). Greek Mythography in the Roman World. Oxford
University Press. ISBN 0-19-517121-7.
^ Zerubavel, Eviatar (1989). The Seven
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.
^ "earth, n". Oxford English Dictionary. 1989. Retrieved
^ 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 and Astrophysics. 18 (1): 77–113.
^ 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.
^ Inaba, S.; Ikoma, M. (2003). "Enhanced Collisional Growth of a
Protoplanet that has an Atmosphere".
Astronomy and Astrophysics. 410
(2): 711–723. Bibcode:2003A&A...410..711I.
^ 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.
^ 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 .
^ 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 .
^ Chambers, J. (2011). "Terrestrial
Planet Formation". In S. Seager.
Exoplanets. University of Arizona Press, Tucson, AZ.
pp. 297–317. Bibcode:2010exop.book..297C.
^ Dutkevitch, Diane (1995). "The
Evolution of Dust in the Terrestrial
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
Photoevaporation from the Central Source". The
Astrophysical Journal. 585 (2): L143–L146.
arXiv:astro-ph/0302042 . Bibcode:2003astro.ph..2042M.
^ 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
^ 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.
^ 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 Satellite Page (Now
Also The Giant
Planet Satellite and
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.
^ Johnson, Michele; Harrington, J.D. (February 26, 2014). "NASA's
Kepler Mission Announces a
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].
^ 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 .
^ Drake, Frank (2003-09-29). "The Drake Equation Revisited".
Astrobiology Magazine. Archived from the original on 2011-06-28.
^ "Artist's View of a Super-
Jupiter around a Brown Dwarf (2M1207)".
Retrieved 22 February 2016.
^ Weintraub, David A. (2014), Is
Pluto a Planet?: A Historical Journey
through the Solar System, Princeton University Press, p. 226,
^ Basri, G.; Brown, E. M. (May 2006), "Planetesimals to Brown Dwarfs:
What is a Planet?", Annual Review of
Earth and Planetary Sciences, 34:
193–216, arXiv:astro-ph/0608417 , Bibcode:2006AREPS..34..193B,
^ 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.
^ Lissauer, J. J. (1987). "Timescales for Planetary Accretion and the
Structure of the Protoplanetary disk". Icarus. 69 (2): 249–265.
^ 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 Press Release
^ Clavin, Whitney (November 9, 2005). "A
Planet with Planets? Spitzer
Finds Cosmic Oddball". Spitzer
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".
Astrophysics. 558 (7): L7. arXiv:1310.1936 .
^ 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.
^ 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 .
^ Britt, Robert Roy (2004-09-10). "Likely First Photo of
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 into a
Planet in a Millisecond
Pulsar Binary". Science. 333 (6050): 1717–20. arXiv:1108.5201 .
^ 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].
^ 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.
^ 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.
^ "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
Inclination and Color in the Classical Kuiper Belt".
Astrophysical Journal. 566 (2): L125. arXiv:astro-ph/0201040 .
^ 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.
^ Goldstein, R. M.; Carpenter, R. L. (1963). "Rotation of Venus:
Period Estimated from Radar Measurements". Science. 139 (3558):
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 and Neptune".
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 formation". Annual Review of
Astronomy and Astrophysics. 31. (A94-12726 02–90) (1): 129–174.
^ Strobel, Nick. "
Planet tables". astronomynotes.com. Retrieved
^ Zarka, Philippe; Treumann, Rudolf A.; Ryabov, Boris P.; Ryabov,
Vladimir B. (2001). "Magnetically-Driven Planetary Radio Emissions and
Application to Extrasolar Planets". Astrophysics &
277 (1/2): 293–300. Bibcode:2001Ap&SS.277..293Z.
^ Faber, Peter; Quillen, Alice C. (2007-07-12). "The Total Number of
Giant Planets in Debris Disks with Central Clearings".
^ Brown, Michael E. (2006). "The Dwarf Planets". California Institute
of Technology. Retrieved 2008-02-01.
^ How One
Astronomer Became the Unofficial
^ Jason T Wright; Onsi Fakhouri; Marcy; Eunkyu Han; Ying Feng; John
Asher Johnson; Howard; Fischer; Valenti; Anderson, Jay; Piskunov,
Nikolai (2010). "The
Orbit Database". Publications of the
Astronomical Society of the Pacific. 123 (902): 412–422.
arXiv:1012.5676 [astro-ph.SR]. Bibcode:2011PASP..123..412W.
^ a b "Planetary Interiors". Department of Physics, University of
Oregon. Retrieved 2008-08-23.
^ Elkins-Tanton, Linda T. (2006).
Jupiter and Saturn. New York:
Chelsea House. ISBN 0-8160-5196-8.
^ Podolak, M.; Weizman, A.; Marley, M. (December 1995). "Comparative
Uranus and Neptune". Planetary and
Space Science. 43 (12):
^ 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 .
^ Zeilik, Michael A.; Gregory, Stephan A. (1998). Introductory
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 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.
^ Gefter, Amanda (2004-01-17). "Magnetic planet". Astronomy. Retrieved
^ Grasset, O.; Sotin C.; Deschamps F. (2000). "On the internal
structure and dynamic of Titan". Planetary and
Space Science. 48
(7–8): 617–636. Bibcode:2000P&SS...48..617G.
^ Fortes, A. D. (2000). "Exobiological implications of a possible
ammonia-water ocean inside Titan". Icarus. 146 (2): 444–452.
^ 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.
^ Thérèse, Encrenaz (2004). The
Solar System (Third ed.). Springer.
pp. 388–390. ISBN 3-540-00241-3.
Wikimedia Commons has media related to Planets.
Wikiquote has quotations related to: Planet
Look up planet in Wiktionary, the free dictionary.
International Astronomical Union
International Astronomical Union website
Planet Quest –
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
The Solar System
S/2015 (136472) 1
Solar System objects
By discovery date
Gravitationally rounded objects
Possible dwarf planets
first discovered: Ceres
Planets beyond Neptune
List of crewed spacecraft
List of probes
Outline of the Solar System
Solar System → Local Interstellar Cloud → Local
Bubble → Gould Belt → Orion Arm → Milky
Milky Way subgroup → Local Group → Virgo
Supercluster → Laniakea Supercluster → Observable
universe → Universe
Each arrow (→) may be read as "within" or "part of".
Methods of detecting exoplanets
Sizes and types
Brown dwarf boundary
Ultra-short period planets
Ultra-short period planets (USP)
Formation and evolution
Planets in globular clusters
Circumstellar habitable zone
Extraterrestrial liquid water
Habitability of natural satellites
Catalog of Nearby Habitable Systems
Exoplanet Data Explorer
Extrasolar Planets Encyclopaedia
Stars with proplyds
List of exoplanets
Discovered exoplanets by year
Carl Sagan Institute
Exoplanet phase curves
Exoplanet System Science
Planets in science fiction
Sudarsky's gas giant classification
Discoveries of exoplanets
Themes and subjects
Chronology of the universe
1: Creation -
Big Bang and cosmogony
2: Stars - creation of stars
3: Elements - creation of chemical elements inside dying stars
4: Planets - formation of planets
Life - abiogenesis and evolution of life
6: Humans - development of Homo sapiens
7: Agriculture - Agricultural Revolution
Modernity - modern era
Big History Project
Crash Course Big History
Cynthia Stokes Brown