Changes

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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'', http://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 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: ehttp://www.iau.org/iau0603.414.0.html, IAU 2006 General Assembly: Result of the IAU resolution votes

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.'''}}

'''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.'''}}

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]]:

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, http://arxiv.org/ftp/astro-ph/papers/0608/0608359.pdf

''"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, http://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.

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.

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.

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]]

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. http://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, http://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.http://cfa-www.harvard.edu/~kenyon/pf/dd/Dusty Rings & Icy Planet Formation, Smithsonian Astrophysical Observatory. Planet Formation on the Fast Track, http://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]].

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. http://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]].)

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. [http://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]].

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==

==Within the Solar System==

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:

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 http://www.space.com/scienceastronomy/060228_stern_interview.html

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==

==Beyond the Solar System==

===Extrasolar planets===

===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, http://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]] http://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, http://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.

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.

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', http://www.spacetoday.org/DeepSpace/Stars/Planets/PlanetFindingMissions.html, Future American and European Planet Finding Missions  

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, http://www.astrobio.net/news/article610.html

  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"===

===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.

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 http://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. http://www.space.com/scienceastronomy/planet_photo_040910.html, Likely First Photo of Planet Beyond the Solar System

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.  

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.  

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.

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====

====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. http://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. http://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.http://www.nasa.gov/vision/universe/starsgalaxies/spitzer-20061012.html|title=NASA's Spitzer Sees Day and Night on Exotic World

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===

===Secondary characteristics===

==External links==

==External links==

* [http://www.iau.org International Astronomical Union]

* [https://www.iau.org International Astronomical Union]

* [http://www.fourmilab.ch/cgi-bin/uncgi/Solar/ Solar System Live] (an interactive [[orrery]])

* [https://www.fourmilab.ch/cgi-bin/uncgi/Solar/ Solar System Live] (an interactive [[orrery]])

* [http://janus.astro.umd.edu/javadir/orbits/ssv.html Solar System Viewer] (animation)

* [https://janus.astro.umd.edu/javadir/orbits/ssv.html Solar System Viewer] (animation)

* [http://www.sky-pics.net/ Pictures of the Solar System]

* [https://www.sky-pics.net/ Pictures of the Solar System]

* [http://planetquest.jpl.nasa.gov/ NASA Planet Quest]

* [https://planetquest.jpl.nasa.gov/ NASA Planet Quest]

*[http://www.co-intelligence.org/newsletter/comparisons.html Illustration comparing the sizes of the planets with each other, the sun, and other stars]

*[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===

===Definition and reclassification debate===

* [http://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

* [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.

* Steven Soter's article "''What is a Planet''" in [[Scientific American]], January 2007, pp 34-41.

* Dan Green's page on [http://cfa-www.harvard.edu/cfa/ps/icq/ICQPluto.html planet classification]

* Dan Green's page on [https://cfa-www.harvard.edu/cfa/ps/icq/ICQPluto.html planet classification]

* Stern & Levinson's article [http://www.boulder.swri.edu/~hal/planet_def.html "Regarding the criteria for planethood and proposed planetary classification schemes."]

* Stern & Levinson's article [https://www.boulder.swri.edu/~hal/planet_def.html "Regarding the criteria for planethood and proposed planetary classification schemes."]

* [http://www.spacedaily.com/news/outerplanets-04b.html Gravity Rules: The Nature and Meaning of Planethood]; S. Alan Stern; [[March 22]], [[2004]]

* [https://www.spacedaily.com/news/outerplanets-04b.html Gravity Rules: The Nature and Meaning of Planethood]; S. Alan Stern; [[March 22]], [[2004]]

* [http://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://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

* [http://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/sci/tech/4795755.stm BBC: "Planets plan boost tally 12" 2006-08-16]

* [http://news.bbc.co.uk/1/hi/world/5282440.stm BBC: "Pluto loses status as a planet" 2006-08-24]

* [https://news.bbc.co.uk/1/hi/world/5282440.stm BBC: "Pluto loses status as a planet" 2006-08-24]

* [http://news.bbc.co.uk/2/hi/science/nature/5283956.stm BBC: "Pluto vote 'hijacked' in revolt" 2006-08-25]

* [https://news.bbc.co.uk/2/hi/science/nature/5283956.stm BBC: "Pluto vote 'hijacked' in revolt" 2006-08-25]