Original URL: http://www.theregister.co.uk/2006/08/16/pluto_planet_q/

Solar system to get new planets?

IAU sets criteria for planetary club membership

By Lucy Sherriff

Posted in Science, 16th August 2006 11:30 GMT

If astronomers approve a new proposal for a definition of a planet, not only will Pluto retain its status as the ninth planet, but the solar system will gain three more.

The question of Pluto's planethood has been the subject of a long-running, and ultimately frustrating debate. Frustrating, because until now there has been no officially accepted definition of a planet.

Step forward the massed ranks of the International Astronomers Union (currently having its regular, once-every-three-yearly, knees-up in Prague) with the following definition, hammered out over two, very long, years:

"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."

This means anything massing more than 5x1020kg with a diameter greater than 800 km will automatically count as a planet of some kind. But borderline cases, the IAU stresses "would have to be established by observation".

The definition also sets out a class structure for different kinds of planets, to distinguish between the inner planets and ore distant bodies in less circular orbits. Thus, anything with an orbit longer than 200 years would be classed as a "pluton". Two of the three new planets would fall into the pluton category.

Qualifying orbiting bodies include the lovely 2003 UB313, otherwise known as Xena. However, now that official planethood beckons (even if it is a kind of lesser, plutonic planethood) the warrior princess might just end up being renamed. Officially, of course, it still carries its 2003 Ub etc moniker. Nice.

Ceres and Charon* will also join the list. Twelve other candidate planets are on the IAU's watch list.

The astronomers will actually vote on the definition on 23 August. ®

* Bootnote: Yes, we spotted that, too. Although the definition rules that objects that are satellites of other planets are not planets themselves, a footnote adds:

"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 barycentre resides outside the primary. Secondary objects not satisfying these criteria are "satellites". Under this definition, Pluto's companion Charon is a planet, making Pluto-Charon a double planet."

As a result, you will probably start to see Charon being referred to as Pluto's companion rather than its moon.