This artist’s impression displays a dozen candidate planets currently listed on the International Astronomical Union’s watchlist next to the Earth for scale. The number of planets in our solar system could increase significantly in coming years. Or not. First, the IAU must determine what a planet actually is. Credit: The IAU / Martin Kornmesser

Here’s the latest from Prague, where Sky & Telescope’s Richard Fienberg reports on the ongoing debate between astronomers seeking to pin down what defines a planet:

The key provision of the twice-revised definition reads as follows: “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 the dominant object in its local population zone, and (c) is in orbit around the Sun.” (For now astronomers are trying to define what “planet” means only in our own solar system, avoiding the complication of planets around other stars or, worse, floating in free space.)

Whereas last week’s wording proposed a class of Pluto-like planets called “plutons” in highly elongated, tilted, 200-plus-year orbits, the new wording instead defines a class of hyphenated “dwarf-planets” that meet criteria (a) and (c) above, but not (b). This class includes Pluto, the slightly larger object provisionally known as 2003 UB₃₁₃, and probably several other icy bodies in the Kuiper Belt beyond Neptune. It also includes Ceres, the biggest of the rocky asteroids circling the Sun between the orbits of Mars and Jupiter.

After reading through the details, I rather prefer this to the original draft proposal unveiled last week. I suspect it still has problems, but perhaps fewer complications. I’m only confident of two things here, though: 1) all bets are off come this Thursday when the final vote takes place; 2) I’m really glad it’s not my job to determine what is or is not a planet. While it’s all too easy to snipe from the comfort of my monitor’s glow, hammering out these ground rules is no simple task.

Added Aug 23: Video is available of yesterday’s proceedings, illustrating the revisions underway to the IAU’s initial draft of their planetary guidelines. Click the play icon to watch in Windows media format. [Running time: 1:03:02]

Further reading:

• Planetary Definition Showdown! - Space.com
• What’s in a name? - Jim Bell, guest commentator at The Planetary Society weblog
• Time to Make a Decision - Robert Naeye, Sky & Telescope