The International Astronomical Union (IAU) released a draft resolution today on the definition of a planet. The seven-member panel agreed unanimously (some say miraculously so) on the following verbage:
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.[*]If this definition is adopted, there would be twelve (12) planets in the solar system: the familiar nine plus Ceres (the largest of the Asteroids), 2003 UB313 (provisional designation for the larger-than-pluto object announced last year sometimes referred to as "Xena" or "Lila"), and perhaps most surprisingly Charon (Pluto's first "moon" discovered in 1978 would be elevated to make Pluto-Charon the first double-planet system--with additional two small moons.) The 26th General Assembly of the IAU will consider this question at its meeting in Prague on August 24th, 2006.
*For two or more objects comprising a multiple object system... a secondary object satisfying these conditions is also designated a planet if the system barycentre resides outside the primary.
It should be noted that the world Ceres (pardon the pun) was originally designated a planet when it was discovered in 1801; it was demoted about fifty years later when it became clear that there were millions of asteroids between Mars and Jupiter. A few years ago, with the discovery of thousands of trans-Neptunian objects (also called Kuiper Belt Objects), Pluto looked ready to suffer the same fate. Instead, the IAU working group decided that it was an error to require that planets have unique orbital characteristics.
I applaud this move because it applies a quantitative test and also recognizes that the solar system is not an empty Copernican orrery of concentric circles but rather a complex mish-mash of millions of bodies on all kinds of orbits. We know now that even the classical planets share their orbits: Jupiter shares its path with two clumps of "Trojan" asteroids, and it is a curious fact that the Moon's orbit is always concave to the Sun, which exerts twice the gravitational force on the Moon as the Earth. (But the barycenter of the Moon's orbit lies within the Earth, so it still counts as a satellite.)
Science should not stand still, and should be willing to change outmoded definitions regardless of their popularity. Now we just need to get a decent name for 2003 UB313 and its satellite.