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A planet is an
astronomical body Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science Natural science is a branch of science Science (from the Latin word ''scientia'', meaning "knowled ...
orbit In celestial mechanics, an orbit is the curved trajectory of an physical body, object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an satellite, artificial satellite around an object or po ...

orbit
ing a
star A star is an astronomical object consisting of a luminous spheroid of plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral) or heliotrope, a mineral aggregate * Quark ...

star
or
stellar remnant In astronomy, the term compact star (or compact object) refers collectively to white dwarfs, neutron stars, and black holes. It would grow to include exotic stars if such hypothetical, dense bodies are confirmed to exist. All compact objects hav ...
that is massive enough to be
rounded
rounded
by its own
gravity Gravity (), or gravitation, is a by which all things with or —including s, s, , and even —are attracted to (or ''gravitate'' toward) one another. , gravity gives to s, and the causes the s of the oceans. The gravitational attracti ...

gravity
, is not massive enough to cause
thermonuclear fusion Thermonuclear fusion is the process of atoms combining or “fusing” together with huge amounts of heat. There are two forms of thermonuclear fusion: ''uncontrolled'', in which the resulting energy is released in an uncontrolled manner, as it is ...
, and – according to the
International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a Non-governmental organization, nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting ...
but not all planetary scientists – has cleared its neighbouring region of
planetesimal Planetesimals are solid objects thought to exist in protoplanetary disks and debris disks. Per the Chamberlin–Moulton planetesimal hypothesis, they are believed to form out of cosmic dust grains. Believed to have formed in the Solar System ab ...
s.This
definition A definition is a statement of the meaning of a term (a word In linguistics, a word of a spoken language can be defined as the smallest sequence of phonemes that can be uttered in isolation with semantic, objective or pragmatics, practical ...
is drawn from two separate
IAU The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a Non-governmental organization, nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting ...
declarations; a formal definition agreed by the IAU in 2006 (IAU Resolution 5A), and an informal working definition proposed in a position statement by an IAU Working Group in 2001/2003 for objects outside of the Solar System (no corresponding IAU resolution). The official 2006 definition applies only to the Solar System, whereas the 2003 working definition applies to planets around other stars. The extrasolar planet issue was deemed too complex to resolve at the 2006 IAU conference.
The term ''planet'' is ancient, with ties to
history History (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approxima ...

history
,
astrology Astrology is a pseudoscience that claims to divination, divine information about human affairs and terrestrial events by studying the movements and relative positions of Celestial objects in astrology, celestial objects. Astrology has be ...
,
science Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is something that is truth, true. The usual test for a statement of ...

science
,
mythology Myth is a folklore genre Folklore is the expressive body of culture shared by a particular group of people; it encompasses the tradition A tradition is a belief A belief is an Attitude (psychology), attitude that something is the ca ...

mythology
, and
religion Religion is a social Social organisms, including humans, live collectively in interacting populations. This interaction is considered social whether they are aware of it or not, and whether the exchange is voluntary/involuntary. Etymology ...

religion
. Apart from Earth itself, five planets in the
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

Solar System
are often visible to the
naked eye Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnification, magnifying, Optical telescope#Light-gathering power, light-collecting optical instrument, such as a telescope or micr ...
. These planets were regarded by many early cultures as divine or as emissaries of
deities A deity or god is a supernatural The supernatural encompasses supposed phenomena or entities that are not subject to the . This term is attributed to , such as s, s, , and . It also includes claimed abilities embodied in or provided by suc ...

deities
. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects that ranged in size, shape, orbital plane, and other characteristics. In 2006, the
International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a Non-governmental organization, nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting ...
(IAU) officially adopted a resolution defining planets within the Solar System. This definition is controversial because it excludes many objects of
planetary mass In astronomy, planetary mass is a measure of the mass of a planet-like astronomical object. Within the Solar System, planets are usually measured in the astronomical system of units, where the unit of mass is the solar mass (), the mass of the Sun ...
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 that were once considered ''planets'' by the scientific community are no longer viewed as such under the current definition. Some of the excluded objects include
Ceres Ceres most commonly refers to: * Ceres (dwarf planet) Ceres (; minor-planet designation: 1 Ceres) is the smallest recognized dwarf planet, the closest dwarf planet to the Sun, and the List of notable asteroids, largest object in the main astero ...
,
Pallas
Pallas
,
Juno Juno commonly refers to: *Juno (mythology), the Roman goddess of marriage and queen of the gods *Juno (film), ''Juno'' (film), 2007 Juno may also refer to: Arts, entertainment and media Fictional characters *Juno, in the film ''Jenny, Juno'' *Jun ...
, Vesta (all of which are objects in the solar asteroid belt), and
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It was the first and the largest Kuiper belt object to be discovered. After Pluto wa ...

Pluto
(the first
trans-Neptunian object A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet A minor planet is an astronomical object in direct orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, objec ...
discovered).
Ptolemy Claudius Ptolemy (; grc-koi, Κλαύδιος Πτολεμαῖος, , ; la, Claudius Ptolemaeus; AD) was a mathematician A mathematician is someone who uses an extensive knowledge of mathematics Mathematics (from Greek: ) includes ...
thought that the planets orbited
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

Earth
in
deferent and epicycle In the Hipparchian, Ptolemaic, and Copernican systems of astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, c ...
motions. Although the idea that the planets orbited the Sun had been suggested many times and had much scientific backing, it wasn’t until the 17th century that this view was supported by the first concrete evidence, which came in the form of the first
telescopic A telescope is an instrument designed for the observation of remote objects. Telescope(s) also may refer to: Music * The Telescopes, a British psychedelic band * Telescope (album), ''Telescope'' (album), by Circle, 2007 * The Telescope (album), '' ...
astronomical observations, performed by
Galileo Galilei Galileo di Vincenzo Bonaiuti de' Galilei (; 15 February 1564 – 8 January 1642) was an Italian astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the ...

Galileo Galilei
. About the same time, by careful analysis of pre-telescopic observational data collected by
Tycho Brahe Tycho Brahe ( ; born Tyge Ottesen Brahe; 14 December 154624 October 1601) was a Danish astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. T ...

Tycho Brahe
,
Johannes Kepler Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe as ...

Johannes Kepler
found the planets' orbits were
elliptical
elliptical
rather than
circular Circular may refer to: * The shape of a circle A circle is a shape consisting of all point (geometry), points in a plane (mathematics), plane that are at a given distance from a given point, the Centre (geometry), centre; equivalently it is ...
. As observational tools improved,
astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, planets, natural satellite, moons, comets and galaxy, g ...

astronomer
s saw that, like Earth, each of the planets rotated around an axis tilted with respect to its
orbital pole An orbital pole is either point at the ends of an imaginary line segment that runs through the center of an orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet ...
and some shared such features as
ice cap In glaciology Lateral moraine on a glacier joining the Gorner Glacier, Zermatt">Gorner_Glacier.html" ;"title="moraine on a glacier joining the Gorner Glacier">moraine on a glacier joining the Gorner Glacier, Zermatt, Swiss Alps. The moraine is ...
s and
season A season is a division of the year based on changes in weather Weather is the state of the atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or ...

season
s. Since the dawn of the
Space Age The Space Age is a period encompassing the activities related to the Space Race The Space Race was a 20th-century competition between two Cold War The Cold War was a period of geopolitical tension between the United States and the ...
, close observations by
space probe A space probe or a spaceprobe is a robotic spacecraft that doesn't Earth orbit, orbit the Earth (planet), Earth, but instead explores farther into outer space. A space probe may approach the Moon; travel through interplanetary space; planetary ...
s have found that Earth and the other planets share characteristics such as
volcanism Volcanism (or volcanicity) is the phenomenon of eruption of molten rock (magma) onto the Earth#Surface, surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics and volcanic gases erupt through a break in the surface called ...

volcanism
,
hurricane A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation Atmospheric circulation is the large-scale movement of Atmosphere of Earth, air and together with oc ...

hurricane
s,
tectonics Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of orogeny, mountain building, the growth and behavior of the strong, old cores of contin ...
, and even
hydrology Hydrology (from Ancient Greek, Greek wikt:ὕδωρ, ὕδωρ, ''hýdōr'' meaning "water" and wikt:λόγος, λόγος, ''lógos'' meaning "study") is the scientific study of the movement, distribution, and management of water on Earth and ...
. Planets in the Solar System are divided into two main types: large low-density
giant planet The giant planets constitute a diverse type of planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, ...
s and smaller rocky terrestrials. There are eight planets in the Solar System according to the IAU definition. In order of increasing distance from the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

Sun
, they are the four terrestrials,
Mercury Mercury usually refers to: * Mercury (planet) Mercury is the smallest planet in the Solar System and the closest to the Sun. Its orbit around the Sun takes 87.97 Earth days, the shortest of all the Sun's planets. It is named after the Roman g ...

Mercury
,
Venus Venus is the second planet from the Sun. It is named after the Venus (mythology), Roman goddess of love and beauty. As List of brightest natural objects in the sky, the brightest natural object in Earth's night sky after the Moon, Venus can ...

Venus
, Earth, and
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, being larger than only Mercury (planet), Mercury. In English, Mars carries the name of the Mars (mythology), Roman god of war and is often referred to ...

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

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

Saturn
,
Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus, who, according to Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and ...

Uranus
, and
Neptune Neptune is the eighth and farthest-known Solar planet from the Sun. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times the mass of Earth, slightly mo ...

Neptune
. Six of the planets are orbited by one or more
natural satellite A natural satellite is in the most common usage, an astronomical body Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science Natural science is a branch ...

natural satellite
s, the two exceptions being Mercury and Venus. Several thousands of planets have been discovered orbiting around other stars in the
Milky Way The Milky Way is the galaxy A galaxy is a gravitation Gravity (), or gravitation, is a natural phenomenon by which all things with mass Mass is both a property Property (''latin: Res Privata'') in the Abstract and con ...

Milky Way
. These are referred to as "
extrasolar planets An exoplanet or extrasolar planet is a planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, is not ma ...
" or "exoplanets.” As of , known extrasolar planets in
planetary systems A planetary system is a set of gravitationally bound non-stellar objects in or out of orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet around a star or ...
have been discovered. The planetary systems figure includes multiple planetary systems. The exoplanets range in size from to
gas giant A gas giant is a giant planet The giant planets constitute a diverse type of planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilib ...
s about twice as large as Jupiter. More than 100 planets are the same size as Earth, nine of which are at the same from their star as Earth is from the Sun, i.e. they are in the
circumstellar habitable zone In astronomy and astrobiology, the circumstellar habitable zone (CHZ), or simply the habitable zone, is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure.J. F. Kast ...
. On 20 December 2011, the
Kepler Space Telescope The Kepler space telescope is a deactivated space telescope A space telescope or space observatory is a telescope located in outer space Outer space is the expanse that exists beyond Earth and between astronomical object, celestial b ...

Kepler Space Telescope
team reported the discovery of the first Earth-sized extrasolar planets,
Kepler-20e Kepler-20e is an exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917, but was not recognized as such. The first confirmation of detection occurred in 19 ...

Kepler-20e
and
Kepler-20f Kepler-20f (also known by its Kepler Object of Interest designation ''KOI-070.05'') is an exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917, but was no ...

Kepler-20f
, orbiting a Sun-like star,
Kepler-20 Kepler-20 is a star 929 light-years from Earth in the constellation Lyra with a system of six known planets. The apparent magnitude of this star is 12.51, so it cannot be seen with the unaided eye. Viewing it requires a telescope with an ape ...
. A 2012 study, analyzing
gravitational microlensing Gravitational microlensing is an astronomy, astronomical phenomenon due to the gravitational lens effect. It can be used to detect objects that range from the mass of a planet to the mass of a star, regardless of the light they emit. Typically, a ...

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-likeData for
G-type star In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectr ...
s like the Sun is not available. This statistic is an extrapolation from data on K-type stars.
stars is thought to have an Earth-sized planet in its habitable zone.


History

The idea of planets has evolved over its history, from the divine lights of antiquity to the earthly objects of the scientific age. The concept has expanded to include worlds not only in the Solar System, but in hundreds of other extrasolar systems. The ambiguities inherent in defining planets have led to much scientific controversy. The five
classical planet In classical antiquity, the seven classical planets or seven sacred luminaries are the seven moving astronomical objects in the sky visible to the naked eye: the Moon The Moon is Earth's only proper natural satellite. At one-quarter ...
s of the
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

Solar System
, being visible to the naked eye, have been known since ancient times and have had a significant impact on
mythology Myth is a folklore genre Folklore is the expressive body of culture shared by a particular group of people; it encompasses the tradition A tradition is a belief A belief is an Attitude (psychology), attitude that something is the ca ...

mythology
,
religious cosmology Religious cosmology is an explanation of the origin, evolution, and eventual fate of the universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of ...
, and ancient
astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses mathematics, phys ...
. In ancient times, astronomers noted how certain lights moved across the sky, as opposed to the "
fixed stars The fixed stars ( la, stellae fixae) compose the background of astronomical object In astronomy, an astronomical object or celestial object is a naturally occurring physical object, physical entity, association, or structure that exists i ...
", which maintained a constant relative position in the sky. Ancient Greeks called these lights (, "wandering stars") or simply (, "wanderers"), from which today's word "planet" was derived. In
ancient Greece Ancient Greece ( el, Ἑλλάς, Hellás) was a civilization belonging to a period of History of Greece, Greek history from the Greek Dark Ages of the 12th–9th centuries BC to the end of Classical Antiquity, antiquity ( AD 600). This era wa ...
,
China China (), officially the People's Republic of China (PRC; ), is a country in East Asia East Asia is the eastern region of Asia Asia () is Earth's largest and most populous continent, located primarily in the Eastern Hemisphere ...
,
Babylon ''Bābili(m)'' * sux, 𒆍𒀭𒊏𒆠 * arc, 𐡁𐡁𐡋 ''Babil'' * grc-gre, Βαβυλών ''Babylṓn'' * he, בָּבֶל ''Bavel'' * peo, 𐎲𐎠𐎲𐎡𐎽𐎢 ''Bābiru'' * elx, 𒀸𒁀𒉿𒇷 ''Babili'' *Kassite The Kassites ...

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


Babylon

The first civilization known to have a functional theory of the planets were the
Babylonia Babylonia () was an and based in central-southern which was part of Ancient Persia (present-day and ). A small -ruled state emerged in 1894 BCE, which contained the minor administrative town of . It was merely a small provincial town dur ...
ns, who lived in
Mesopotamia Mesopotamia ( grc, Μεσοποταμία ''Mesopotamíā''; ar, بِلَاد ٱلرَّافِدَيْن ; syc, ܐܪܡ ܢܗܪ̈ܝܢ, or , ) is a historical region of Western Asia situated within the Tigris–Euphrates river system, in th ...

Mesopotamia
in the first and second millennia BC. The oldest surviving planetary astronomical text is the Babylonian
Venus tablet of Ammisaduqa The Venus tablet of Ammisaduqa ('' Enuma Anu Enlil'' Tablet 63) is the record of astronomical observations of Venus Venus is the second planet from the Sun. It is named after the Venus (mythology), Roman goddess of love and beauty. As List of ...
, 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 MUL.APIN () is the conventional title given to a Babylonian literature, Babylonian compendium that deals with many diverse aspects of Babylonian astronomy and Babylonian astrology, astrology. It is in the tradition of earlier Babylonian star cat ...
is a pair of
cuneiform Cuneiform is a Logogram, logo-Syllabary, syllabic writing system, script that was used to write several languages of the Ancient Near East. The script was in active use from the early Bronze Age until the beginning of the Common Era. It is nam ...

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 Western astrology is the system of astrology most popular in Western countries. Western astrology is historically based on Ptolemy's ''Tetrabiblos'' (2nd century CE), which in turn was a continuation of Hellenistic astrology, Hellenistic and ult ...
. The ''
Enuma anu enlil Enuma Anu Enlil ( ,'' The Assyrian Dictionary'', volume 7 (I/J) – ''inūma'', The Oriental Institute, Chicago 1960, s. 160. ''When he godsAnu , image=File:Cuneiform sumer dingir.svg , caption=Ur III Sumerian cuneiform Cuneiform is a Logo ...
'', written during the
Neo-Assyrian The Neo-Assyrian Empire (Assyrian cuneiform: ''mat Aš-šur KI'', "Country of the Assur, city of Ashur (god), god Aššur"; also phonetically ''mat Aš-šur'') was an Iron Age Mesopotamian empire, in existence between 911 and 609 BC, and becam ...
period in the 7th century BC, comprises a list of
omen An omen (also called ''portent'') is a phenomenon A phenomenon (; plural phenomena) is an observable In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis ...

omen
s and their relationships with various celestial phenomena including the motions of the planets.
Venus Venus is the second planet from the Sun. It is named after the Venus (mythology), Roman goddess of love and beauty. As List of brightest natural objects in the sky, the brightest natural object in Earth's night sky after the Moon, Venus can ...

Venus
,
Mercury Mercury usually refers to: * Mercury (planet) Mercury is the smallest planet in the Solar System and the closest to the Sun. Its orbit around the Sun takes 87.97 Earth days, the shortest of all the Sun's planets. It is named after the Roman g ...

Mercury
, and the outer planets
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, being larger than only Mercury (planet), Mercury. In English, Mars carries the name of the Mars (mythology), Roman god of war and is often referred to ...

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

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

Saturn
were all identified by Babylonian astronomers. These would remain the only known planets until the invention of the
telescope A telescope is an optical instrument An optical instrument (or "optic" for short) is a device that processes light waves (or photons), either to enhance an image for viewing or to analyze and determine their characteristic properties. Common ...

telescope
in early modern times.


Greco-Roman astronomy

The ancient Greeks initially did not attach as much significance to the planets as the Babylonians. The
Pythagoreans Pythagoreanism originated in the 6th century BC, based on the teachings and beliefs held by Pythagoras and his followers, the Pythagoreans. Pythagoras established the first Pythagorean community in Crotone, Italy. Early Pythagorean communities spr ...
, in the 6th and 5th centuries BC appear to have developed their own independent planetary theory, which consisted of the Earth, Sun, Moon, and planets revolving around a "Central Fire" at the center of the Universe.
Pythagoras Pythagoras of Samos, or simply ; in Ionian Greek () was an ancient Ionians, Ionian Ancient Greek philosophy, Greek philosopher and the eponymous founder of Pythagoreanism. His political and religious teachings were well known in Magna Graec ...

Pythagoras
or
Parmenides Parmenides of Elea (; grc-gre, Παρμενίδης ὁ Ἐλεάτης; ) was a pre-Socratic Greek philosopher A philosopher is someone who practices philosophy. The term ''philosopher'' comes from the grc, φιλόσοφος, , translit ...

Parmenides
is said to have been the first to identify the evening star (
Hesperos In Greek mythology, Hesperus (; grc, Ἕσπερος, Hésperos) is the Evening Star, the planet Venus in the evening. He is the son of the dawn goddess Eos (Roman mythology, Roman Aurora (mythology), Aurora) and is the half-brother of her other s ...
) and morning star (
Phosphoros Phosphorus (Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximate ...
) as one and the same (
Aphrodite Aphrodite; , , ) is an ancient Greek goddess associated with love Love encompasses a range of strong and positive emotion Emotions are mental state, psychological states brought on by neurophysiology, neurophysiological changes, ...

Aphrodite
, Greek corresponding to Latin
Venus Venus is the second planet from the Sun. It is named after the Venus (mythology), Roman goddess of love and beauty. As List of brightest natural objects in the sky, the brightest natural object in Earth's night sky after the Moon, Venus can ...

Venus
), though this had long been known by the Babylonians. In the 3rd century BC,
Aristarchus of Samos Aristarchus of Samos (; grc-gre, Ἀρίσταρχος ὁ Σάμιος, ''Aristarkhos ho Samios''; ) was an ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient ...

Aristarchus of Samos
proposed a
heliocentric Heliocentrism is the astronomical Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies celestial objects and phenomena. It uses mathematics ...
system, according to which Earth and the planets revolved around the Sun. The geocentric system remained dominant until the
Scientific Revolution The Scientific Revolution was a series of events that marked the emergence In philosophy Philosophy (from , ) is the study of general and fundamental questions, such as those about reason, Metaphysics, existence, Epistemology, ...

Scientific Revolution
. By the 1st century BC, during the
Hellenistic period The Hellenistic period spans the period of History of the Mediterranean region, Mediterranean history between the death of Alexander the Great in 323 BC and the emergence of the Roman Empire, as signified by the Battle of Actium in 31  ...
, the Greeks had begun to develop their own mathematical schemes for predicting the positions of the planets. These schemes, which were based on geometry rather than the arithmetic of the Babylonians, would eventually eclipse the Babylonians' theories in complexity and comprehensiveness, and account for most of the astronomical movements observed from Earth with the naked eye. These theories would reach their fullest expression in the ''
Almagest The ''Almagest'' is a 2nd-century Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is appr ...
'' written by
Ptolemy Claudius Ptolemy (; grc-koi, Κλαύδιος Πτολεμαῖος, , ; la, Claudius Ptolemaeus; AD) was a mathematician A mathematician is someone who uses an extensive knowledge of mathematics Mathematics (from Greek: ) includes ...
in the 2nd century CE. So complete was the domination of Ptolemy's model that it superseded all previous works on astronomy and remained the definitive astronomical text in the Western world for 13 centuries. To the Greeks and Romans there were seven known planets, each presumed to be circling Earth according to the complex laws laid out by Ptolemy. They were, in increasing order from Earth (in Ptolemy's order and using modern names): the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. ''Note: select the Etymology tab ''
Cicero Marcus Tullius Cicero ( ; ; 3 January 106 BC – 7 December 43 BC) was a Ancient Rome, Roman statesman, lawyer, scholar, philosopher and Academic skepticism, Academic Skeptic, who tried to uphold optimate principles during crisis of ...

Cicero
, in his ''
De Natura Deorum ''De Natura Deorum'' (''On the Nature of the Gods'') is a philosophical dialogue by Roman Academic Skeptic philosopher A philosopher is someone who practices philosophy. The term ''philosopher'' comes from the grc, φιλόσοφος, , tran ...
'', enumerated the planets known during the 1st century BCE using the names for them in use at the time: :"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 Phaethon (; grc, Φαέθων, Phaéthōn, ), also spelled Phaëthon, was the son of the Oceanid Clymene (mother of Phaethon), Clymene and the solar deity, sun god Helios in Greek mythology. His name was also used by the Ancient Greek , Ancient ...
); 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 Pyroeis (Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 ...
), 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 Phosphorus (Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximate ...
) in Greek, and
Lucifer Lucifer is one of various figures in folklore Folklore is the expressive body of culture shared by a particular group of people; it encompasses the tradition A tradition is a belief A belief is an attitude Attitude may refer to: ...

Lucifer
in Latin, when it is preceding the sun, but Ἕσπερος (
Hesperos In Greek mythology, Hesperus (; grc, Ἕσπερος, Hésperos) is the Evening Star, the planet Venus in the evening. He is the son of the dawn goddess Eos (Roman mythology, Roman Aurora (mythology), Aurora) and is the half-brother of her other s ...
) 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

In 499 CE, the Indian astronomer
Aryabhata Aryabhata (, ISO: ) or Aryabhata I (476–550 CE) was the first of the major mathematician A mathematician is someone who uses an extensive knowledge of mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such ...
propounded a planetary model that explicitly incorporated
Earth's rotation Earth's rotation or Earth's spin is the rotation A rotation is a circular movement of an object around a center (or point) of rotation. The plane (geometry), geometric plane along which the rotation occurs is called the ''rotation plan ...
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 . Aryabhata's followers were particularly strong in
South India South India is a region consisting of the southern part of India India (Hindi: ), officially the Republic of India (Hindi: ), is a country in South Asia. It is the List of countries and dependencies by population, second-most populous ...

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. In 1500,
Nilakantha Somayaji Keļallur Nilakantha Somayaji (14 June 1444 – 1544), also referred to as Keļallur Comatiri, was a major mathematician A mathematician is someone who uses an extensive knowledge of mathematics Mathematics (from Ancient Greek, Gree ...
of the
Kerala school of astronomy and mathematics The Kerala school of astronomy and mathematics or the Kerala school was a school of mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algeb ...
, 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, Jupiter and Saturn orbit the Sun, which in turn orbits Earth, similar to the
Tychonic system File:Tychonian system.svg, The Tychonic system shown in colour, with the objects that rotate around the Earth shown on blue orbits, and the objects that rotate around the Sun shown on orange orbits. Around all is a sphere of fixed stars, stars, wh ...
later proposed by
Tycho Brahe Tycho Brahe ( ; born Tyge Ottesen Brahe; 14 December 154624 October 1601) was a Danish astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. T ...

Tycho Brahe
in the late 16th century. Most astronomers of the Kerala school who followed him accepted his planetary model.


Medieval Muslim astronomy

In the 11th century, the
transit of Venus A transit of Venus across the Sun takes place when the planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own ...

transit of Venus
was observed by
Avicenna Ibn Sina ( fa, ابن سینا), also known as Abu Ali Sina (), Pur Sina (), and often known in the West as Avicenna (;  – June 1037), was a Persian polymath who is regarded as one of the most significant physicians, astronomers, t ...

Avicenna
, who established that
Venus Venus is the second planet from the Sun. It is named after the Venus (mythology), Roman goddess of love and beauty. As List of brightest natural objects in the sky, the brightest natural object in Earth's night sky after the Moon, Venus can ...

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 Mercury A transit of Mercury across the Sun takes place when the planet Mercury (planet), Mercury passes directly (Transit (astronomy), transits) between the Sun and a superior planet, becoming visible against (and hence obscuring a small portion of) t ...
and Venus by the
Maragha Maragheh ( fa, مراغه, az, ماراغا ), also Romanized as Marāgheh; also known as Marāgha), is an ancient city and capital of Maragheh County, East Azerbaijan Province East Azerbaijan Province ( fa, استان آذربایجان ...

Maragha
astronomer Qotb al-Din Shirazi in the 13th century. Ibn Bajjah could not have observed a transit of Venus, because none occurred in his lifetime.


European Renaissance

With the advent of the
Scientific Revolution The Scientific Revolution was a series of events that marked the emergence In philosophy Philosophy (from , ) is the study of general and fundamental questions, such as those about reason, Metaphysics, existence, Epistemology, ...

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 orbited the Sun when the heliocentric model of
Copernicus Nicolaus Copernicus (; pl, Mikołaj Kopernik; german: link=no, Niclas Koppernigk, modern: ''Nikolaus Kopernikus''; 19 February 1473 – 24 May 1543) was a Renaissance The Renaissance ( , ) , from , with the same meanings. was a ...

Copernicus
,
Galileo Galileo di Vincenzo Bonaiuti de' Galilei ( , ; 15 February 1564 – 8 January 1642), commonly referred to as Galileo, was an astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific qu ...

Galileo
and
Kepler Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German , , , and writer on music. He is a key figure in the 17th-century , best known for his , and his books ', ', and '. These works also provided one of the foundations for ...

Kepler
gained sway. Thus, Earth became included in the list of planets, whereas the Sun and Moon were excluded. At first, when the first satellites of Jupiter and 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.


19th century

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 Ceres most commonly refers to: * Ceres (dwarf planet) Ceres (; minor-planet designation: 1 Ceres) is the smallest recognized dwarf planet, the closest dwarf planet to the Sun, and the List of notable asteroids, largest object in the main astero ...
, ,
Juno Juno commonly refers to: *Juno (mythology), the Roman goddess of marriage and queen of the gods *Juno (film), ''Juno'' (film), 2007 Juno may also refer to: Arts, entertainment and media Fictional characters *Juno, in the film ''Jenny, Juno'' *Jun ...
, and Vesta) were very different from the traditional ones. These bodies shared the same region of space between Mars and Jupiter (the
asteroid belt The asteroid belt is a torus-shaped region in the Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizin ...

asteroid belt
), and had a much smaller mass; as a result they were reclassified as "
asteroid An asteroid is a minor planet of the Solar System#Inner solar system, inner Solar System. Historically, these terms have been applied to any astronomical object orbiting the Sun that did not resolve into a disc in a telescope and was not observ ...

asteroid
s". In the absence of any formal definition, a "planet" came to be understood as any "large" body that orbited the Sun. Because there was a dramatic size gap between the asteroids and the planets, and the spate of new discoveries seemed to have ended after the discovery of Neptune in 1846, there was no apparent need to have a formal definition.


20th century

In the 20th century,
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It was the first and the largest Kuiper belt object to be discovered. After Pluto wa ...

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 Raymond Arthur Lyttleton FRS (7 May 1911 – 16 May 1995) was a British mathematician and theoretical astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the sco ...
suggested that Pluto may be an escaped satellite of
Neptune Neptune is the eighth and farthest-known Solar planet from the Sun. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times the mass of Earth, slightly mo ...

Neptune
, and
Fred Whipple Fred Lawrence Whipple (November 5, 1906 – August 30, 2004) was an American astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe ...
suggested in 1964 that Pluto may be a comet. As it was still larger than all known asteroids and the population of dwarf planets and other trans-Neptunian objects was not well observed, it kept its status until 2006. In 1992, astronomers
Aleksander Wolszczan Aleksander Wolszczan (born 29 April 1946 in Szczecinek, Poland) is a Polish astronomy, astronomer. He is the co-discoverer of the first extrasolar planets and pulsar planets. Early life and education Wolszczan was born on 29 April 1946 in Szcz ...

Aleksander Wolszczan
and
Dale Frail Dale A. Frail is a Canadian astronomer An astronomer is a in the field of who focuses their studies on a specific question or field outside the scope of . They observe s such as s, s, , s and – in either (by analyzing the data) or . Exam ...
announced the discovery of planets around a
pulsar A pulsar (from ''pulsating radio source'') is a highly magnetized rotating neutron star A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the s ...

pulsar
,
PSR B1257+12 PSR B1257+12, previously designated PSR 1257+12, alternatively designated PSR J1300+1240, is a millisecond pulsar located 2,300 light-years from the Sun in the constellation of Virgo (constellation), Virgo, rotating at about 161 times per ...
. This discovery is generally considered to be the first definitive detection of a planetary system around another star. Then, on October 6, 1995,
Michel Mayor Michel Gustave Édouard Mayor (; born 12 January 1942) is a Switzerland, Swiss astrophysicist and professor emeritus at the University of Geneva's Department of Astronomy. He formally retired in 2007, but remains active as a researcher at the O ...
and Didier Queloz of the Geneva Observatory announced the first definitive detection of an exoplanet orbiting an ordinary main sequence, 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 13 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 planets.


21st century

With the discovery during the latter half of the 20th century of more objects within the Solar System and large objects around other stars, disputes arose over what should constitute a planet. There were particular disagreements over whether an object should be considered a planet if it was part of a distinct population such as a asteroid belt, belt, or if it was large enough to generate energy by the
thermonuclear fusion Thermonuclear fusion is the process of atoms combining or “fusing” together with huge amounts of heat. There are two forms of thermonuclear fusion: ''uncontrolled'', in which the resulting energy is released in an uncontrolled manner, as it is ...
of deuterium. 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 50000 Quaoar, Quaoar, 90377 Sedna, Sedna, and Eris (dwarf planet), Eris, were heralded in the popular press as the tenth planet. The announcement of Eris in 2005, an object 27% more massive than Pluto, created the impetus for an official definition of a planet. Acknowledging the problem, the IAU set about creating the IAU definition of planet, definition of planet, and produced one in August 2006. The number of planets dropped to the eight significantly larger bodies that had Clearing the neighbourhood, 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 Ceres most commonly refers to: * Ceres (dwarf planet) Ceres (; minor-planet designation: 1 Ceres) is the smallest recognized dwarf planet, the closest dwarf planet to the Sun, and the List of notable asteroids, largest object in the main astero ...
,
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It was the first and the largest Kuiper belt object to be discovered. After Pluto wa ...

Pluto
and Eris). Dwarf planets had been proposed as a category of small planet (as opposed to planetoids as sub-planetary objects), and planetary geology, planetary geologists continue to treat them as planets despite the IAU definition. The number of dwarf planets even among known objects is not certain, but there is general consensus on Ceres in the asteroid belt and on at least eight trans-Neptunians: Quaoar, Sedna, 90482 Orcus, Orcus, Pluto, Haumea, Eris, Makemake, and 225088 Gonggong, Gonggong. Planetary geologists also often include the nineteen known planetary-mass moons as "satellite planets", including Earth's Moon. Some go even further and include relatively large, geologically evolved bodies that are nonetheless not very round today, such as Pallas and Vesta.Emily Lakdawalla et al.
What Is A Planet?
The Planetary Society, 21 April 2020


Extrasolar planets

There is no official definition of extrasolar planets. In 2003, the
International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a Non-governmental organization, nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting ...
(IAU) Working Group on Extrasolar Planets issued a position statement, but this position statement was never proposed as an official IAU resolution and was never voted on by IAU members. The positions statement incorporates the following guidelines, mostly focused upon the boundary between planets and brown dwarfs: # 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 natural abundance, 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 System. # Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed or where they are located. # Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate). This working definition was amended by the IAU's Commission F2: Exoplanets and the Solar System in August 2018. The official working definition of an ''exoplanet'' is now as follows: The IAU noted that this definition could be expected to evolve as knowledge improves. One definition of a sub-brown dwarf is a planet-mass object that formed through cloud collapse rather than accretion (astrophysics), 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 disc, 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. One study suggests that objects above formed through gravitational instability and should not be thought of as planets. 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 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 . 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. As of 2011 the Extrasolar Planets Encyclopaedia included objects up to 25 Jupiter masses, saying, "The fact that there is no special feature around in the observed mass spectrum reinforces the choice to forget this mass limit". As of 2016 this limit was increased to 60 Jupiter massesExoplanets versus brown dwarfs: the CoRoT view and the future
Jean Schneider, 4 Apr 2016
based on a study of mass–density relationships. The Exoplanet Data Explorer includes objects up to 24 Jupiter masses with the advisory: "The 13 Jupiter-mass distinction by the IAU Working Group is physically unmotivated for planets with rocky cores, and observationally problematic due to the sin i ambiguity." The NASA Exoplanet Archive includes objects with a mass (or minimum mass) equal to or less than 30 Jupiter masses. Another criterion for separating planets and brown dwarfs, rather than deuterium fusion, formation process or location, is whether the core pressure is dominated by Lateral earth pressure#Coulomb theory, coulomb pressure or electron degeneracy pressure.


2006 IAU definition of planet

The matter of the lower limit was addressed during the 2006 meeting of the International Astronomical Union#The XXVIth General Assembly and the definition of a planet, IAU's General Assembly. After much debate and one failed proposal, 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 inside the Solar System that * is in orbit around the Sun, * has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and * has clearing the neighbourhood, cleared the neighbourhood around its orbit. Therefore, the Solar System has eight planets: 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 satellite A natural satellite is in the most common usage, an astronomical body Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science Natural science is a branch ...

natural satellite
s 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 planets''. 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 observable.


Margot's criterion

Astronomer Jean-Luc Margot proposed a mathematical criterion that determines whether an object can clear its orbit during the lifetime of its host star, based on the mass of the planet, its semimajor axis, and the mass of its host star. The formula produces a value called that is greater than 1 for planets. The eight known planets and all known exoplanets have values above 100, while Ceres, Pluto, and Eris have values of 0.1, or less. Objects with values of 1 or more are 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 The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

Solar System
bodies once generally considered to be planets but no longer considered as such by the IAU, as well as whether they would be considered planets under alternative definitions, such as Soter's 2006 definition that favors dynamical dominance or Stern's 2002 and 2017 definitions that favor having a shape dominated by gravity. The reporting of newly discovered large Kuiper belt objects as planets – particularly Eris (dwarf planet), Eris – triggered the August 2006 IAU decision on what a planet is.


Mythology and naming

The names for the planets in the Western world are derived from the naming practices of the Romans, which ultimately derive from those of the Greeks and the Babylonians. In
ancient Greece Ancient Greece ( el, Ἑλλάς, Hellás) was a civilization belonging to a period of History of Greece, Greek history from the Greek Dark Ages of the 12th–9th centuries BC to the end of Classical Antiquity, antiquity ( AD 600). This era wa ...
, the two great luminaries the Sun and the Moon were called ''Helios'' and ''Selene'', two ancient Titan (mythology), Titanic deities; the slowest planet (Saturn) was called ''Phaenon, Phainon'', the shiner; followed by ''
Phaethon Phaethon (; grc, Φαέθων, Phaéthōn, ), also spelled Phaëthon, was the son of the Oceanid Clymene (mother of Phaethon), Clymene and the solar deity, sun god Helios in Greek mythology. His name was also used by the Ancient Greek , Ancient ...
'' (Jupiter), "bright"; the red planet (Mars) was known as ''
Pyroeis Pyroeis (Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 ...
'', the "fiery"; the brightest (Venus) was known as ''Phosphorus (morning star), Phosphoros'', the light bringer; and the fleeting final planet (Mercury) was called '' Stilbon'', the gleamer. The Greeks also assigned each planet to one among their pantheon of gods, the Twelve Olympians, Olympians and the earlier Titan (mythology), Titans: * Helios and Selene were the names of both planets and gods, both of them Titan (mythology), Titans (later supplanted by Twelve Olympians, Olympians Apollo and Artemis); * Phainon was sacred to Cronus, the Titan (mythology), 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 Aphrodite; , , ) is an ancient Greek goddess associated with love Love encompasses a range of strong and positive emotion Emotions are mental state, psychological states brought on by neurophysiology, neurophysiological changes, ...

Aphrodite
, the goddess of love; and * Stilbon with its speedy motion, was ruled over by Hermes, messenger of the gods and god of learning and wit. The Greek practice of grafting their gods' names onto the planets was almost certainly borrowed from the Babylonians. The Babylonians named Hesperus#Variant names, Phosphoros [Venus] after their goddess of love, ''Ishtar''; Pyroeis [Mars] after their god of war, ''Nergal'', Stilbon [Saturn] after their god of wisdom Nabu, and Phaethon [Jupiter] 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-European mythology, Indo-Europeans, shared with them a Roman mythology, common pantheon under different names but lacked the rich narrative traditions that Greek poetic culture had given Greek mythology, 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: ''Mercury (mythology), Mercurius'' (for Hermes), ''Venus (mythology), Venus'' (Aphrodite), ''Mars (mythology), Mars'' (Ares), ''Jupiter (mythology), Iuppiter'' (Zeus) and ''Saturn (mythology), Saturnus'' (Cronus). When subsequent planets were discovered in the 18th and 19th centuries, the naming practice was retained with ''Neptune (mythology), Neptūnus'' (Poseidon). Uranus is unique in that it is named for a Uranus (mythology), Greek deity rather than his Caelus, Roman counterpart. Some Ancient Rome, Romans, following a belief possibly originating in
Mesopotamia Mesopotamia ( grc, Μεσοποταμία ''Mesopotamíā''; ar, بِلَاد ٱلرَّافِدَيْن ; syc, ܐܪܡ ܢܗܪ̈ܝܢ, or , ) is a historical region of Western Asia situated within the Tigris–Euphrates river system, in th ...

Mesopotamia
but developed in Hellenistic Egypt, believed that the seven gods after whom the planets were named took hourly shifts in looking after affairs on Earth. The order of shifts went Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon (from the farthest to the closest planet). Therefore, the first day was started by Saturn (1st hour), second day by Sun (25th hour), followed by Moon (49th hour), Mars, Mercury, Jupiter and Venus. Because each day was named by the god that started it, this is also the order of the Week-day names, days of the week in the Roman calendar after the Nundinae, 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 ''Týr, Tīw'' (Tuesday), ''Woden, Wōden'' (Wednesday), ''Thor, Þunor'' (Thursday), and ''Frige, Frīġ'' (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, there is no tradition of naming it after a god. (The same is true, in English at least, of the Sun and the Moon, though they are no longer generally considered planets.) The name originates from the Old English word ''eorþe'', which was the word for "ground" and "dirt" as well as the Earth itself. As with its equivalents in the other Germanic languages, it derives ultimately from the Proto-Germanic word ''erþō'', as can be seen in the English ''earth'', the German ''Erde'', the Dutch ''aarde'', and the Scandinavian ''jord''. Many of the Romance languages retain the old Roman word ''Terra (mythology), 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, ''Gaia (mythology), 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 (mythology), ''Ketu''. China and the countries of eastern Asia historically subject to Chinese cultural sphere, Chinese cultural influence (such as Japan, Korea and Vietnam) use a naming system based on the Wuxing (Chinese philosophy), five Chinese elements: Water (classical element), water (Mercury), Metal (classical element), metal (Venus), Fire (classical element), fire (Mars), Wood (classical element), wood (Jupiter) and Earth (classical element), 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 one," Venus is כוכב נוגה ''Kokhav Nogah'' or "the bright planet," Mercury is כוכב ''Kokhav'' or "the planet" (given its lack of distinguishing features), Mars is מאדים ''Ma'adim'' or "the red one," and Saturn is שבתאי ''Shabbatai'' or "the resting one" (in reference to its slow movement compared to the other visible planets). The odd one out is Jupiter, called צדק ''Tzedeq'' or "justice". 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. In Arabic, Mercury is عُطَارِد (''ʿUṭārid'', cognate with Ishtar / Astarte), Venus is الزهرة (''az-Zuhara'', "the bright one", an epithet of the goddess Al-'Uzzá), Earth is الأرض (''al-ʾArḍ'', from the same root as wikt:ארץ, eretz), Mars is اَلْمِرِّيخ (''al-Mirrīkh'', meaning "featherless arrow" due to its Retrograde and prograde motion, retrograde motion), Jupiter is المشتري (''al-Muštarī'', "the reliable one", from Akkadian language, Akkadian) and Saturn is زُحَل (''Zuḥal'', "withdrawer").


Formation

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 (astrophysics), 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
planetesimal Planetesimals are solid objects thought to exist in protoplanetary disks and debris disks. Per the Chamberlin–Moulton planetesimal hypothesis, they are believed to form out of cosmic dust grains. Believed to have formed in the Solar System ab ...
s form, and these accelerate the accretion process by drawing in additional material by their gravitational attraction. These concentrations become ever denser until they collapse inward under gravity to form protoplanets. After a planet reaches a mass somewhat larger than
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, being larger than only Mercury (planet), Mercury. In English, Mars carries the name of the Mars (mythology), Roman god of war and is often referred to ...

Mars
' mass, it begins to accumulate an extended atmosphere, greatly increasing the capture rate of the planetesimals by means of Drag (physics), atmospheric drag. Depending on the accretion history of solids and gas, a
giant planet The giant planets constitute a diverse type of planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, ...
, an ice giant, or a terrestrial planet may result. When the protostar has grown such that it ignites to form a
star A star is an astronomical object consisting of a luminous spheroid of plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral) or heliotrope, a mineral aggregate * Quark ...

star
, the surviving disk is removed from the inside outward by photoevaporation, the solar wind, Poynting–Robertson effect, 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 satellite A natural satellite is in the most common usage, an astronomical body Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science Natural science is a branch ...

natural satellite
s of planets through a process of gravitational capture, or remain in belts of other objects to become either dwarf planets or small Solar System body, 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. 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 Atmospheric escape, 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.


Solar System

According to the IAU definition of planet, IAU definition, there are eight planets in the Solar System, which are in increasing distance from the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

Sun
: # Planet_symbols, ☿
Mercury Mercury usually refers to: * Mercury (planet) Mercury is the smallest planet in the Solar System and the closest to the Sun. Its orbit around the Sun takes 87.97 Earth days, the shortest of all the Sun's planets. It is named after the Roman g ...

Mercury
# Planet_symbols, ♀
Venus Venus is the second planet from the Sun. It is named after the Venus (mythology), Roman goddess of love and beauty. As List of brightest natural objects in the sky, the brightest natural object in Earth's night sky after the Moon, Venus can ...

Venus
# Planet_symbols, 🜨
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

Earth
# Planet_symbols, ♂
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, being larger than only Mercury (planet), Mercury. In English, Mars carries the name of the Mars (mythology), Roman god of war and is often referred to ...

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

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

Saturn
# Planet_symbols, ⛢
Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus, who, according to Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and ...

Uranus
# Planet_symbols, ♆
Neptune Neptune is the eighth and farthest-known Solar planet from the Sun. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times the mass of Earth, slightly mo ...

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: * Terrestrial planet, Terrestrials: Planets that are similar to Earth, with bodies largely composed of Rock (geology), 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, and 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. 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 definition of 'planet', 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. These include the eight classical planets, as well as two more populations. Nine objects are generally agreed to be dwarf planets, with some others being disputed list of possible dwarf planets, candidates: they are gravitationally rounded, but do not clear their orbits. In increasing order of average distance from the Sun, they are: # ' # ' # ' # ' # ' # ' # ' # ' # ' Ceres is the largest object in the
asteroid belt The asteroid belt is a torus-shaped region in the Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizin ...

asteroid belt
, between the orbits of Mars and Jupiter. The other eight all orbit beyond Neptune. Orcus, Pluto, Haumea, Quaoar, and Makemake orbit in the Kuiper belt, which is a second asteroid belt beyond the orbit of Neptune. Gonggong and Eris orbit in the scattered disc, which is somewhat further out and, unlike the Kuiper belt, is unstable towards interactions with Neptune. Sedna is the largest known detached object, a population that never comes close enough to the Sun to interact with any of the classical planets: the origins of their orbits are still being debated. There are also at least nineteen planetary-mass moons or satellite planets, i.e. moons large enough to take on ellipsoidal shapes. The nineteen generally agreed are: * One satellite of Earth – ''the Moon'' * Four satellites of Jupiter – ''Io (moon), Io'', ''Europa (moon), Europa'', ''Ganymede (moon), Ganymede'', and ''Callisto (moon), Callisto'' * Seven satellites of Saturn – ''Mimas (moon), Mimas'', ''Enceladus'', ''Tethys (moon), Tethys'', ''Dione (moon), Dione'', ''Rhea (moon), Rhea'', ''Titan (moon), Titan'', and ''Iapetus (moon), Iapetus'' * Five satellites of Uranus – ''Miranda (moon), Miranda'', ''Ariel (moon), Ariel'', ''Umbriel (moon), Umbriel'', ''Titania (moon), Titania'', and ''Oberon (moon), Oberon'' * One satellite of Neptune – ''Triton (moon), Triton'' * One satellite of Pluto – ''Charon (moon), Charon''


Planetary attributes


Exoplanets

An exoplanet (extrasolar planet) is a planet outside the Solar System. In early 1992, radio astronomers
Aleksander Wolszczan Aleksander Wolszczan (born 29 April 1946 in Szczecinek, Poland) is a Polish astronomy, astronomer. He is the co-discoverer of the first extrasolar planets and pulsar planets. Early life and education Wolszczan was born on 29 April 1946 in Szcz ...

Aleksander Wolszczan
and
Dale Frail Dale A. Frail is a Canadian astronomer An astronomer is a in the field of who focuses their studies on a specific question or field outside the scope of . They observe s such as s, s, , s and – in either (by analyzing the data) or . Exam ...
announced the discovery of two planets orbiting the
pulsar A pulsar (from ''pulsating radio source'') is a highly magnetized rotating neutron star A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the s ...

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 planet The giant planets constitute a diverse type of planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, ...
s that survived the supernova and then decayed into their current orbits. The first confirmed discovery of an extrasolar planet orbiting an ordinary main-sequence star occurred on 6 October 1995, when
Michel Mayor Michel Gustave Édouard Mayor (; born 12 January 1942) is a Switzerland, Swiss astrophysicist and professor emeritus at the University of Geneva's Department of Astronomy. He formally retired in 2007, but remains active as a researcher at the O ...
and Didier Queloz of the University of Geneva announced the detection of an exoplanet around 51 Pegasi. From then until the Kepler (spacecraft), 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. 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 Gravitational microlensing is an astronomy, astronomical phenomenon due to the gravitational lens effect. It can be used to detect objects that range from the mass of a planet to the mass of a star, regardless of the light they emit. Typically, a ...

gravitational microlensing
data, estimates an Arithmetic mean, average of at least 1.6 bound planets for every star in the Milky Way. On 20 December 2011, the
Kepler Space Telescope The Kepler space telescope is a deactivated space telescope A space telescope or space observatory is a telescope located in outer space Outer space is the expanse that exists beyond Earth and between astronomical object, celestial b ...

Kepler Space Telescope
team reported the discovery of the first Terrestrial planet, Earth-size exoplanets,
Kepler-20e Kepler-20e is an exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917, but was not recognized as such. The first confirmation of detection occurred in 19 ...

Kepler-20e
and
Kepler-20f Kepler-20f (also known by its Kepler Object of Interest designation ''KOI-070.05'') is an exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917, but was no ...

Kepler-20f
, orbiting a Sun-like star,
Kepler-20 Kepler-20 is a star 929 light-years from Earth in the constellation Lyra with a system of six known planets. The apparent magnitude of this star is 12.51, so it cannot be seen with the unaided eye. Viewing it requires a telescope with an ape ...
. Around 1 in 5 Sun-like stars have an "Earth-sized"For the purpose of this 1 in 5 statistic, Earth-sized means 1–2 Earth radii planet in the habitableFor 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). 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 Extraterrestrial life, intelligent, communicating civilizations that exist in the
Milky Way The Milky Way is the galaxy A galaxy is a gravitation Gravity (), or gravitation, is a natural phenomenon by which all things with mass Mass is both a property Property (''latin: Res Privata'') in the Abstract and con ...

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 Mercury usually refers to: * Mercury (planet) Mercury is the smallest planet in the Solar System and the closest to the Sun. Its orbit around the Sun takes 87.97 Earth days, the shortest of all the Sun's planets. It is named after the Roman g ...

Mercury
, the closest planet to the Sun at 0.4 astronomical unit, 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 Neptune is the eighth and farthest-known Solar planet from the Sun. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times the mass of Earth, slightly mo ...

Neptune
is 30 AU from the Sun and takes 165 years to orbit, but there are exoplanets that are hundreds of astronomical unit, AU from their star and take more than a thousand years to orbit, e.g. 1RXS1609 b.


Planetary-mass objects

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 Clearing the neighbourhood, 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

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.


Rogue planets

Several computer simulations of stellar and planetary system formation have suggested that some objects of planetary mass would be ejected into interstellar space. Such objects are typically called ''rogue planets''.


Sub-brown dwarfs

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 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.


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 objects, planetary-mass object. An example is a Jupiter-mass object orbiting the
pulsar A pulsar (from ''pulsating radio source'') is a highly magnetized rotating neutron star A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the s ...

pulsar
PSR J1719-1438. These shrunken white dwarfs may become a helium planet or carbon planet.


Satellite planets

Some large satellites (moons) are of similar size or larger than the planet
Mercury Mercury usually refers to: * Mercury (planet) Mercury is the smallest planet in the Solar System and the closest to the Sun. Its orbit around the Sun takes 87.97 Earth days, the shortest of all the Sun's planets. It is named after the Roman g ...

Mercury
, e.g. Jupiter's Galilean moons and Titan (moon), 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. The term ''satellite planet'' is sometimes used for planet-sized satellites.


Captured planets

Rogue planets in Open cluster, 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.


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

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 Gravity (), or gravitation, is a by which all things with or —including s, s, , and even —are attracted to (or ''gravitate'' toward) one another. , gravity gives to s, and the causes the s of the oceans. The gravitational attracti ...

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 ''Orbital eccentricity, 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. * 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 Pluto are at far more extreme angles to it. The points at which a planet crosses above and below its reference plane are called its ascending node, 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.


Axial tilt

Planets also have varying degrees of axial tilt; they lie at an angle to the reference plane, plane of their inclination, 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 stars.


Rotation

The planets rotate around invisible axes through their centres. A planet's rotation period is known as a day, 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 Poles of astronomical bodies#Geographic poles, 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 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. There is great variation in the length of day between the planets, with Venus taking 243 Julian day, days to rotate, and the giant planets only a few hours. The rotational periods of extrasolar planets are not known. However, for Hot Jupiter, "hot" Jupiters, their proximity to their stars means that they are Tidal locking, 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.


Orbital clearing

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
planetesimal Planetesimals are solid objects thought to exist in protoplanetary disks and debris disks. Per the Chamberlin–Moulton planetesimal hypothesis, they are believed to form out of cosmic dust grains. Believed to have formed in the Solar System ab ...
s in its orbit. In effect, it orbits its star in isolation, as opposed to sharing its orbit with a multitude of similar-sized objects. This characteristic was mandated as part of the
IAU The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a Non-governmental organization, nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting ...
's official 2006 definition of planet, definition of a planet in August, 2006. This criterion excludes such planetary bodies as
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It was the first and the largest Kuiper belt object to be discovered. After Pluto wa ...

Pluto
, Eris (dwarf planet), Eris and
Ceres Ceres most commonly refers to: * Ceres (dwarf planet) Ceres (; minor-planet designation: 1 Ceres) is the smallest recognized dwarf planet, the closest dwarf planet to the Sun, and the List of notable asteroids, largest object in the main astero ...
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 discs.


Physical characteristics


Size and shape

A planet's size is defined at least by an average radius (e.g., Earth radius (unit), Earth radius, Jupiter radius, etc.); polar and equatorial radii of a spheroid or more general triaxial ellipsoidal shapes are often estimated (e.g., reference ellipsoid). Derived quantities include the flattening, surface area, and volume. Knowing further the rotation rate and mass, allows the calculation of normal gravity.


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
star A star is an astronomical object consisting of a luminous spheroid of plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral) or heliotrope, a mineral aggregate * Quark ...

star
s. While the lower stellar mass limit is estimated to be around 75 times that of
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...

Jupiter
(), the upper planetary mass limit for planethood is only roughly 13 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 limit is not universally agreed upon and the Extrasolar Planets Encyclopaedia includes objects up to 60 , and the Exoplanet Data 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 A pulsar (from ''pulsating radio source'') is a highly magnetized rotating neutron star A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the s ...

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 the Moon.


Internal differentiation

Every planet began its existence in an entirely fluid state; in early formation, the denser, heavier materials sank to the centre, leaving the lighter materials near the surface. Each therefore has a Planetary differentiation, differentiated interior consisting of a dense planetary core surrounded by a Mantle (geology), mantle that either is or was a fluid. The terrestrial planets are sealed within hard Crust (geology), 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 silicates.
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...

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

Saturn
are believed to have cores of rock and metal surrounded by mantles of metallic hydrogen.
Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus, who, according to Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and ...

Uranus
and
Neptune Neptune is the eighth and farthest-known Solar planet from the Sun. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times the mass of Earth, slightly mo ...

Neptune
, which are smaller, have rocky cores surrounded by mantles of water, ammonia, methane and other Volatiles, ices. The fluid action within these planets' cores creates a geodynamo that generates a magnetic field.


Atmosphere

All of the Solar System planets except
Mercury Mercury usually refers to: * Mercury (planet) Mercury is the smallest planet in the Solar System and the closest to the Sun. Its orbit around the Sun takes 87.97 Earth days, the shortest of all the Sun's planets. It is named after the Roman g ...

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
hurricane A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation Atmospheric circulation is the large-scale movement of Atmosphere of Earth, air and together with oc ...

hurricane
s (on Earth), planet-wide dust storms (on Mars), a greater-than-Earth-sized Anticyclonic storm, anticyclone on Jupiter (called the Great Red Spot), and Great Dark Spot, 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 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 appear to have very similar temperatures, indicating that that planet's atmosphere effectively redistributes the star's energy around the planet.


Magnetosphere

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 electrical conductivity, 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 Venus and Mars lack such a magnetic field. In addition, the moon of Jupiter Ganymede (moon), 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 crewed 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 of rotation, 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. Several planets or dwarf planets in the Solar System (such as Neptune and Pluto) have orbital periods that are in Orbital resonance, resonance with each other or with smaller bodies (this is also common in satellite systems). All except Mercury and Venus have
natural satellite A natural satellite is in the most common usage, an astronomical body Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science Natural science is a branch ...

natural satellite
s, 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 (moon), 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 'Rings of Saturn#Propeller moonlets, 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 Earth masses.


See also


Notes


References


External links


International Astronomical Union website

Photojournal NASA

NASA Planet Quest – Exoplanet Exploration


* * [http://www.boulder.swri.edu/~hal/planet_def.html "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) {{Featured article Observational astronomy Planetary science Planets, Concepts in astronomy