Astroblog: Planetary Proposal Preserves Pluto

Wednesday, August 16, 2006

Planetary Proposal Preserves Pluto

A news item in the International Astronomical Union site reveals a proposed definition of planets that will keep Pluto as a planet.

(1) 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) shape¹, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.²

This keeps Pluto as a planet, and UB313 becomes a planet... as does the asteroid Ceres, surprisingly. This definition is different from the one Stuart has discussed.

# posted by Ian Musgrave @ 10:18 pm

Comments:

Under this definition, Charon is also a planet. This might seem to be in violation of (b). However, since Charon is about 1/7 of the mass of Pluto, the center of mass of the system is between the two bodies (rather than being inside one body, as is the case in the Earth/Moon system). Therefore Pluto and Charon are mutually orbiting each other and Charon as a planet does not violate (b).

# posted by

Ed : 10:48 am

If you look at the full definitions, and the IAU explanation, Charon is a planet because the common barycentre of the Pluto Charon system is between Pluto and Charon, not inside Pluto. It's a little clunky, but I tink it does the job.

# posted by

Ian Musgrave : 8:56 pm