Astronomers to vote on potential new planets
{{date|August 16, 2006}}
[[Image:Solar sys.jpg|right|thumb|200px|Our solar system, showing the Sun, Inner Planets, Asteroid Belt, Outer Planets, and a comet. If astronomers endorse a new proposal for the definition of the word "planet", the planets in our solar system would be Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon and 2003<sub>UB313</sub>.]]

The [[w:International Astronomical Union|International Astronomical Union]] (IAU) has offered up a new [[w:Definition of planet|definition of the word "planet"]] that could potentially increase [[w:Solar system|our solar system]]'s nine [[w:planet|planets]] to at least twelve. According to the proposed definition, "A planet is a [[w:Astronomical object|celestial body]] that (a) has sufficient mass for its self-[[w:gravity|gravity]] to overcome rigid body forces so that it assumes a [[w:Hydrostatic equilibrium|hydrostatic equilibrium]] (nearly round) shape, and (b) is in [[w:Planetary orbit|orbit]] around a [[w:star|star]], and is neither a star nor a [[w:satellite|satellite]] of a planet." Roughly speaking, the former includes objects over 5 x 10<sup>20</sup> kilograms (1/12,000th of Earth's mass) and 800 kilometers in diameter, but all borderline cases would require confirmation by observation of their shape.

According to this new definition, [[w:1 Ceres|Ceres]] (an [[w:asteroid|asteroid]] in the [[w:asteroid belt|asteroid belt]], between [[w:Mars|Mars]] and [[w:Jupiter|Jupiter]]), [[w:2003 UB313|2003<sub>UB313</sub>]] (an astronomical object beyond [[w:Pluto|Pluto]] which has previously been called the [[w:Tenth planet|tenth planet]]), and Pluto's [[w:Natural satellite|moon]] [[w:Charon (moon)|Charon]] may be dubbed planets. A dozen other candidates, like [[w:90377 Sedna|Sedna]], [[w:90482 Orcus|Orcus]], [[w:50000 Quaoar|Quaoar]], [[w:2003 EL61|2003 EL<sub>61</sub>]] ''etc''., along with the asteroids [[w:4 Vesta|Vesta]], [[w:2 Pallas|Pallas]] and [[w:10 Hygiea|Hygiea]] are awaiting evaluation by the IAU.

The draft resolution also introduces the term "[[w:Pluton (astronomy)|pluton]]", which refers to a growing subcategory of planets that have orbits around the sun that take at least 200 years to complete - effectively this will mean planets that orbit beyond [[w:Neptune|Neptune]]. Plutons differ from [[w:classical planet|classical planets]]: their orbit is highly tilted, [[w:Orbital eccentricity|eccentric]] and not circular, which suggests they have a different origin, the main reason why [[w:Astronomer|astronomers]] are interested in them.

The word "planet" comes from the [[w:Ancient Greek|ancient Greek]] word for "wanderer", because it was known in ancient times that certain lights in the sky moved in relation to other stars. However, since then no formal definition of the word planet was agreed upon. With the advent of powerful telescopes on the ground and in space, knowledge about heavenly bodies became more complex, stressing the need for unambiguous definitions. The issue came to a head in 2005 with the discovery of the [[w:trans-Neptunian object|trans-Neptunian object]] 2003<sub>UB313</sub> (unofficially termed Xena by its discoverer, [[w:Michael E. Brown|Michael E. Brown]]). 2003<sub>UB313</sub> is a body larger than the smallest accepted planet, Pluto.

The suggested definition is not universally popular: some astronomers would like to draw the line at Neptune, and wouldn't classify icy dwarfs like Pluto as a planet, but as a trans-Neptunian object (similar to the plutons in the draft resolution) part of the [[w:Kuiper belt|Kuiper belt]]. Robin Catchpole, from the [[w:Institute of Astronomy, Cambridge|Institute of Astronomy, Cambridge]], said in an interview: "The public are very clear about what they understand by "planets". Those are the big, dominant bodies in the Solar System that we're all familiar with, the eight - or nine if you include Pluto. I think including more is going to add confusion to the public, but not really be particularly useful for astronomers."

[[Image:2003UB313_August16_2006.gif|left|thumb|400px|Celestial object 2003<sub>UB313</sub> viewed in relation to the outer planets and the sun, with current position on August 16, 2006. AU = [[w:Astronomical unit|Astronomical unit]], almost 150 million kilometers.]]
Pluto is now being considered to be moved off the list of classical planets, but as the prototype of the new plutons category. The ninth planet (as it is termed, but maybe not for long), discovered in 1930, is a curiosity among planets for more than one reason: because Charon is so big in comparison, both are considered twin planets by some.

The current proposal is the result of two years work of the Planet Definition Committee of the IAU, which is responsible for the naming of astronomical objects. The 26th IAU General Assembly in [[w:Prague|Prague]] plans to vote on the proposal on Thursday, August 24 of 2006. When asked if he was confident the proposal would get the necessary two thirds of the votes, Professor [[w:Owen Gingerich|Owen Gingerich]], Chair of the IAU Planet Definition Committee, replied: "I'm sure it will be controversial to those with a stake in some other solution, but I hope we will get an overwhelming endorsement."

==Sources==

*{{source-pr|url=http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601_release.html
|title=The IAU draft definition of "planet" and "plutons"
|pub=International Astronomical Union
|date=August 16, 2006}}

*{{source|url=http://news.bbc.co.uk/2/hi/science/nature/4795755.stm
|title=Planets plan boosts tally to 12
|author=Paul Rincon
|pub=BBC News Online
|date=August 16, 2006}}

*{{source|url=http://edition.cnn.com/2006/TECH/space/08/16/new.planets.ap/index.html
|title=Proposal would increase planets from 9 to 12
|author=
|pub=CNN
|date=August 16, 2006}}

*{{source|url=http://www.space.com/scienceastronomy/060816_planet_definition.html
|title=Nine Planets Become 12 with Controversial New Definition
|author=Robert Roy Britt
|pub=Space.com
|date=August 16, 2006}}

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[[Category:Space]]
[[Category:Science and technology]]

[[Category:Astronomy]]
[[Category:Europe]]
[[Category:Czech Republic]]
[[Category:Physics]]
[[Category:Pluto]]
[[Category:Kuiper belt]]
[[Category:Charon]]

[[ca:Astrònoms de tot el món decideixen el nombre de planetes del sistema solar]]
[[de:Neue Definition des Wortes „Planet“ würde deren Anzahl in unserem Sonnensystem erhöhen]]
[[es:Astrónomos de todo el mundo deciden si son 12 los planetas del sistema solar]]
[[fr:L'Union astronomique internationale envisage de donner une définition précise du terme Planète]]
[[it:Consiglio degli astronomi verso l'accordo: 3 nuovi pianeti nel sistema solare]]
[[ja:太陽系の惑星が12個になる可能性―国際天文学連合の新定義案]]
[[pl:Propozycja nowej definicji planety]]
[[ru:В Солнечной системе станет больше планет]]
[[uk:Вчені пропонують збільшити кількість планет Сонячної Системи]]
Our solar system, showing the Sun, Inner Planets, Asteroid Belt, Outer Planets, and a comet. If astronomers endorse a new proposal for the definition of the word "planet", the planets in our solar system would be Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon and 2003UB313.

The International Astronomical Union (IAU) has offered up a new definition of the word "planet" that could potentially increase our solar system's nine planets to at least twelve. According to the proposed definition, "A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet." Roughly speaking, the former includes objects over 5 x 1020 kilograms (1/12,000th of Earth's mass) and 800 kilometers in diameter, but all borderline cases would require confirmation by observation of their shape.

According to this new definition, Ceres (an asteroid in the asteroid belt, between Mars and Jupiter), 2003UB313 (an astronomical object beyond Pluto which has previously been called the tenth planet), and Pluto's moon Charon may be dubbed planets. A dozen other candidates, like Sedna, Orcus, Quaoar, 2003 EL61 etc., along with the asteroids Vesta, Pallas and Hygiea are awaiting evaluation by the IAU.

The draft resolution also introduces the term "pluton", which refers to a growing subcategory of planets that have orbits around the sun that take at least 200 years to complete - effectively this will mean planets that orbit beyond Neptune. Plutons differ from classical planets: their orbit is highly tilted, eccentric and not circular, which suggests they have a different origin, the main reason why astronomers are interested in them.

The word "planet" comes from the ancient Greek word for "wanderer", because it was known in ancient times that certain lights in the sky moved in relation to other stars. However, since then no formal definition of the word planet was agreed upon. With the advent of powerful telescopes on the ground and in space, knowledge about heavenly bodies became more complex, stressing the need for unambiguous definitions. The issue came to a head in 2005 with the discovery of the trans-Neptunian object 2003UB313 (unofficially termed Xena by its discoverer, Michael E. Brown). 2003UB313 is a body larger than the smallest accepted planet, Pluto.

The suggested definition is not universally popular: some astronomers would like to draw the line at Neptune, and wouldn't classify icy dwarfs like Pluto as a planet, but as a trans-Neptunian object (similar to the plutons in the draft resolution) part of the Kuiper belt. Robin Catchpole, from the Institute of Astronomy, Cambridge, said in an interview: "The public are very clear about what they understand by "planets". Those are the big, dominant bodies in the Solar System that we're all familiar with, the eight - or nine if you include Pluto. I think including more is going to add confusion to the public, but not really be particularly useful for astronomers."

Astronomical unit, almost 150 million kilometers.
Pluto is now being considered to be moved off the list of classical planets, but as the prototype of the new plutons category. The ninth planet (as it is termed, but maybe not for long), discovered in 1930, is a curiosity among planets for more than one reason: because Charon is so big in comparison, both are considered twin planets by some.

The current proposal is the result of two years work of the Planet Definition Committee of the IAU, which is responsible for the naming of astronomical objects. The 26th IAU General Assembly in Prague plans to vote on the proposal on Thursday, August 24 of 2006. When asked if he was confident the proposal would get the necessary two thirds of the votes, Professor Owen Gingerich, Chair of the IAU Planet Definition Committee, replied: "I'm sure it will be controversial to those with a stake in some other solution, but I hope we will get an overwhelming endorsement."

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ca:Astrònoms de tot el món decideixen el nombre de planetes del sistema solar
de:Neue Definition des Wortes „Planet“ würde deren Anzahl in unserem Sonnensystem erhöhen
es:Astrónomos de todo el mundo deciden si son 12 los planetas del sistema solar
fr:L'Union astronomique internationale envisage de donner une définition précise du terme Planète
it:Consiglio degli astronomi verso l'accordo: 3 nuovi pianeti nel sistema solare
Space Science and technology Astronomy Europe Czech Republic Physics Pluto Kuiper belt Charon