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This page needs a lot of work. The fragment below seems to be separate for some reason.--rdavis 20:47, 3 September 2010 (UTC)

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This definition has since been widely usedby astronomers when publishing discoveries in [[journal]]s, See for example the list of references for: Butler, R. P. ''et al'', https://exoplanets.org/, Catalog of Nearby Exoplanets, University of California and the Carnegie Institution, although it remains a temporary yet effective, working definition until a more permanent one is formally adopted. It also did not address the dispute over the lower mass limit and steered clear of the controversy regarding objects within the [[Solar System]].

This matter was finally addressed during the 2006 meeting of the IAU's General Assembly. After much debate and one failed proposal, the assembly voted to pass a resolution that [[2006 definition of planet|defined planets within the Solar System]] as: ehttps://www.iau.org/iau0603.414.0.html, IAU 2006 General Assembly: Result of the IAU resolution votes

'''A celestial body that is (a) in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a [[hydrostatic equilibrium]] (nearly round) shape, and (c) has [[clearing the neighbourhood|cleared the neighbourhood]] around its orbit.'''}}

Under this definition, the Solar System is considered to have eight planets. Bodies which fulfill the first two conditions but not the third (such as Pluto and Eris) are classified as [[dwarf planet]]s, providing they are not also [[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 modern 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. Asteroids and comets, including KBOs, differ from planets in that they can collide with each other and with planets."'' What is a Planet, Astronomical Journal, https://arxiv.org/ftp/astro-ph/papers/0608/0608359.pdf

In the aftermath of the IAU's 2006 vote, there has been criticism of the new definition,<ref>{{cite web, https://www.space.com/scienceastronomy/060824_planet_definition.html] and some astronomers have ev[en stated that they will not use it.//www.space.com/scienceastronomy/060831_planet_definition.html. Part of the dispute centres around the belief that point (c) (clearing its orbit) should not have been listed, and that those objects now categorised as dwarf planets should actually be part of a broader planetary definition. The next IAU [[conference]] is not until 2009, when modifications could be made to the definition, also possibly including extrasolar planets.

Beyond the scientific community, Pluto has held a strong cultural significance for many in the general public considering its planetary status during most of the 20th century, in a similar way to Ceres and its kin in the 1800s. More recently, the discovery of Eris was widely reported in the [[mass media|media]] as the "[[tenth planet]]". The reclassification of all three objects as dwarf planets has attracted much media and public attention.

==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]]&mdash;a process of sticky collision&mdash;dust particles in the disk steadily accumulate mass to form ever-larger bodies. Local concentrations of mass known as [[planetesimal]]s form, and these accelerate the accretion process by drawing in additional material by their gravitational attraction. These concentrations become ever more dense until they collapse inward under gravity to form [[protoplanet]]

When the protostar has grown such that it ignites to form a [[star]], the surviving disk is removed from the inside outward by photoevaporation, the [[solar wind]], [[Poynting-Robertson effect|Poynting-Robertson drag]] and other effects. https://www.astro.umass.edu/theses/dianne/thesis.html . The Evolution of Dust in the Terrestrial Planet Region of Circumstellar Disks Around publisher =University of Massachusetts Amherst, I.; Johnstone, D.; Murray, N., Halting Planet Migration by Photoevaporation from the Central Source, The Astrophysical Journal, https://adsabs.harvard.edu/abs/2003astro.ph..2042M .  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. B.https://cfa-www.harvard.edu/~kenyon/pf/dd/Dusty Rings & Icy Planet Formation, Smithsonian Astrophysical Observatory. Planet Formation on the Fast Track, https://www.sciencenews.org/articles/20030125/bob9.asp. Those objects that have become massive enough will capture most matter in their orbital neighbourhoods to become planets. Meanwhile, protoplanets that have avoided collisions may become [[natural satellite]]s of planets through a process of gravitational capture, or remain in belts of other objects to become either [[dwarf planet]]s or [[small solar system 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 [[comet]]s. https://home.tiac.net/~cri/1998/planet.html, The Standard Model of Planet Formation, (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 our own, it is becoming possible to elaborate, revise or even replace this account. The level of [[metallicity];a astronomical term describing the abundance of [[isotope]]s with an [[atomic number]] greater than 2 (Helium)&mdash;is now believed to determine the likelihood that a star will have planets. [https://cfa-www.harvard.edu/press/pr0404.html], Lifeless Suns Dominated The Early Universe, Harvard-Smithsonian Center for Astrophysic.  Hence it is thought less likely that a metal-poor, [[population II star]] will possess a more substantial planetary system than a metal-rich [[population I star]].

==Within the Solar System==

According to the [[IAU]]'s current definitions. According to the IAU" alone sounds really awkward there are eight planets in the Solar System. In increasing distance from the [[Sun]], they are:
                                                                                               
The larger bodies of the Solar System can be divided into categories based on their composition:
* '''[[Terrestrial planet|Terrestrial]]s''': Planets (and possibly dwarf planets) that are similar to Earth &mdash; with bodies largely composed of [[Rock (geology)|rock]]: Mercury, Venus, Earth and Mars. If including dwarf planets, [[Ceres (dwarf planet)|Ceres]] would also be counted, with as many as three other [[asteroids]] that might be added.
* '''[[Gas giant]]s''': Planets with a composition largely made up of [[gas]]eous material and are significantly more massive than terrestrials: Jupiter, Saturn, Uranus, Neptune. [[Ice giant]]s are a sub-class of gas giants, distinguished from gas giants by their depletion in hydrogen and helium, and a significant composition of rock and ice: Uranus and Neptune.
* '''[[Ice dwarf]]s''': Objects that are composed mainly of ice, and do not have planetary mass. The dwarf planets [[Pluto]] and [[Eris (dwarf planet)|Eris]] are ice dwarfs, and several dwarf planetary candidates also qualify.

===Dwarf planets===

Before the [[2006 redefinition of planet|August 2006 decision]], several objects were proposed by astronomers, including at one stage by the [[International Astronomical Union|IAU]], as planets. However in 2006 several of these objects were reclassified as [[dwarf planet]]s, objects distinct from planets. Currently three dwarf planets in the [[Solar System]] are recognized by the IAU: [[Ceres (dwarf planet)|Ceres]], [[Pluto]] and [[Eris (dwarf planet)|Eris]]. Several other objects in both the [[asteroid belt]] and the [[Kuiper belt]] are under consideration, with as many as 50 that could eventually qualify. There may be as many as 200 that could be discovered once the Kuiper Belt has been fully explored. Dwarf planets share many of the same [[characteristics]] as planets, although notable differences remain&mdash;namely that they are not [[clearing the neighbourhood|dominant in their orbits]]. Their attributes are:

By definition, all dwarf planets are members of larger [[populations]]. Ceres is the largest body in the [[asteroid belt]], while Pluto is a member of the [[Kuiper belt]] and Eris is a member of the [[scattered disc]]. According to [[Michael E. Brown|Mike Brown]] there may soon be over forty [[trans-Neptunian objects]] that qualify as dwarf planets under the IAU's recent definition. Behind the Pluto Mission: An Interview with Project Leader https://www.space.com/scienceastronomy/060228_stern_interview.html

==Beyond the Solar System==
===Extrasolar planets===

Since the 1988 discovery of [[Gamma Cephei|Gamma Cephei Ab]], a number of confirmed discoveries have been made of planets orbiting stars other than the Sun. Of the 239 [[extrasolar planet]]s discovered by August 2007, most have masses which are comparable to or larger than Jupiter's.<ref name="Encyclopedia" Interactive Extra-solar Planets Catalog, The Extrasolar Planets Encyclopedia, https://exoplanet.eu/catalog.php |last=Schneider |first=Jean |date=[[December 11]], [[2006]] |accessdate=2006-12-11}} Exceptions include a number of planets discovered orbiting burned-out star remnants called [[pulsar]]s, such as [[PSR B1257 plus 12|PSR B1257+12]],<ref>{{cite news | title=Scientists reveal smallest extra-solar planet yet found [[February 11]], [[2005]] https://www.spaceflightnow.com/news/n0502/11planet/ the planets orbiting the stars [[Mu Arae]], [[55 Cancri]] and [[GJ 436]] which are approximately Neptune-sized, N.; Bouchy, F.; Vauclair, S.; Queloz, D.; Mayor, M. Fourteen Times the Earth, https://www.eso.org/public/outreach/press-rel/pr-2004/pr-22-04.html, and a planet orbiting [[Gliese 876]] that is estimated to be about 6 to 8 times as massive as the Earth and is probably rocky in composition.

It is far from clear if the newly discovered large planets would resemble the gas giants in the Solar System or if they are of an entirely different type as yet unknown, like ammonia giants or carbon planets. In particular, some of the newly discovered planets, known as [[hot Jupiter]]s, orbit extremely close to their parent stars, in nearly circular orbits. They therefore receive much more [[solar radiation|stellar radiation]] than the gas giants in the Solar System, which makes it questionable whether they are the same type of planet at all. There is also a class of hot Jupiters that orbit so close to their star that their atmospheres are slowly blown away in a comet-like tail: the [[Chthonian planet]]s.

More detailed observation of extrasolar planets will require a new generation of instruments, including [[space telescope]]s. Currently the [[CoRoT]] spacecraft is searching for stellar luminosity variations due to [[Astronomical transit|transiting planets]]. Several projects have also been proposed to create an array of [[space telescope]]s to search for extrasolar planets with masses comparable to the Earth. These include the proposed NASA's [[Kepler Mission]], [[Terrestrial Planet Finder]], and [[Space Interferometry Mission]] programs, the [[European Space Agency|ESA]]'s [[Darwin (ESA)|Darwin]], and the CNES', https://www.spacetoday.org/DeepSpace/Stars/Planets/PlanetFindingMissions.html, Future American and European Planet Finding Missions 
 The [[New Worlds Imager]] is an occulting device that may work in conjunction with the [[James Webb Space Telescope]]. However, funding for some of these projects remains uncertain. The frequency of occurrence of such terrestrial planets is one of the variables in the [[Drake equation]] which estimates the number of [[extraterrestrial intelligence|intelligent, communicating civilizations]] that exist in our galaxy. The Drake Equation Revisited, Astrobiology Magazine, https://www.astrobio.net/news/article610.html

===Interstellar "planets"===
Several [[computer simulation]]s of stellar and planetary system formation have suggested that some [[planemo|objects of planetary mass]] would be ejected into interstellar [[space]]. Some scientists have argued that such objects found roaming in deep space should be classed as "planets". However, many others argue that only planemos that directly orbit [[star]]s should qualify as planets, preferring to use the terms "planetary body", "planetary mass object" or "planemo" for similar free-floating objects (as well as planetary-sized moons). The [[International Astronomical Union|IAU's]] working definition on extrasolar planets takes no position on the issue. The discoverers of the bodies mentioned above decided to avoid the debate over what constitutes a planet by referring to the objects as planemos. However, the original IAU proposal for the 2006 definition of planet favoured the star-orbiting criterion, although the final draft avoided the issue.

For a brief time in 2006, astronomers believed they had found a binary system of such objects, [[Oph 162225-240515]], which the discoverers described as "planemos". However, recent analysis, The Wide Brown Dwarf Binary Oph 1622-2405 and Discovery of A Wide, Low Mass Binary in Ophiuchus (Oph 1623-2402): A New Class of Young Evaporating Wide Binaries https://fr.arxiv.org/PS_cache/astro-ph/pdf/0608/0608574.pdf.,  ''et al'' of the objects has determined that their masses are each greater than 13 Jupiter-masses, making the pair [[brown dwarf]]s. https://www.space.com/scienceastronomy/planet_photo_040910.html, Likely First Photo of Planet Beyond the Solar System

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

===Dynamic characteristics===
====Orbit====
[[Image:Eris Orbit.svg|thumb|250 px|left|The orbits of the planets compared to the trans-Neptunian objects [[Eris (dwarf planet)|Eris]] and [[Pluto]]. Note the extreme elongation of both objects' orbits in relation to those of the planets ([[orbital eccentricity|eccentricity]]), as well as their large angles to the ecliptic ([[inclination]])]]
All the planets revolve around their star. In the Solar System, all the planets orbit in sync with the Sun's rotation. It is not yet known whether all extrasolar planets follow this pattern. 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 the Sun; the farther a planet is from its star, not only the longer the distance it must travel, but also the slower its speed, as it is less affected by the star's gravity. Because no planet's orbit is perfectly circular, the distance of each varies over the course of its year. Its closest distance to its is called its [[periastron]] ([[perihelion]] in the Solar System), while its farthest distance from the star is called its [[apastron]] ([[aphelion]] in the Solar System). As a planet approaches periastron, its speed increases as the pull of its star's gravity strengthens; as it reaches apastron, its speed decreases. 

Each planet's orbit is delineated by a set of [[orbital elements|elements]]:

*The points at which a planet crosses above and below the ecliptic are called its [[ascending node|ascending]] and [[descending node]]s. 
*The ''[[Orbital eccentricity|eccentricity]]'' of an orbit describes how elongated a planet's orbit is. Planets with low eccentricities have more circular orbits, while planets with a high eccentricities have more elliptical orbits. The planets in our 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 necessarily the same as its apasteron, as no planet's orbit has its star at its exact centre.

*In our Solar System, the ''[[inclination]]'' of a planet tells how far above or below the plane of Earth's orbit (called the [[ecliptic]]) a planet's orbit lies. The eight planets of our Solar System all lie very close to the ecliptic; comets and [[Kuiper belt object]]s like [[Pluto]] are at far more extreme angles to it.

====Axial tilt====
Planets also have varying degrees of [[axial tilt]]; they lie at an angle to the [[reference plane|plane]] of the [[inclination|Sun's equator]]. This causes the amount of sunlight received by each hemisphere to vary over the course of its year; when the northern hemisphere points away from the Sun, the southern hemisphere points towards it, and vice versa. Each planet therefore possesses [[season]]s; changes to the climate over the course of its year. The point at which each hemisphere is farthest/nearest from the Sun 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. 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.

====Rotation====
The planets also rotate around invisible axes through their centres. A planet's [[rotation period]] is known as its [[day]]. All the planets rotate in a counter-clockwise direction, except for Venus, which [[Retrograde and direct motion|rotates clockwise]] (Uranus, because of its extreme axial tilt, can be said to be rotating either clockwise or anti-clockwise, depending on whether one states it to be inclined 82° from the ecliptic in one direction, or 98° in the opposite direction). There is great variation in the length of day between the planets, with Venus taking 243 Earth days to rotate, and the gas giants only a few hours. Their close proximity to their stars means that most extrasolar planets discovered to date are [[tidal lock|tidelocked]]; their orbits are in sync with their rotations. They only ever show one face to their stars.

===Physical characteristics===
====Hydrostatic equilibrium====
One of a planet's defining characteristics is that it is large 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 size, 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. 
====Internal diffrentiation====
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]] which either is or was a [[fluid]]. The terrestrial planets are sealed within hard [[Crust (geology)|crusts]], but in the gas giants the mantle simply dissolves into the upper cloud layers. The terrestrial planets possess cores of magnetic elements such as [[iron]] and [[nickel]], and mantles of [[silicates]]. [[Jupiter]] and [[Saturn]] are believed to possess cores of rock and metal surrounded by mantles of [[metallic hydrogen]]. [[Uranus]] and [[Neptune]], which are smaller, possess rocky cores surrounded by mantles of [[water]], [[ammonia]], [[methane]] and other ices.
====Atmospheres====
All of the planets have [[atmosphere]]s as their large masses mean gravity is strong enough to keep gaseous particles close to the surface. The larger gas giants are massive enough to keep large amounts of the light gases [[Hydrogen]] and [[Helium]] close by, although these gases mostly float into [[space]] around the smaller planets. Earth's atmosphere is greatly different to the other planets because of the various life processes that have transpired there, while the atmosphere of Mercury has mostly, although not entirely, been blasted away by the [[solar wind]]. Planetary atmospheres are affected by the varying degrees of energy received from either the Sun or their interiors, leading to the formation of dynamic [[weather system]]s such as [[hurricane]]s, (on Earth), planet-wide [[dust storm]]s (on Mars) and [[Great Red Spot|Earth-sized anticyclone]]s (on Jupiter). At least one extrasolar planet, [[HD 189733b]], has been shown to possess such a weather system, similar to the Great Red Spot on Jupiter but twice as large. https://www.cfa.harvard.edu/press/2007/pr200713.html, First Map of an Extrasolar Planet|work=Harvard-Smithsonian Center for Astrophysics.  Hot Jupiters have been shown to be losing their atmospheres into space due to stellar radiation, much like the tails of comets. https://hubblesite.org/newscenter/archive/releases/2007/07/full/|title=Hubble Probes Layer-cake Structure of Alien World's Atmosphere. These planets have vast differences in temperature between their day and night sides which produce supersonic windspeeds.https://www.nasa.gov/vision/universe/starsgalaxies/spitzer-20061012.html|title=NASA's Spitzer Sees Day and Night on Exotic World

===Secondary characteristics===
Many of the planets have [[natural satellite]]s, often called "moons."  Mercury and Venus have no moons, the Earth has one, and Mars has two, but the [[gas giant]]s all have numerous moons in complex planetary systems. Many gas giant moons have similar features to the terrestrial planets and dwarf planets, and some have been studied for signs of life.

The four largest planets in the Solar System 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 '[[moonlet]]s' 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 which fell below their parent planet's [[Roche limit]] and were torn apart by [[tidal forces]].

==External links==
* [https://www.iau.org International Astronomical Union]
* [https://www.fourmilab.ch/cgi-bin/uncgi/Solar/ Solar System Live] (an interactive [[orrery]])
* [https://janus.astro.umd.edu/javadir/orbits/ssv.html Solar System Viewer] (animation)
* [https://www.sky-pics.net/ Pictures of the Solar System]
* [https://planetquest.jpl.nasa.gov/ NASA Planet Quest]
*[https://www.co-intelligence.org/newsletter/comparisons.html Illustration comparing the sizes of the planets with each other, the sun, and other stars]

===Definition and reclassification debate===
* [https://www.ciw.edu/IAU/div3/wgesp/definition.html Working definition of "planet"] from [[International Astronomical Union|IAU]] WGESP &mdash; the lower bound remained a matter of consensus in February 2003
* Steven Soter's article "''What is a Planet''" in [[Scientific American]], January 2007, pp 34-41.
* Dan Green's page on [https://cfa-www.harvard.edu/cfa/ps/icq/ICQPluto.html planet classification]
* Stern & Levinson's article [https://www.boulder.swri.edu/~hal/planet_def.html "Regarding the criteria for planethood and proposed planetary classification schemes."]
* [https://www.spacedaily.com/news/outerplanets-04b.html Gravity Rules: The Nature and Meaning of Planethood]; S. Alan Stern; [[March 22]], [[2004]]
* [https://www.iau.org/STATUS_OF_PLUTO.238.0.html IAU Press Release 01/99 "The status of Pluto: A Clarification"]; [[International Astronomical Union|IAU]], 1999-02-03
* [https://news.bbc.co.uk/1/hi/sci/tech/4795755.stm BBC: "Planets plan boost tally 12" 2006-08-16]
* [https://news.bbc.co.uk/1/hi/world/5282440.stm BBC: "Pluto loses status as a planet" 2006-08-24]
* [https://news.bbc.co.uk/2/hi/science/nature/5283956.stm BBC: "Pluto vote 'hijacked' in revolt" 2006-08-25]