A few years ago (in 2006) a gang of astronomers called IAU decided on rules for planets, as they wanted to exclude objects in an area they call the Kuiper belt, where there may be a hundred thousand bodies and dozens the size of Pluto or even larger. Why the IAU hates them no one knows, but they voted in these rules to exclude them as planets--
(1) A planet is a celestial body that
a. is in orbit around the Sun
b. has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
c. has cleared the neighborhood around its orbit.
(2) A "dwarf planet" is a celestial body that
a. is in orbit around the Sun,
b. has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,
c. has not cleared the neighborhood around its orbit, and
d. is not a satellite.
(3) All other objects orbiting the Sun (except satellites) shall be referred to collectively as "Small Solar System Bodies".
By these rules Pluto and Ceres are excluded because Pluto spends a good bit of time in the Kuiper belt and Ceres resides in the asteroid belt. It would be easy to make Pluto a planet and exclude all other known bodies by adding a few words--
1 c. has cleared the neighborhood around its orbit or has at least five moons
2 c. has not cleared the neighborhood around its orbit and has fewer than five moons
Which is ad hoc, but 1c and 2c were ad hoc to begin with.
Another option for Pluto--
1 c. has cleared the neighborhood around its orbit, or has at least one moon with sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium
Edited by Turnbuckle, 28 October 2015 - 12:50 PM.