.comment-link {margin-left:.6em;}

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.

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).
 
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.
 
Post a Comment



<< Home

This page is powered by Blogger. Isn't yours?