Planet

Un article de Vev.

Modèle:Two other uses

www.iau2006.org/mirror/www.iau.org/iau0603/index.html | year = 2006 | accessdate = 2007-04-30 }}</ref><ref name=WSGESP>You must specify title = and url = when using {{cite web}}.

. IAU arxiv.org/ftp/astro-ph/papers/0608/0608359.pdf//www.dtm.ciw.edu/boss/definition.html
   (2001)
     
   
 

. Retrieved on 2006-05-25. </ref>//www.iau2006.org/mirror/www.iau.org/iau0603/index.html | year = 2006 | accessdate = 2007-04-30 }}</ref><ref name=WSGESP>You must specify title = and url = when using {{cite web}}.

. IAU arxiv.org/ftp/astro-ph/papers/0608/0608359.pdf//www.dtm.ciw.edu/boss/definition.html
   (2001)
     
   
 

. Retrieved on 2006-05-25. </ref>

The term planet is an ancient one having ties to history, science, myth, and religion. The planets were originally seen as a divine presence; as emissaries of the gods. Even today, many people continue to believe the movement of the planets affects their lives, although such a causation is rejected by the scientific community. As scientific knowledge advanced, the human perception of the planets changed over time, incorporating a number of disparate objects. Even now there is no uncontested definition of what a planet is. In 2006, the IAU officially adopted a resolution defining planets within the Solar System. This definition has been both praised and criticized, and remains disputed by some scientists.

The planets were initially thought to orbit the Earth in circular motions; after the development of the telescope, the planets were determined to orbit the Sun, and their orbits were found to be elliptical. As observational tools improved, astronomers saw that, like Earth, the planets rotated around tilted axes and shared such features as ice-caps and seasons. Since the dawn of the space age, close observation by probes has found that Earth and the other planets share characteristics such as volcanism, hurricanes, tectonics and even hydrology. Since 1992, and the discovery of hundreds of extrasolar planets, scientists are beginning to observe similar features across the galaxy.

exoplanet.eu/catalog.php |last=Schneider |first=Jean |date=October 30, 2006 |accessdate=2006-10-31}}</ref> Planets are generally divided into two main types: large, low-density gas giants and smaller, rocky terrestrials.//exoplanet.eu/catalog.php |last=Schneider |first=Jean |date=October 30, 2006 |accessdate=2006-10-31}}</ref> Planets are generally divided into two main types: large, low-density gas giants and smaller, rocky terrestrials.

Sommaire 1 Etymology 2 History 3 Definition and disputes 4 Formation 5 Within the Solar System 5.1 Dwarf planets 6 Beyond the Solar System 6.1 Extrasolar planets 6.2 Interstellar "planets" 7 Attributes 7.1 Dynamic characteristics 7.1.1 Orbit 7.1.2 Rotation 7.1.3 Axial tilt 7.1.4 Orbital clearance 7.2 Physical characteristics 7.2.1 Mass 7.2.2 Internal differentiation 7.2.3 Atmospheres 7.3 Secondary characteristics 8 See also 9 References 10 External links 10.1 Definition and reclassification debate

Etymology

www.m-w.com/dictionary/planet|title=Definition of planet|publisher=Merriam-Webster OnLine|accessdate=2007-07-23}}</ref><ref> Words For Our Modern Age: Especially words derived from Latin and Greek sources

. Wordsources.info  
 

 

. Retrieved on 2007-07-23. </ref>//www.m-w.com/dictionary/planet|title=Definition of planet|publisher=Merriam-Webster OnLine|accessdate=2007-07-23}}</ref><ref> Words For Our Modern Age: Especially words derived from Latin and Greek sources

. Wordsources.info  
 

 

. Retrieved on 2007-07-23. </ref> The Greeks gave the planets names: the farthest was called Phainon, the shiner, while below it was Phaethon, the bright one. The red planet was known as Pyroeis, "fiery", while the brightest was known as Phosphoros, the light bringer, and the fleeting final planet was called Stilbon, the gleamer. However, the Greeks also made each planet sacred to one of their pantheon of gods, the Olympians: Phainon was sacred to Kronos, the Titan who fathered the Olympians, while Phaethon was sacred to Zeus, his son who deposed him as king. Ares, son of Zeus and god of war, was given dominion over Pyroeis, while Aphrodite, goddess of love, ruled over bright Phosphoros, and Hermes ruled over Stilbon.<ref name=astronomy>Modèle:Cite book</ref>

members.optusnet.com.au/~gtosiris/page9k.html|year=2007|accessdate=2007-04-30}}</ref> The Babylonians named Phosphoros after their goddess of love, Ishtar, Pyroeis after their god of war, Nergal, and Phaethon after their chief god, Marduk.<ref name= nergal> Ross , Kelley L.

       (2005)
     
   
 
.    The Days of the Week 
. The Friesian School 
   

. Retrieved on 2007-04-30. </ref> There are too many concordances between Greek and Babylonian naming conventions for them to have arisen separately.<ref name=astronomy /> There does, however, appear to have been some confusion in translation. For instance, the Babylonian Nergal was a god of war, and the Greeks, seeing this aspect of Nergal's persona, identified him with Ares, their god of war. However, Nergal, unlike Ares, was also a god of the dead and a god of pestilence.<ref name= nergal />//members.optusnet.com.au/~gtosiris/page9k.html|year=2007|accessdate=2007-04-30}}</ref> The Babylonians named Phosphoros after their goddess of love, Ishtar, Pyroeis after their god of war, Nergal, and Phaethon after their chief god, Marduk.<ref name= nergal> Ross , Kelley L.

       (2005)
     
   
 
.    The Days of the Week 
. The Friesian School 
   

. Retrieved on 2007-04-30. </ref> There are too many concordances between Greek and Babylonian naming conventions for them to have arisen separately.<ref name=astronomy /> There does, however, appear to have been some confusion in translation. For instance, the Babylonian Nergal was a god of war, and the Greeks, seeing this aspect of Nergal's persona, identified him with Ares, their god of war. However, Nergal, unlike Ares, was also a god of the dead and a god of pestilence.<ref name= nergal />

Today, most people in the western world know the planets by names derived from the Olympian pantheon of gods; however, because of the influence of the Roman Empire and, later, the Catholic Church, they are known by their Roman (or Latin) names, rather than the Greek. The Romans, who, like the Greeks, were Indo-Europeans, shared with them a common pantheon under different names but lacked the rich narrative traditions that Greek poetic culture had given their gods. During the later period of the Roman Republic, Roman writers borrowed much of the Greek narratives and applied them to their own pantheon, to the point where they became virtually indistinguishable.<ref>Modèle:Cite book</ref> When the Romans studied Greek astronomy, they gave the planets their own gods' names. To the Greeks and Romans, there were five known planets; each presumed to be circling the Earth according to the complex laws laid out by Claudius Ptolemy in the 2nd century. They were, in increasing order from Earth (according to Ptolemy): Mercury (Hermes), Venus (Aphrodite), Mars (Ares), Jupiter (Zeus), and Saturn (Kronos). Although strictly the term "planetai" referred only to those five objects, the term was often expanded to include the Sun and the Moon.<ref>You must specify title = and url = when using {{cite web}}.

  Atsma , Aaron 
     
 

       (2007
www.theoi.com/Titan/AstraPlaneta.html//www.theoi.com/Titan/AstraPlaneta.html)
     
   
 
.  
. Theoi Project 
   

. Retrieved on 2007-02-25. </ref> When subsequent planets were discovered in the 18th and 19th centuries, the naming practice was retained: Uranus (Ouranos) and Neptune (Poseidon). The Greeks still use their original names for the planets.

Some Romans, following a belief imported from Mesopotamia into Hellenistic Egypt,<ref>You must specify title = and url = when using {{cite web}}.

  Arnett , Bill 
     
 

       (2006)
     
   
 
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. Retrieved on 2007-04-30. </ref> believed that the seven gods after whom the planets were named took hourly shifts in looking after affairs on Earth. The order of shifts began with Jupiter and worked inwards; as a result, a list of which god had charge of the first hour in each day became Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn, i.e. the usual weekday name order.<ref name="weekdays">Modèle:Cite journal</ref> Sunday, Monday, and Saturday are straightforward translations of these Roman names. In English the other days were renamed after Tiw, (Tuesday) Wóden (Wednesday), Thunor (Thursday), and Fríge (Friday), Anglo-Saxon gods considered similar or equivalent to Mars, Mercury, Jupiter, and Venus respectively.

www.etymonline.com/index.php?search=earth&searchmode=none|accessdate=2007-10-02}}</ref> as can be seen in the English Earth, the German Erde, the Dutch Aarde, and the Scandinavian Jorde. The same is true for the Sun and the Moon, though they are no longer considered planets.//www.etymonline.com/index.php?search=earth&searchmode=none|accessdate=2007-10-02}}</ref> as can be seen in the English Earth, the German Erde, the Dutch Aarde, and the Scandinavian Jorde. The same is true for the Sun and the Moon, though they are no longer considered planets.

Some non-European cultures use their own planetary naming systems. India uses a naming system based on the Navagraha, which incorporates the seven traditional planets (Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn) and the ascending and descending lunar nodes Rahu and Ketu. China, and the countries of eastern Asia subject to Chinese cultural influence, such as Japan, Korea and Vietnam, use a naming system based on the five Chinese elements.<ref name="weekdays"/>

History

Modèle:Seealso

galileo.rice.edu/sci/theories/copernican_system.html | title =Copernican System | publisher =The Galileo Project | accessdate = 2007-04-08 }}</ref> while the Sun and Moon were not. At the end of the 17th century, when the first satellites of Saturn were discovered, the terms "planet" and "satellite" were at first used interchangeably, although "satellite" would gradually become more prevalent in the following century.<ref>Modèle:Cite journal PIG//galileo.rice.edu/sci/theories/copernican system.html</ref> while the Sun and Moon were not. At the end of the 17th century, when the first satellites of Saturn were discovered, the terms "planet" and "satellite" were at first used interchangeably, although "satellite" would gradually become more prevalent in the following century.<ref>Modèle:Cite journal PIG</ref> Until the mid-19th century, any newly discovered object orbiting the Sun was listed with the planets by the scientific community, and the number of "planets" swelled rapidly towards the end of that period.

aa.usno.navy.mil/hilton/AsteroidHistory/minorplanets.html | title =When Did the Asteroids Become Minor Planets? | publisher =U.S. Naval Observatory | accessdate = 2007-04-08 }}</ref>//aa.usno.navy.mil/hilton/AsteroidHistory/minorplanets.html | title =When Did the Asteroids Become Minor Planets? | publisher =U.S. Naval Observatory | accessdate = 2007-04-08 }}</ref>

However, in the 20th century, Pluto was discovered. After initial observations led to the belief it was larger than Earth, the recently-created IAU accepted the object as a planet. Further monitoring found the body was actually much smaller, but, as it was still larger than all known asteroids and seemingly did not exist within a larger population, it kept its status for some seventy years.<ref>You must specify title = and url = when using {{cite web}}.

  Green , Daniel W.E. 
     
 

       (2005
www.cfa.harvard.edu/icq/ICQPluto.html//www.cfa.harvard.edu/icq/ICQPluto.html)
     
   
 
.  
. Harvard-Smithosonian Center for Astrophysics 
   

. Retrieved on 2007-05-11. </ref>

In the 1990s and early 2000s, there was a flood of discoveries of similar objects in the same region of the Solar System. Like Ceres and the asteroids before it, Pluto was found to be just one small body in a population of thousands. A growing number of astronomers argued for it to be declassified as a planet, since many similar objects approaching its size were found. The discovery of Eris, a more massive object widely publicised as the tenth planet, brought things to a head. The IAU set about creating the definition of planet, and eventually produced one in 2006. The number of planets dropped to the eight significantly larger bodies that had cleared their orbit (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus & Neptune), and a new class of dwarf planets was created, initially containing three objects (Ceres, Pluto and Eris).<ref>Modèle:Cite paper</ref>

Definition and disputes

Main article: Definition of planet

With the discovery during the latter half of the twentieth century of more objects within the Solar System and large objects around other stars, disputes arose over what should constitute a planet. There was particular disagreement over whether an object should be considered a planet if it was part of a distinct population such as a belt, or if it was large enough to generate energy by the thermonuclear fusion of deuterium. <imagemap> Image:EightTNOs.png|thumb|275px|The largest Trans-Neptunian objects that prompted the IAU's decision.

1. Earth

rect 646 1714 2142 1994 The Earth

1. Eris and Dysnomia

circle 226 412 16 Dysnomia circle 350 626 197 (136199) Eris

1. Pluto and Charon

circle 1252 684 86 Charon circle 1038 632 188 (134340) Pluto

1. 2005 FY9

circle 1786 614 142 (136472) 2005 FY9

1. 2003 EL61

circle 2438 616 155 (136108) 2003 EL61

1. Sedna

circle 342 1305 137 (90377) Sedna

1. Orcus

circle 1088 1305 114 (90482) Orcus

1. Quaoar

circle 1784 1305 97 (50000) Quaoar

1. Varuna

circle 2420 1305 58 (20000) Varuna

desc none

1. - setting this to "bottom-right" will display a (rather large) icon linking to the graphic, if desired

1. Notes:
2. Details on the new coding for clickable images is here: mw:Extension:ImageMap
3. While it may look strange, it's important to keep the codes for a particular system in order. The clickable coding treats the first object created in an area as the one on top.
4. Moons should be placed on "top" so that their smaller circles won't disappear "under" their respective primaries.

</imagemap> In 2003, The International Astronomical Union (IAU) Working Group on Extrasolar Planets made a position statement on the definition of a planet that incorporated a working definition:<ref name=WSGESP>You must specify title = and url = when using {{cite web}}.

. IAU
   (2001)
     
   
 

. Retrieved on 2006-05-25. </ref>

1. Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 times the mass of Jupiter for objects with the same isotopic abundance as the Sun)<ref>Modèle:Cite journal</ref> 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 our Solar System.
2. Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed nor where they are located.
3. 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 definition has since been widely used by astronomers when publishing discoveries in journals,<ref>See for example the list of references for: You must specify title = and url = when using {{cite web}}.

  Butler, R. P. et al
   
 

       (2006
www.theoi.com/Titan/AstraPlaneta.html//exoplanets.org/)
     
   
 
.  
. University of California and the Carnegie Institution 
   

. Retrieved on 2007-07-23. </ref> 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 defined planets within the Solar System as:<ref name=IAU>You must specify title = and url = when using {{cite web}}.

  Staff
   
 

       (2006
www.cfa.harvard.edu/icq/ICQPluto.html//www.iau.org/iau0603.414.0.html)
     
   
 
.  
. IAU 
   

. Retrieved on 2007-05-11. </ref>

Modèle:Quotation

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 planets, providing they are not also natural satellites of other planets. Originally an IAU committee had proposed a definition that would have included a much larger number of planets as it did not include (c) as a criterion. After much discussion, it was decided via a vote that those bodies should instead be classified as dwarf 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:

Modèle:Quotation

In the aftermath of the IAU's 2006 vote, there has been criticism of the new definition,<ref>You must specify title = and url = when using {{cite web}}.

  Britt , Robert Roy 
     
 

     (August 24, 2006)
   
.  
. Space.com
www.theoi.com/Titan/AstraPlaneta.html//www.space.com/scienceastronomy/060824_planet_definition.html

. Retrieved on 2006-08-24. </ref> and some astronomers have even stated that they will not use it.<ref>You must specify title = and url = when using {{cite web}}.

  Britt , Robert Roy 
     
 

     (August 31, 2006)
   
.  
. Space.com
www.theoi.com/Titan/AstraPlaneta.html//www.space.com/scienceastronomy/060831_planet_definition.html

. Retrieved on 2006-08-24. </ref> 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 media as the "tenth planet". The reclassification of all three objects as dwarf planets has attracted much media and public attention.<ref> Moskowitz , Clara

  . 
 "
   Scientist who found '10th planet' discusses downgrading of Pluto
   
 " , Stanford news
  , October 18, 2006
www.theoi.com/Titan/AstraPlaneta.html//news-service.stanford.edu/news/2006/october18/mbrown-101806.html
 
  . Retrieved on 2007-05-11
 . </ref>

Formation

Main article: Planetary 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—a process of sticky collision—dust particles in the disk steadily accumulate mass to form ever-larger bodies. Local concentrations of mass known as planetesimals form, and these accelerate the accretion process by drawing in additional material by their gravitational attraction. These concentrations become ever more dense until they collapse inward under gravity to form protoplanets.<ref>Modèle:Cite journal</ref> After a planet reaches a diameter larger than the Earth's moon, it begins to accumulate an extended atmosphere, greatly increasing the capture rate of the planetesimals by means of atmospheric drag.<ref>Modèle:Cite journal</ref>

When the protostar has grown such that it ignites to form a star, the surviving disk is removed from the inside outward by photoevaporation, the solar wind, Poynting-Robertson drag and other effects.<ref>You must specify title = and url = when using {{cite web}}.

  Dutkevitch , Diane 
     
 

     (May 1995
www.cfa.harvard.edu/icq/ICQPluto.html//www.astro.umass.edu/theses/dianne/thesis.html)
   
.  
. University of Massachusetts Amherst 
   

. Retrieved on 2007-05-13. </ref><ref>Modèle:Cite journal</ref> 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.<ref>You must specify title = and url = when using {{cite web}}.

  Kenton, S.; Bromley, B.
arxiv.org/ftp/astro-ph/papers/0608/0608359.pdf//cfa-www.harvard.edu/~kenyon/pf/dd/
   
 

.  
. Smithsonian Astrophysical Observatory 
   

. Retrieved on 2006-07-25.

</ref><ref> Cowen , Ron

  . 
 "
   Planet Formation on the Fast Track
   
 " , Science News
  , January 25, 2003
www.theoi.com/Titan/AstraPlaneta.html//www.sciencenews.org/articles/20030125/bob9.asp
 
  . Retrieved on 2006-07-25
 . </ref> 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 satellites of planets through a process of gravitational capture, or remain in belts of other objects to become either dwarf planets 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 comets.<ref>You must specify title = and url = when using {{cite web}}.

  Musgrave , Ian 
     
 

     (June 1, 1998
arxiv.org/ftp/astro-ph/papers/0608/0608359.pdf//home.tiac.net/~cri/1998/planet.html)
   
.  

. Retrieved on 2006-07-23. </ref> (Smaller planets will lose any atmosphere they gain through various 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 isotopes with an atomic number greater than 2 (Helium)—is now believed to determine the likelihood that a star will have planets.<ref> Aguilar, D.; Pulliam, C.

  . 
 "
   Lifeless Suns Dominated The Early Universe
   
 " , Harvard-Smithsonian Center for Astrophysics
  , January 6, 2004
arxiv.org/ftp/astro-ph/papers/0608/0608359.pdf//cfa-www.harvard.edu/press/pr0404.html
 
  . Retrieved on 2006-08-26
 . </ref> 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

Main article: Solar System

According to the IAU's current definitions there are eight planets in the Solar System. The largest is Jupiter, at 318 Earth masses. The smallest is Mercury, at 0.055 Earth masses. In increasing distance from the Sun, the planets are:

1. Image:Mercury symbol.svg Mercury
2. Image:Venus symbol.svg Venus
3. Image:Earth symbol.svg Earth
4. Image:Mars symbol.svg Mars
5. Image:Jupiter symbol.svg Jupiter
6. Image:Saturn symbol.svg Saturn
7. Image:Uranus symbol.svg Uranus
8. Image:Neptune symbol.svg Neptune

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

• Terrestrials: Planets (and possibly dwarf planets) that are similar to Earth — with bodies largely composed of rock: Mercury, Venus, Earth and Mars.
• Gas giants: Planets with a composition largely made up of gaseous material and are significantly more massive than terrestrials: Jupiter, Saturn, Uranus, Neptune. Ice giants 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.

Planetary attributes
	Name	Equatorial diameterModèle:Ref label	MassModèle:Ref label	Orbital radius (AU)	Orbital period (years)	Inclination to Sun's equator (°)	Orbital eccentricity	Rotation period (days)	Moons	Rings	Atmosphere
Terrestrials	Mercury	0.382	0.06	0.39	0.24	3.38	0.206	58.64	—	no	minimal
	Venus	0.949	0.82	0.72	0.62	3.86	0.007	-243.02	—	no	CO2, N2
	EarthModèle:Ref label	1.00	1.00	1.00	1.00	7.25	0.017	1.00	1	no	N2, O2
	Mars	0.532	0.11	1.52	1.88	5.65	0.093	1.03	2	no	CO2, N2
Gas giants	Jupiter	11.209	317.8	5.20	11.86	6.09	0.048	0.41	63	yes	H2, He
	Saturn	9.449	95.2	9.54	29.46	5.51	0.054	0.43	60	yes	H2, He
	Uranus	4.007	14.6	19.22	84.01	6.48	0.047	-0.72	27	yes	H2, He
	Neptune	3.883	17.2	30.06	164.8	6.43	0.009	0.67	13	yes	H2, He
Modèle:Note label Measured relative to the Earth. Modèle:Note label See Earth article for absolute values.

Dwarf planets

Main article: Dwarf planet

Before the August 2006 decision, several objects were proposed by astronomers, including at one stage by the IAU, as planets. However in 2006 several of these objects were reclassified as dwarf planets, objects distinct from planets. Currently three dwarf planets in the Solar System are recognized by the IAU: Ceres, Pluto and 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—namely that they are not dominant in their orbits. Their attributes are:

Dwarf planetary attributes
Name	Equatorial diameterModèle:Ref label	MassModèle:Ref label	Orbital radius (AU)	Orbital period (years)	Inclination to ecliptic (°)	Orbital eccentricity	Rotation period (days)	Moons	Rings	Atmosphere
Ceres	0.08	0.0002	2.76	4.60	10.59	0.080	0.38	0	no	none
Pluto	0.19	0.0022	39.48	248.09	17.14	0.249	-6.39	3	no	temporary
Eris	0.19	0.0025	67.67	~557	44.19	0.442	~0.3	1	?	temporary
Modèle:Note label Measured relative to the Earth.

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 Mike Brown there may soon be over forty trans-Neptunian objects that qualify as dwarf planets under the IAU's recent definition.<ref> Amburn , Brad

  . 
 "
   Behind the Pluto Mission: An Interview with Project Leader Alan Stern
   
 " , www.space.com
  , February 28, 2006
www.theoi.com/Titan/AstraPlaneta.html//www.space.com/scienceastronomy/060228_stern_interview.html
 
  . Retrieved on 2006-11-01
 . </ref>

Beyond the Solar System

Extrasolar planets

Main article: Extrasolar planet

exoplanet.eu/catalog.php |last=Schneider |first=Jean |date=December 11, 2006 |accessdate=2006-12-11}}</ref> Exceptions include a number of planets discovered orbiting burned-out star remnants called pulsars, such as PSR B1257+12,<ref>Modèle:Cite news//exoplanet.eu/catalog.php</ref> Exceptions include a number of planets discovered orbiting burned-out star remnants called pulsars, such as PSR B1257+12,<ref> Kennedy , Barbara

  . 
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   Scientists reveal smallest extra-solar planet yet found
   
 " , SpaceFlight Now
  , February 11, 2005
www.theoi.com/Titan/AstraPlaneta.html//www.spaceflightnow.com/news/n0502/11planet/
 
  . Retrieved on 2006-07-28
 . </ref> the planets orbiting the stars Mu Arae, 55 Cancri and GJ 436 which are approximately Neptune-sized,<ref>   Santos, N.; Bouchy, F.; Vauclair, S.; Queloz, D.; Mayor, M.
     
   
  . 
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   Fourteen Times the Earth
   
 " , ESO
  , August 25, 2004
www.theoi.com/Titan/AstraPlaneta.html//www.eso.org/public/outreach/press-rel/pr-2004/pr-22-04.html
exoplanet.eu/star.php?st=Gliese+876
 
  . Retrieved on 2007-09-10
 . </ref>//exoplanet.eu/star.php?st=Gliese+876|accessdate=2007-09-10}}</ref>

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 Jupiters, orbit extremely close to their parent stars, in nearly circular orbits. They therefore receive much more 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 planets.<ref>Modèle:Cite journal</ref>

More detailed observation of extrasolar planets will require a new generation of instruments, including space telescopes. Currently the CoRoT spacecraft is searching for stellar luminosity variations due to transiting planets. Several projects have also been proposed to create an array of space telescopes 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 ESA's Darwin, and the CNES' PEGASE.<ref>You must specify title = and url = when using {{cite web}}.

  Staff
   
 

       (2004
www.cfa.harvard.edu/icq/ICQPluto.html//www.spacetoday.org/DeepSpace/Stars/Planets/PlanetFindingMissions.html)
     
   
 
.  
. Space Today Online 
   

. Retrieved on 2007-07-23. </ref> The New Worlds Mission 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 intelligent, communicating civilizations that exist in our galaxy.<ref> Drake , Frank

  . 
 "
   The Drake Equation Revisited
   
 " , Astrobiology Magazine
  , September 29, 2003
www.theoi.com/Titan/AstraPlaneta.html//www.astrobio.net/news/article610.html
 
  . Retrieved on 2007-07-23
 . </ref>

Interstellar "planets"

Main article: Interstellar planetary mass object

Several computer simulations of stellar and planetary system formation have suggested that some objects of planetary mass would be ejected into interstellar space.<ref>Modèle:Cite journal</ref> Some scientists have argued that such objects found roaming in deep space should be classed as "planets". However, others have suggested that they could be low-mass stars.<ref>You must specify title = and url = when using {{cite web}}.

  Clavin, Whitney
   
 

       (2005)
     
   
 
.  
. Spitzer Science Centerwww.theoi.com/Titan/AstraPlaneta.html//www.nasa.gov/vision/universe/starsgalaxies/spitzerf-20051129.html

. Retrieved on 2006-05-25. </ref> The IAU's working definition on extrasolar planets takes no position on the issue.

In 2005, astronomers announced the discovery of Cha 110913-773444, the smallest brown dwarf found to date, at only seven times Jupiter's mass. Since it was not found in orbit around a fusing star, it is a sub-brown dwarf according to the IAU's working definition. However, some astronomers believe it should be referred to as a planet.<ref>You must specify title = and url = when using {{cite web}}.

  Clavin, Whitney
   
 

       (2005)
     
   
 
.  
. Spitzer Science Centerwww.theoi.com/Titan/AstraPlaneta.html//www.nasa.gov/vision/universe/starsgalaxies/spitzerf-20051129.html

. Retrieved on 2006-05-25. </ref>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", or "planetary mass objects". However, recent analysis<ref>You must specify title = and url = when using {{cite web}}.

  Close, L. M. et al
   
 

       (2007)
     
   
 
.  

. Retrieved on 2007-04-30. </ref> of the objects has determined that their masses are each greater than 13 Jupiter-masses, making the pair brown dwarfs.<ref>{{cite web

www.theoi.com/Titan/AstraPlaneta.html//www.space.com/scienceastronomy/planet_photo_040910.html
| title=Likely First Photo of Planet Beyond the Solar System
| first=Robert Roy | last=Britt| publisher=Space.com
| date=September 10, 2004
| accessdate=2007-03-13}}</ref>

Attributes

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, whilst others are also common to extrasolar planets.

Dynamic characteristics

Modèle:Seealso

Orbit

All planets revolve around stars. 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.<ref name="young">Modèle:Cite book</ref> 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, 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.<ref>Modèle:Cite book</ref>

Each planet's orbit is delineated by a set of elements:

adsabs.harvard.edu/abs/2007arXiv0708.0335M|year=2007|accessdate=2007-09-08}}</ref><ref> Kuiper Belt Objects

. The Astrophysics Spectator

 

. Retrieved on 2007-09-08. </ref>//adsabs.harvard.edu/abs/2007arXiv0708.0335M|year=2007|accessdate=2007-09-08}}</ref><ref> Kuiper Belt Objects

. The Astrophysics Spectator

 

. Retrieved on 2007-09-08. </ref>

• 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.<ref name="young"/>

www.journals.uchicago.edu/ApJ/journal/issues/ApJL/v566n2/15859/15859.html|//www.journals.uchicago.edu/ApJ/journal/issues/ApJL/v566n2/15859/15859.html| accessdate=2007-09-08}}</ref> The points at which a planet crosses above and below the ecliptic are called its ascending and descending nodes.<ref name="young"/> Other orbital elements used to describe the orientation of a planet's orbit within our Solar System include the argument of periapsis and longitude of the ascending node.<ref name="young"/>

Rotation

www.sciencemag.org/cgi/content/abstract/139/3558/910|accessdate=2007-09-09}}</ref> (Uranus is generally said to be rotating clockwise as well<ref> MJS Belton, RJ Terrile

       (1984)
     
   
 
.    Rotational properties of Uranus and Neptune 
. Kitt Peak National Observatory

. Retrieved on 2007-09-09. </ref> though because of its extreme axial tilt, it 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).<ref>Modèle:Cite book</ref> 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.<ref> Planet tables

. Retrieved on 2007-09-09. </ref> The rotational periods of extrasolar planets are not known; however their close proximity to their stars means that hot Jupiters are tidelocked; their orbits are in sync with their rotations. This means they only ever show one face to their stars, with one side in perpetual day, the other in perpetual night.<ref> Philippe Zarka, Rudolf A. Treumann et al.

       (2001)
     
   
 
.    Magnetically-Driven Planetary Radio Emissions and Application to Extrasolar Planets 

. Retrieved on 2007-09-08. </ref>//www.sciencemag.org/cgi/content/abstract/139/3558/910|accessdate=2007-09-09}}</ref> (Uranus is generally said to be rotating clockwise as well<ref> MJS Belton, RJ Terrile

       (1984)
     
   
 
.    Rotational properties of Uranus and Neptune 
. Kitt Peak National Observatory

. Retrieved on 2007-09-09. </ref> though because of its extreme axial tilt, it 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).<ref>Modèle:Cite book</ref> 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.<ref> Planet tables

. Retrieved on 2007-09-09. </ref> The rotational periods of extrasolar planets are not known; however their close proximity to their stars means that hot Jupiters are tidelocked; their orbits are in sync with their rotations. This means they only ever show one face to their stars, with one side in perpetual day, the other in perpetual night.<ref> Philippe Zarka, Rudolf A. Treumann et al.

       (2001)
     
   
 
.    Magnetically-Driven Planetary Radio Emissions and Application to Extrasolar Planets 

. Retrieved on 2007-09-08. </ref>

Axial tilt

Planets also have varying degrees of axial tilt; they lie at an angle to the plane of the their stars' equators. This causes the amount of light received by each hemisphere to vary over the course of its year; when the northern hemisphere points away from its star, the southern hemisphere points towards it, and vice versa. Each planet therefore possesses seasons; changes to the climate over the course of its year. The point at which each hemisphere is 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. 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.<ref name=Weather>You must specify title = and url = when using {{cite web}}.

  Harvey , Samantha 
     
 

     (May 1, 2006
www.theoi.com/Titan/AstraPlaneta.html//solarsystem.nasa.gov/scitech/display.cfm?ST_ID=725)
   
.  
. NASA
www.journals.uchicago.edu/cgi-bin/resolve?id=doi:10.1086/432834&erFrom=6796329409371401737Guest 
   

. Retrieved on 2007-09-08. </ref>//www.journals.uchicago.edu/cgi-bin/resolve?id=doi:10.1086/432834&erFrom=6796329409371401737Guest| accessdate=2007-09-08}}</ref>

Orbital clearance

arxiv.org/PS_cache/arxiv/pdf/0706/0706.1684v1.pdf|accessdate=2007-09-15}}</ref>//arxiv.org/PS_cache/arxiv/pdf/0706/0706.1684v1.pdf|accessdate=2007-09-15}}</ref>

Physical characteristics

Mass

www.gps.caltech.edu/~mbrown/dwarfplanets/|author=Mike Brown|work=CalTech|year=2006|accessdate=2007-09-09}}</ref>//www.gps.caltech.edu/~mbrown/dwarfplanets/|author=Mike Brown|work=CalTech|year=2006|accessdate=2007-09-09}}</ref>

Mass is also the prime attribute by which planets are distinguished from stars. The upper mass limit for planethood, beyond which it achieves conditions suitable for nuclear fusion, is roughly 13 times Jupiter's mass. No objects of such mass exist in our Solar System; however a number of extrasolar planets lie at that threshold. The Extrasolar Planets Encyclopedia lists the largest planets discovered to date as HD 38529 c, at 12.7 Jupiter masses, AB Pic b, at 13.5 Jupiter masses, HD 162020 b at 13.75 Jupiter masses, and HD 13189 b, at 14 Jupiter masses. A number of objects of higher mass are also listed, but since they lie above the fusion threshold, they would be better described as brown dwarfs.<ref name=threshold> [? Interactive Extra-solar Planets Catalog]

. The Extrasolar Planets Encyclopedia
   (2007)
     
   
 

. Retrieved on 2007-12-02. </ref>

The smallest known planet, excluding dwarf planets and satellites, is [[PSR B1257+12]] a, one of the first extrasolar planets discovered, which was found in 1992 in orbit around a pulsar. Its mass is roughly half that of the planet Mercury.<ref name=threshold />

Internal differentiation

abyss.uoregon.edu/~js/ast121/lectures/lec16.html|accessdate=2007-09-10}}</ref> 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.<ref>Modèle:Cite book//abyss.uoregon.edu/~js/ast121/lectures/lec16.html</ref> 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.<ref>Modèle:Cite book</ref> Uranus and Neptune, which are smaller, possess rocky cores surrounded by mantles of water, ammonia, methane and other ices.<ref>Modèle:Cite journal</ref> The fluid action within these planets' cores creates a geodynamo that generates a magnetic field.<ref name=terrestrial />

Atmospheres

All of the planets have atmospheres 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.<ref>Modèle:Cite journal</ref> Earth's atmosphere is greatly different to the other planets because the various life processes that have transpired there have introduced massive amounts of free oxygen,<ref> Anne E. Egger, M.A./M.S.

.    Earth's Atmosphere: Composition and Structure 
. VisionLearning.com

. Retrieved on 2006-12-26. </ref> while the atmosphere of Mercury has mostly, although not entirely, been blasted away by the solar wind.<ref>Hunten D.M., Shemansky D.E., Morgan T.H. (1988), The Mercury atmosphere, In: Mercury (A89-43751 19-91). University of Arizona Press, p. 562-612</ref> 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 systems such as hurricanes, (on Earth), planet-wide dust storms (on Mars) and Earth-sized anticyclones (on Jupiter).<ref name=Weather /> 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.<ref>You must specify title = and url = when using {{cite web}}.

  Aguilar, D. A.; Pulliam, C.
www.theoi.com/Titan/AstraPlaneta.html//www.cfa.harvard.edu/press/2007/pr200713.html
   
 

     (May 9, 2007)
   
.  
. Harvard-Smithsonian Center for Astrophysics

. Retrieved on 2007-08-15. </ref> Hot Jupiters have been shown to be losing their atmospheres into space due to stellar radiation, much like the tails of comets.<ref>You must specify title = and url = when using {{cite web}}.

  Weaver, D.; Villard, R.
www.theoi.com/Titan/AstraPlaneta.html//hubblesite.org/newscenter/archive/releases/2007/07/full/
   
 

     (January 31, 2007)
   
.  
. Hubblesite

. Retrieved on 2007-08-15. </ref> These planets have vast differences in temperature between their day and night sides which produce supersonic windspeeds.<ref>You must specify title = and url = when using {{cite web}}.

  Clavin, W.; Watanabe, S.
   
 

     (October 12, 2006
www.theoi.com/Titan/AstraPlaneta.html//www.nasa.gov/vision/universe/starsgalaxies/spitzer-20061012.html)
   
.  
. NASA

. Retrieved on 2007-08-15. </ref>

Secondary characteristics

www.newscientist.com/article.ns?id=dn1647 Bacterial explanation for Europa's rosy glow], NewScientist.com (11 December 2001)</ref>//www.newscientist.com/article.ns?id=dn1647 Bacterial explanation for Europa's rosy glow], NewScientist.com (11 December 2001)</ref>

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 '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 which fell below their parent planet's Roche limit and were torn apart by tidal forces.<ref>Modèle:Cite journal</ref><ref>Modèle:Cite book</ref>

www.nasa.gov/vision/universe/starsgalaxies/spitzerf-20051129.html|accessdate=2007-12-14}}</ref>//www.nasa.gov/vision/universe/starsgalaxies/spitzerf-20051129.html|accessdate=2007-12-14}}</ref>

See also

References

External links

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

www.iau.org International Astronomical Union]//www.ciw.edu/IAU/div3/wgesp/definition.html Working definition of "planet"] from IAU WGESP — 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. www.boulder.swri.edu/~hal/planet_def.html "Regarding the criteria for planethood and proposed planetary classification schemes."]//www.boulder.swri.edu/~hal/planet_def.html "Regarding the criteria for planethood and proposed planetary classification schemes."] www.iau.org International Astronomical Union]//www.spacedaily.com/news/outerplanets-04b.html Gravity Rules: The Nature and Meaning of Planethood]; S. Alan Stern; March 22, 2004 www.iau.org International Astronomical Union]//www.iau.org/STATUS_OF_PLUTO.238.0.html IAU Press Release 01/99 "The status of Pluto: A Clarification"]; IAU, 1999-02-03 www.iau.org International Astronomical Union]//news.bbc.co.uk/1/hi/sci/tech/4795755.stm BBC: "Planets plan boost tally 12" 2006-08-16] www.iau.org International Astronomical Union]//news.bbc.co.uk/1/hi/world/5282440.stm BBC: "Pluto loses status as a planet" 2006-08-24] www.iau.org International Astronomical Union]//news.bbc.co.uk/2/hi/science/nature/5283956.stm BBC: "Pluto vote 'hijacked' in revolt" 2006-08-25]

Modèle:Footer SolarSystemModèle:Link FA

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