IAU definition of planet
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In 2006, the International Astronomical Union proposed that the term "planet" be redefined to include other objects beyond the traditional nine planets considered a part of the solar system.[1] Members of the IAU will vote on the proposal on August 24, 2006 in Prague, the Czech Republic.
The three new planets would be Charon (formerly considered a moon of Pluto; now considered a double planet), Ceres (first considered a planet, then an asteroid), and the recently discovered 2003 UB313 (nicknamed Xena). It is quite possible that, after more searching, astronomers will discover more objects that meet this new definition.
The draft proposal
The IAU published its definition proposal on August 16, 2006. This definition loosely follows the second option considered by the 19-member panel in 2005. In its exact wording, it is:
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.
The IAU proposal requires that three new worlds immediately become planets: Ceres, Charon and 2003 UB313. A further dozen are possible candidates to join the list, pending refinements of our knowledge regarding their physical properties. Some objects in the second list are more likely to eventually be christened as "planets" than others. Despite what has been claimed in the media, this proposal does not automatically leave our solar system with 12 planets. Mike Brown, the discoverer of Sedna and 2003 UB313, has claimed that this definition could eventually comprise up to 53 planets in our system alone, with hundreds, or even thousands, not yet discovered.[2]
A pair of objects are considered a double planet system if they independently satisfy the definition of a planet, and the common center of gravity of the system (known as the barycenter) lies outside either body.[3] Pluto and Charon are the only known double planet system in the solar system. Other planetary satellites (such as the Earth's Moon) might be in hydrostatic equilibrium but are not defined as a planet, since the barycenter lies within the more massive planet.
The term "minor planet" is abandoned, replaced by the categories "small Solar System body" and a new classification, "pluton". The latter signifies those planets with highly inclined orbits with large eccentricities and an orbital period of more than 200 years, i.e. those orbiting beyond Neptune. Pluto is the archetypal pluton. The term "dwarf planet", which can be used to describe all planets smaller than the eight "classical planets" in orbit around the Sun, is not an official IAU classification. [4] Those objects underneath the "spherical" threshold are termed "Small Solar System Bodies." The IAU has not ruled on what separates a planet from a brown dwarf. [5] The final decision is expected by August 24, 2006.[6]
This redefinition of the term could also lead to changes in classification for the trans-Neptunian objects 2003 EL61, 2005 FY9, Sedna, Orcus, Quaoar, Varuna, 2002 TX300, Ixion, 2002 AW197, and the asteroids Vesta, Pallas and Hygiea.
See also
References
- ^ Connor, Steve (2006-08-16). "Solar system to welcome three new planets". New Zealand Herald.
{{cite news}}: Check date values in:|date=(help) - ^ Mike Brown (2006). "How Many Planets Are There?". CalTech. Retrieved 2006-08-16.
- ^ Robert Roy Britt (2006). "Nine Planets Become 12 with Controversial New Definition". Space.com. Retrieved 2006-08-16.
- ^ "Draft Resolution 5 for GA-XXVI: Definition of a Planet". International Austronomical Union. 2006. Retrieved 2006-08-16.
- ^ "Planet Definition" Questions & Answers Sheet". International Austronomical Union. 2006. Retrieved 2006-08-16.
- ^ Gareth Cook (2006). "Nine no longer: Panel declares 12 planets". Boston Globe. Retrieved 2006-08-16.
External links
- The IAU draft definition of "planet" and "plutons", International Astronomical Union press release.
- Working Group on Extrasolar Planets - Defintion of a "Planet" POSITION STATEMENT ON THE DEFINITION OF A "PLANET" (IAU) 2003
- Q&A New planets proposal - BBC Online
