More Planethood Debate

Is the New Horizon probe flying to a planet? The mission's target is Pluto, but in just the past two years Pluto has been defined by groups of astronomers as a planet, a dwarf planet, and a plutoid. No kidding.

Next week, another group of astronomers will meet in Laurel, Maryland, to try to come to a consensus on the definition of "planet." This bizarre little controversy really caught fire in 2006 when the International Astronomical Union decided Pluto would no longer have planet status. The IAU's case is based on the facts that Pluto is much smaller than any other planet-- indeed, smaller than some moons, including our own. The case can also be made that Pluto is simply the closest in a new class of objects orbiting in the Kuiper Belt beyond Neptune. We are beginning to find others, and New Horizon may visit one or more after its Pluto flyby.

On the other hand, many argue Pluto is a substantial, spherical body that orbits a star; it is, therefore, a planet. If that definition means a few more bodies in the Solar System get uograded to planets, they say, so be it.

Whatever the outcome at Laurel, the conference will look at extrasolar planets, as well, and try to find consensus that covers bodies everywhere. One further quirk in the IAU's decision is that its definition of planethood applied only to this solar system, leaving open the possibility that the definition in other solar systems could be different. Laurel, hopefully, will bring that position into the modern age.