An executive committee formed by the International Astronomical Union has approved a new definition of what a planet is, and by the new definition, the Solar System will have 12 planets, up from the present nine. Three bodies will be upgraded to the planet status: Charon, Ceres (formerly an asteroid), and Xena. The decision of the executive panel will be put on vote on August 24 by the delegates composing the IAU.
The new definition of a planet is: an object that circles the Sun, large enough to have gravitational forces that will compress it into a spherical shape, and at is least 250-500 miles in diameter.
The committee also came up with a new designation, called plutons, that encompasses planets that take at least 200 years to orbit the Sun. Plutons include Pluto, Charon and Xena. Ceres would just be an ordinary planet under the new definition, while Pluto and Charon, which orbit close to one another, will be a double planet system.
Many other objects might be upgraded to planet status, including Kuiper Belt Objects such as Sedna, Orcus and Quaoar, as well as asteroids such as Vespa, Pallas and Hygeia.
2 comments:
You said, the official definition of a planet is, "an object that circles the Sun, large enough to have gravitational forces that will compress it into a spherical shape, and is at 250-500 miles in diameter."
However, the IAU says it slightly differently: "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 official definition says nothing at all about the diameter.
You should have mentioned that the diameter given is just a rough guide to know when an object becomes roughly spherical - about 250 miles for KBOs and 500 miles for asteroids, because asteroids are denser than KBOs.
The 250-500 mile-radius was the one initially reported by the press shortly after the executive committe of the IAU broke out the proposed definition.
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