Planet

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 – according to the International Astronomical Union but not all planetary scientists – has cleared its neighbouring region of planetesimals.[b][1][2]

The term planet is ancient, with ties to history, astrology, science, mythology, and religion. Apart from Earth itself, five planets in the Solar System are often visible to 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 (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 current definition, some celestial bodies, such as Ceres, Pallas, Juno and Vesta (each an object in the solar asteroid belt), and Pluto (the first trans-Neptunian object discovered), that were once considered planets by the scientific community, are no longer viewed as planets under the current definition of planet.

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. About the same time, by careful analysis of pre-telescopic observational data collected by Tycho Brahe, Johannes Kepler found the planets' orbits were elliptical rather than circular. As observational tools improved, astronomers saw that, like Earth, each of the planets rotated around an axis tilted with respect to its orbital pole, and some shared such features as ice caps and seasons. Since the dawn of the Space Age, close observation by space probes has found that Earth and the other planets share characteristics such as volcanism, hurricanes, tectonics, and even hydrology.

Planets in the Solar System are divided into two main types: large low-density giant planets, and smaller rocky terrestrials. There are eight planets in the Solar System according to the IAU definition.[1] 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 December 2020, 4,379 known extrasolar planets in 3,237 planetary systems (including 717 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 circumstellar habitable zone.[3][4] On 20 December 2011, the Kepler Space 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[c] stars is thought to have an Earth-sized[d] planet in its habitable[e] zone.[11][12]

History
Further information: History of astronomy, Definition of planet, and Timeline of 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 of the Solar System, 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.[13] Ancient Greeks called these lights πλάνητες ἀστέρες (planētes asteres, "wandering stars") or simply πλανῆται (planētai, "wanderers"),[14] from which today's word "planet" was derived.[15][16][17] In ancient Greece, China, Babylon, and indeed all pre-modern civilizations,[18][19] 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[20] and the apparently common-sense perceptions that 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 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.[21] 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.[22] The Babylonian astrologers also laid the foundations of what would eventually become Western astrology.[23] The Enuma anu enlil, written during the Neo-Assyrian period in the 7th century BC,[24] comprises a list of omens and their relationships with various celestial phenomena including the motions of the planets.[25][26] Venus, Mercury, and the outer planets Mars, Jupiter, and Saturn were all identified by Babylonian astronomers. These would remain the only known planets until the invention of the telescope in early modern times.[27]
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 or 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),[28] though this had long been known by the Babylonians. In the 3rd century BC, 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.[21][29] To the Greeks and Romans there were seven known planets, each presumed to be circling Earth according to the complex laws laid out by Ptolemy. They were, in increasing order from Earth (in Ptolemy's order and using modern names): the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn.[17][29][30]

Cicero, in his De Natura Deorum, enumerated the planets known during the 1st century BCE using the names for them in use at the time:[31]

"But there is most matter for wonder in the movements of the five stars which are falsely called wandering; falsely, because nothing wanders which through all eternity preserves its forward and retrograde courses, and its other movements, constant and unaltered. ... For instance, the star which is farthest from the earth, which is known as the star of Saturn, and is called by the Greeks Φαίνων (Phainon), accomplishes its course in about thirty years, and though in that course it does much that is wonderful, first preceding the sun, and then falling off in speed, becoming invisible at the hour of evening, and returning to view in the morning, it never through the unending ages of time makes any variation, but performs the same movements at the same times. Beneath it, and nearer to the earth, moves the planet of Jupiter, which is called in Greek Φαέθων (Phaethon); it completes the same round of the twelve signs in twelve years, and performs in its course the same variations as the planet of Saturn. The circle next below it is held by Πυρόεις (Pyroeis), which is called the planet of Mars, and traverses the same round as the two planets above it in four and twenty months, all but, I think, six days. Beneath this is the planet of Mercury, which is called by the Greeks Στίλβων (Stilbon); it traverses the round of the zodiac in about the time of the year’s revolution, and never withdraws more than one sign’s distance from the sun, moving at one time in advance of it, and at another in its rear. The lowest of the five wandering stars, and the one nearest the earth, is the planet of Venus, which is called Φωσϕόρος (Phosphoros) in Greek, and Lucifer in Latin, when it is preceding the sun, but Ἕσπερος (Hesperos) when it is following it; it completes its course in a year, traversing the zodiac both latitudinally and longitudinally, as is also done by the planets above it, and on whichever side of the sun it is, it never departs more than two signs’ distance from it."

India
Main articles: 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.[32] 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.[33]

In 1500, Nilakantha Somayaji of the Kerala school of astronomy and mathematics, in his Tantrasangraha, revised Aryabhata's model.[34] In his Aryabhatiyabhasya, a commentary on Aryabhata's Aryabhatiya, he developed a planetary model where Mercury, Venus, Mars, Jupiter and 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.[34][35]
Medieval Muslim astronomy
Main articles: 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.[36] 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 by the Maragha astronomer Qotb al-Din Shirazi in the 13th century.[37] Ibn Bajjah could not have observed a transit of Venus, because none occurred in his lifetime.[38]
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

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

Planets 1854–1930, Solar planets 2006–present
1 Mercury	2 Venus	3 Earth	4 Mars	5 Jupiter	6 Saturn	7 Uranus	8 Neptune

(Solar) planets 1930–2006
1 Mercury	2 Venus	3 Earth	4 Mars	5 Jupiter	6 Saturn	7 Uranus	8 Neptune	9 Pluto

See also: List of former planets

The table below lists Solar System bodies once considered to be planets but no longer considered as such by the IAU, as well as whether they would be considered planets under Stern's 2002 and 2018 definitions.

Body	IAU classification	Geophysical planet?	Notes
Sun	Star	No	Classified as a classical planet (Ancient Greek πλανῆται, wanderers) in classical antiquity and medieval Europe, in accordance with the now-disproved geocentric model.^[70]
Moon	Natural satellite	No (not in equilibrium)
Io, Europa	Natural satellites	Possibly (perhaps in equilibrium due to tidal heating)	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 secondary planets.^[71]
Ganymede, Callisto	Natural satellites	Yes
Titan^[f]	Natural satellite	Yes
Rhea^[g]	Natural satellite	Possibly (excluded 2002)	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.^[71]
Iapetus,^[g], Tethys,^[h] and Dione^[h]	Natural satellites	No
Juno	Asteroid	No	Regarded as planets from their discoveries between 1801 and 1807 until they were reclassified as asteroids during the 1850s.^[73] Ceres was subsequently classified by the IAU as a dwarf planet in 2006.
Pallas	Asteroid	No
Vesta	Asteroid	Formerly
Ceres	Dwarf planet and asteroid	Yes
Astraea, Hebe, Iris, Flora, Metis, Hygiea, Parthenope, Victoria, Egeria, Irene, Eunomia	Asteroids	No	More asteroids, discovered between 1845 and 1851. The rapidly expanding list of bodies between Mars and Jupiter prompted their reclassification as asteroids, which was widely accepted by 1854.^[74]
Pluto	Dwarf planet and Kuiper belt object	Yes	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.

The reporting of newly discovered large Kuiper belt objects as planets - particularly Eris - triggered the August 2006 IAU decision on what a planet is.
Mythology and naming
See also: 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 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: Helios and Selene were 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 of 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.[21]

The Greek practice of grafting their gods' names onto the planets was almost certainly borrowed from the Babylonians. The Babylonians named Phosphoros after their goddess of love, Ishtar; Pyroeis after their god of war, Nergal, Stilbon after their god of wisdom Nabu, and Phaethon after their chief god, Marduk.[75] There are too many concordances between Greek and Babylonian naming conventions for them to have arisen separately.[21] 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.[76]

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.[77] When the Romans studied Greek astronomy, they gave the planets their own gods' names: Mercurius (for Hermes), Venus (Aphrodite), 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 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 (from the farthest to the closest planet).[78] Therefore, the first day was started by Saturn (1st hour), second day by Sun (25th hour), followed by Moon (49th hour), Mars, Mercury, Jupiter and Venus. Because each day was named by the god that started it, this 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.[79] 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 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,[39] there is no tradition of naming it after a god. (The same is true, in English at least, of the Sun and the Moon, though they are no longer generally considered planets.) The name originates from the 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 perhaps around 1300.[80][81] As with its equivalents in the other Germanic languages, it derives ultimately from the Proto-Germanic word ertho, "ground",[81] 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".[82] 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, Budha for Mercury, Shukra for Venus, Mangala for Mars, Bṛhaspati for Jupiter, and Shani for 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, Korea and Vietnam) use a naming system based on the five Chinese elements: water (Mercury), metal (Venus), fire (Mars), wood (Jupiter) and earth (Saturn).[79]

In traditional Hebrew astronomy, the seven traditional planets have (for the most part) descriptive names – the Sun is חמה Ḥammah or "the hot one," the Moon is לבנה Levanah or "the white one," Venus is כוכב נוגה Kokhav Nogah or "the bright planet," Mercury is כוכב Kokhav or "the planet" (given its lack of distinguishing features), Mars is מאדים Ma'adim or "the red one," and Saturn is שבתאי Shabbatai or "the resting one" (in reference to its slow movement compared to the other visible planets).[83] 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.[83]

In Arabic, Mercury is عُطَارِد (ʿUṭārid, cognate with Ishtar / Astarte), Venus is الزهرة (az-Zuhara, "the bright one",[84] an epithet of the goddess Al-'Uzzá[85]), Earth is الأرض (al-ʾArḍ, from the same root as eretz), Mars is اَلْمِرِّيخ (al-Mirrīkh, meaning "featherless arrow" due to its retrograde motion[86]), Jupiter is المشتري (al-Muštarī, "the reliable one", from Akkadian[87]) and Saturn is زُحَل (Zuḥal, "withdrawer"[88]).[89][90]
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.[91] After a planet reaches a mass somewhat larger than Mars' mass, it begins to accumulate an extended atmosphere,[92] greatly increasing the capture rate of the planetesimals by means of atmospheric drag.[93][94] Depending on the accretion history of solids and gas, a giant planet, an ice giant, or a terrestrial planet may result.[95][96][97]
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.[98][99] 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.[100] 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.[101] 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.[102] (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.[103] 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
Solar System – sizes but not distances are to scale
The 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 against the Sun and some sunspots
Main article: Solar System
See also: List of gravitationally rounded objects of the Solar System

According to the IAU definition, there are eight planets in the Solar System, which are in increasing distance from the Sun:

☿ Mercury
♀ Venus
⊕ Earth
♂ Mars
♃ Jupiter
♄ Saturn
♅ Uranus
♆ Neptune

Jupiter is the largest, at 318 Earth masses, whereas Mercury is the smallest, at 0.055 Earth masses.

The planets of the Solar System can be divided into categories based on their composition:

Terrestrials: 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.
Gas giants, 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 Earth masses.
Ice giants, Uranus and Neptune, are primarily composed of low-boiling-point materials such as water, methane, and ammonia, with thick atmospheres of hydrogen and helium. They have a significantly lower mass than the gas giants (only 14 and 17 Earth masses).

The number of geophysical planets in the solar system is unknown - previously considered to be potentially in the hundreds, but now only estimated at only the low double digits.[104]
Planetary attributes
Comparison of the rotation period (sped up 10 000 times, negative values denoting retrograde), flattening and axial tilt of the planets and the Moon (SVG animation)

	Name	Equatorial diameter ^[i]	Mass ^[i]	Semi-major axis (AU)	Orbital period (years)	Inclination to Sun's equator (°)	Orbital eccentricity	Rotation period (days)	Confirmed moons	Axial tilt (°)	Rings	Atmosphere
1.	Mercury	0.383	0.06	0.39	0.24	3.38	0.206	58.65	0	0.10	no	minimal
2.	Venus	0.949	0.81	0.72	0.62	3.86	0.007	−243.02	0	177.30	no	CO₂, N₂
3.	Earth ^(a)	1.000	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	317.83	5.20	11.86	6.09	0.048	0.41	79	3.12	yes	H₂, He
6.	Saturn	9.449	95.16	9.54	29.45	5.51	0.054	0.44	82	26.73	yes	H₂, He
7.	Uranus	4.007	14.54	19.19	84.02	6.48	0.047	−0.72	27	97.86	yes	H₂, He, CH₄
8.	Neptune	3.883	17.15	30.07	164.79	6.43	0.009	0.67	14	29.60	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 December 2020, there are 4,379 confirmed exoplanets in 3,237 systems, with 717 systems having more than one planet.[105][106][107][108]

In early 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12.[46] 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 Candidates – based on 2,740 candidates orbiting 2,036 stars as of 4 November 2013 (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 of the 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 or larger as they were more easily detected. The catalog of Kepler candidate planets consists mostly of planets the size of Neptune and smaller, down to smaller than Mercury.

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 that of Earth is a possible dividing line between the two types of planet.[109] 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 20 December 2011, the Kepler Space 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 stars have an "Earth-sized"[d] planet in the habitable[e] zone, so the nearest would be expected to be within 12 light-years distance from Earth.[11][110] 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.[111]

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, see Ultra-short period planet. The 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.
Planetary-mass objects
Main article: Geophysical definition of 'planet'
See also: List of gravitationally rounded objects of the Solar System

A planetary-mass object (PMO), planemo,[112] 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.[113][114] 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, planetary-mass 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).
Dwarf planets
The dwarf planet Pluto
Main article: Dwarf planet

A dwarf planet is a planetary-mass object that is neither a true planet nor a natural satellite; it is in direct orbit of a star, and is massive enough for its gravity to compress it into a hydrostatically equilibrious shape (usually a spheroid), but has not cleared the neighborhood of other material around its orbit. Planetary scientist and New Horizons principal investigator Alan Stern, who proposed the term 'dwarf planet', has argued that location should not matter and that only geophysical attributes should be taken into account, and that dwarf planets are thus a subtype of planet. The IAU accepted the term (rather than the more neutral 'planetoid') but decided to classify dwarf planets as a separate category of object.[115]
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.[116] Such objects are typically called rogue planets.
Sub-brown dwarfs
Artist's impression of a super-Jupiter around the brown dwarf 2M1207.[117]
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[118] and OTS 44,[119] 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 Jupiter masses, but further observations revised the estimated masses upwards to greater than 13 Jupiter masses, making them brown dwarfs according to the IAU working definitions.[120][121][122]
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.[123] These shrunken white dwarfs may become a helium planet or carbon planet.
Satellite planets
Main article: Satellite planet

Some large satellites (moons) are of similar size or larger than the planet Mercury, e.g. Jupiter's Galilean moons and Titan. Proponents of the geophysical definition of planets argue that location should not matter and that only geophysical attributes should be taken into account in the definition of a planet. Alan Stern proposes the term satellite planet for a planet-sized satellite.[124]
Captured planets

Rogue 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.[125]
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 and Orbital elements
See also: Kepler's laws of planetary motion and Exoplanetology § Orbital parameters
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.[126] The period of one revolution of a planet's orbit is known as its sidereal period or year.[127] 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.[128]

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.[127] Comets and Kuiper belt objects (as well as several extrasolar planets) have very high eccentricities, and thus exceedingly elliptical orbits.[129][130]

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.[127]
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.[131] The eight planets of the Solar System all lie very close to the ecliptic; comets and Kuiper belt objects like Pluto are at far more extreme angles to it.[132] The points at which a planet crosses above and below its reference plane are called its ascending and descending nodes.[127] 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.[127]

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.[133] 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.[134]
Rotation
See also: Exoplanetology § Rotation and axial tilt

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[135] and Uranus,[136] 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.[137] Regardless of which convention is used, Uranus has a retrograde rotation relative to its orbit.

The rotation of a planet can be induced by several factors during formation. A net angular momentum 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.[138] 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.[139] 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.[140]
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.[141]
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.[142]

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 60 Jupiter masses,[58] and the Exoplanet Data Explorer up to 24 Jupiter masses.[143]

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,[144] 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 and Saturn are believed to have cores of rock and metal surrounded by mantles of metallic hydrogen.[145] Uranus and Neptune, which are smaller, have rocky cores surrounded by mantles of water, ammonia, methane and other ices.[146] The fluid action within these planets' cores creates a geodynamo that generates a magnetic field.[144]
Atmosphere
Main articles: Atmosphere and Extraterrestrial atmospheres
See also: Extraterrestrial skies
Earth's atmosphere

All of the Solar System planets except Mercury[147] 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.[148] 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.[149]

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).[133] At least one extrasolar planet, HD 189733 b, has been claimed to have such a weather system, similar to the Great Red Spot but twice as large.[150]

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.[151][152] These planets may have vast differences in temperature between their day and night sides that produce supersonic winds,[153] although the day and night sides of HD 189733 b appear to have very similar temperatures, indicating that that planet's atmosphere effectively redistributes the star's energy around the planet.[150]
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.[154]

Of the eight planets in the Solar System, only Venus and Mars lack such a magnetic field.[154] In addition, the moon of 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 (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 and Neptune are strongly tilted relative the rotational axis and displaced from the centre of the planet.[154]

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.[155]
Secondary characteristics
Main articles: 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 "moons". 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).[156][157][158]

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.[159][160]

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[118] and the sub-brown dwarf OTS 44 was shown to be surrounded by a substantial protoplanetary disk of at least 10 Earth masses.[119]
See also

Astronomy portal Solar System portal Space 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 – A list of lists of planets sorted by diverse attributes
Mesoplanet – A celestial body smaller than Mercury but larger than Ceres
Minor planet – A celestial body smaller than a planet
Planetary habitability – Extent to which a planet is suitable for life as we know it
Planetary mnemonic – A phrase used to remember the names of the planets
Planetary science – Science of astronomical objects apparently in orbit around one or more stellar objects within a few light years – The scientific study of planets
Planets in astrology
Planets in science fiction – Planet that only appears in works of fiction
Theoretical planetology

Notes

According to the IAU definition of planet.
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.
Data for G-type stars like the Sun is not available. This statistic is an extrapolation from data on K-type stars.
For the purpose of this 1 in 5 statistic, Earth-sized means 1–2 Earth radii
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
Both labelled nouvelles planètes (new planets) by Cassini in his Découverte de deux nouvelles planetes autour de Saturne[72]
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").

Measured relative to Earth.

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Thérèse, Encrenaz (2004). The Solar System (Third ed.). Springer. pp. 388–390. ISBN 978-3-540-00241-3.

External links
Wikimedia Commons has media related to Planets.
Wikiquote has quotations related to: Planet
Look up planet in Wiktionary, the free dictionary.

International Astronomical Union website
Photojournal NASA
NASA Planet Quest – 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)
The Planets, BBC Radio 4 discussion with Paul Murdin, Hugh Jones & Carolin Crawford (In Our Time, May 27, 2004)

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Solar System
The Sun, the planets, their moons, and several trans-Neptunian objects

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

Planets

Terrestrials
Mercury Venus Earth Mars Giants
Jupiter Saturn Uranus Neptune Dwarfs
Ceres Pluto Haumea Makemake Eris

Moons

Earth
Moon other near-Earth objects Mars
Phobos Deimos Jupiter
Ganymede Callisto Io Europa all 79 Saturn
Titan Rhea Iapetus Dione Tethys Enceladus Mimas Hyperion Phoebe all 82 Uranus
Titania Oberon Umbriel Ariel Miranda all 27 Neptune
Triton Proteus Nereid all 14 Pluto
Charon Nix Hydra Kerberos Styx Eris
Dysnomia Haumean
Hiʻiaka Namaka Makemake
S/2015 (136472) 1

Lists

Comets Possible Dwarf planets Gravitationally rounded objects Minor planets Natural satellites Solar System models Solar System objects
by size by discovery date Interstellar/Circumstellar molecules

Small
Solar
System
bodies

Comets Damocloids Meteoroids Minor planets
Names and meanings moons Planetesimal Mercury-crossers Venus-crossers Venus trojans Near-Earth objects Earth-crossers Earth trojans Mars-crossers Mars trojans Asteroid belt Asteroids
Ceres Pallas Juno Vesta active first 1000 families exceptional Kirkwood gap Jupiter-crossers Jupiter trojans Centaurs Saturn-crossers Uranus-crossers Uranus trojans Neptune-crossers Neptune trojans Sednoids Cis-Neptunian objects Trans-Neptunian objects Kuiper belt
Cubewanos Plutinos Detached objects Hills cloud Oort cloud Scattered disc

Rings

Jovian Saturnian (Rhean) Charikloan Chironean Uranian Neptunian Haumean

Hypothetical
objects

Fifth giant Nemesis Phaeton Planet Nine Planet V Planet X Subsatellites Theia Tyche Vulcan Vulcanoids

Exploration
(outline)

Colonization Discovery
astronomy historical models timeline Human spaceflight
space stations list Space probes
timeline list Mercury Venus Moon
mining Mars Ceres Asteroids
mining Comets Jupiter Saturn Uranus Neptune Pluto Deep space

Formation
and
evolution

Accretion Accretion disk
Excretion disk Circumplanetary disk Circumstellar disc Circumstellar envelope Coatlicue Cosmic dust Debris disk Detached Objects Disrupted planet EXCEDE Exozodiacal dust Extraterrestrial materials
Sample-return mission Sample curation Giant-impact hypothesis Gravitational collapse Hills Cloud Interplanetary dust cloud Interplanetary medium Interplanetary space Interstellar cloud Interstellar dust Interstellar medium Interstellar space Kuiper belt Merging stars Molecular cloud Nebular hypothesis Oort cloud Outer space Planetary migration Planetary system Planetesimal Planet formation Protoplanetary disk Ring system Rubble pile Scattered disc Star formation

Outline of the Solar System Solar system.jpg Solar System portal Crab Nebula.jpg Astronomy portal The Earth seen from Apollo 17 with transparent background.png Earth Sciences portal

Solar System → Local Interstellar Cloud → Local Bubble → Gould Belt → Orion Arm → Milky Way → Milky Way subgroup → Local Group → Local Sheet → 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 Planet-hosting stars

PDS 70.jpg
Sizes
and
types
Terrestrial

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

Gaseous

Eccentric Jupiter Gas dwarf Helium planet Hot Jupiter Hot Neptune Ice giant Mini-Neptune Super-Neptune Super-Jupiter Super-puff Ultra-hot Jupiter Ultra-hot Neptune

Other types

Blanet Brown dwarf Chthonian planet Circumbinary planet Disrupted planet Double planet Eyeball planet Giant planet Mesoplanet Planemo Planet/Brown dwarf boundary Planetesimal Protoplanet Pulsar planet Sub-brown dwarf Sub-Neptune Ultra-cool dwarf Ultra-short period planet (USP)

Formation
and
evolution

Accretion Accretion disk Asteroid belt Circumplanetary disk Circumstellar disc Circumstellar envelope Cosmic dust Debris disk Detached object Disrupted planet Excretion disk Exozodiacal dust Extraterrestrial materials Extraterrestrial sample curation Giant-impact hypothesis Gravitational collapse Hills cloud Interplanetary dust cloud Interplanetary medium Interplanetary space Interstellar cloud Interstellar dust Interstellar medium Interstellar space Kuiper belt List of interstellar and circumstellar molecules Merging stars Molecular cloud Nebular hypothesis Oort cloud Outer space Planetary migration Planetary system Planetesimal Planet formation Protoplanetary disk Ring system Rubble pile Sample-return mission Scattered disc Star formation

Systems

Exocomet
Interstellar Exomoon
Tidally detached Rogue planet Orbits
Retrograde Trojan Mean-motion resonances Titius–Bode law

Host stars

A B Binary star Brown dwarfs F/Yellow-white dwarfs G/Yellow dwarfs Herbig Ae/Be K/Orange dwarfs M/Red dwarfs Pulsar Red giant Subdwarf B Subgiant T Tauri White dwarfs Yellow giants

Detection

Astrometry Direct imaging
list Microlensing
list Polarimetry Pulsar timing
list Radial velocity
list Transit method
list Transit-timing variation

Habitability

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

Catalogues

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

Lists

Exoplanetary systems
Host stars Multiplanetary systems Stars with proto-planetary discs

Exoplanets
Discoveries Extremes Firsts Nearest Largest Heaviest Terrestrial candidates Kepler Potentially habitable Proper names

Discovered exoplanets by year
before 2000 2000–2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020

Other

Carl Sagan Institute Exoplanet naming convention Exoplanet phase curves Exoplanetary Circumstellar Environments and Disk Explorer Extragalactic planet Fulton gap Geodynamics of terrestrial exoplanets Neptunian Desert Nexus for Exoplanet System Science Planets in globular clusters Planets in science fiction Sudarsky's gas giant classification

Discoveries of exoplanets Search projects

vte

Big History
Themes and subjects

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

Eight thresholds

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 5: Life - abiogenesis and evolution of life 6: Humans - development of Homo sapiens
Stone Age 7: Agriculture - Agricultural Revolution 8: Modernity - modern era

Web-based education

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

Big History (2013 series)

Physics Encyclopedia

World

Index

Hellenica World - Scientific Library

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