Great planet debate to go on for three days starting today at the Johns Hopkins Applied Physics Laboratory

Ceres, perhaps the 13th (out of 13) largest planet in the Solar System if the definition of a planet is changed yet again.

There are a lot of sites reporting about this; here's spaceref.com on the issue.

I'm in agreement with the Mark Sykes on what the definition of a planet should be:

Mark Sykes says that if a non-stellar object is massive enough to be round and orbits a star, it ought to be a planet. The key here is that once an object gets that big, important geophysical processes begin.

Under this scenario, the smallest known planet in the solar system would be Ceres, the largest and most massive object in the asteroid belt between Mars and Jupiter. Ceres is less than half the diameter of Pluto.

By round, Sykes doesn't mean perfectly spherical. In scientific terms, he's talking about the shape of bodies that are in "hydrostatic equilibrium," where the pressure from an essentially fluid interior is balanced against gravity and centrifugal force at the body's surface to round the object. This makes planetary objects fundamentally different from small, irregularly shaped asteroids and comets.

Sykes, director of the Planetary Science Institute, is talking about planets across the universe in general, about Pluto and other small planets in particular, and about planets in the distant reaches of our solar system yet to be discovered. Some of those could even be as large as the Earth, he said.

This is a simple enough definition, as hydrostatic equilibrium is easy to prove, and bodies without it are extremely different from those that don't. Here's a body with it:
(that's Rhea)

and here's one without it:

(that's Janus)

Clearly not the same thing. If Rhea wasn't a moon then it would be classified as a planet. The new definition then would be a bit unfair to large moons, because then massive Titan and Ganymede would be called moons along with Janus and all the rest of the rocky asteroid-like moons. I think a new name would be needed for them in this case, such as a planetoid moon or something along those lines.

The new definition is probably needed because eventually we are going to be discovering smaller and smaller worlds around other stars (equal to and smaller than Earth), and trying to prove whether or not a body has "cleared its orbit" in another solar system is going to be nearly impossible for the time being. Since we have such a clear dividing line between small (lumpy and irregular) and large (round and with hydrostatic equilibrium), it probably doesn't make sense to try to fit in yet another dividing line between the large bodies until they become completely different by becoming gas giants.

By the way, Neil deGrasse Tyson will be there. He's been on The Colbert Report a few times.

This Thursday (Aug. 14), Sykes and astronomer Neil deGrasse Tyson, of the American Museum of Natural History and host of NOVA Science Now, will debate the IAU's actions, the value of different planet definitions, and the place for Pluto and other objects in the solar system. They'll meet as a part of the three-day "Great Planet Debate: Science as a Process" conference at the Johns Hopkins Applied Physics Laboratory near Columbia, Maryland. The conference runs Thursday and Friday, with an educator workshop on Saturday.