UB313... Planet!

The International Astronomical Union will be issueing a formal definition of the term "Planet" next week... and that definition will mean UB313 (which has not been formally named yet) will recieve an official designation as a "Planet".

From the wire services, here's the formal definition:

"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."

In other words, if the object is large enough that its own gravity forces it into a ball, and is in orbit around a star but not another planet, then it is considered a planet. This leaves aside any definition of planet based on an arbitrary ruling of size or mass. It also opens the door for many more objects to become (re)classified as "planets", instead of "small solar system objects" (as asteroids, or "minor planets", will now be called). Indeed, the definition above not only lets UB313 into the Planet Club, but also main-belt asteroid Ceres. And pending better observations of such objects as Sedna, Quaoar, and other outer-solar system (Kuiper Belt) objects, the number of worlds (re)classified as Planets might go up even more.

But wait, it gets even stranger. In a footnote to the planetary definition, we find this:

"For two or more objects comprising a multiple-object system, the primary object is designated a planet if it independently satisfies the conditions above. A secondary object satisfying these conditions is also designated a planet if the system barycenter [center of mass] resides outside the primary. Secondary objects not satisfying these criteria are 'satellites."

This means that if the center of mass of a planet-satellite system resides outside of the primary object, then both objects are considered a "double planet", and each member of the duo are considered planets. In our solar system, there is only one planetary system that fits that definition: Pluto-Charon. So while Pluto has been considered a planet since its discovery, its satellite Charon is about to get the same definition because Charon is sufficiently massive and orbits far enough away that the center of mass lies in space between the two worlds rather than below Pluto's surface (in contrast, the center of mass for our own Earth-Moon system lies just below the surface of the Earth). Effectively, they will collectively be considered a "double planet".

The above definitions are not formal yet; they have to be voted on by the IAU body. But that vote is set for next week. If the definition passes, then our solar system will have 12 objects officially classified as Planet, with probably more yet to come.

For anyone who is interested, here's a quick summary:

What's A Planet? (Sky and Telescope)

EDIT: Here's what Micheal E. Brown (one of UB313's co-discoverers) has to say about the proposed IAU definition:

IAU Proposal

According to him, the IAU definition would mean there are currently 53 planets in our solar system with more yet to come. He refers to it as the "Leave No Ice Ball Behind!" definition.

Well, this is going to get heated.