The IAU, or International Astronomical Union, is about to release its long awaited planet definition on Wednesday, with voting by member astronomers the next day. At stake primarily is the status of the current planet Pluto, as well as other Kuiper Belt Objects like 2003 UB313. After a year's debate, a seven member committee was formed to create the definition.
"Yes, it is very clear within what will be released … [Pluto] is very specifically in or out," said Richard Binzel, an MIT planetary scientist on a seven-person committee that developed the definition.
"I think we have come up with a very reasonable definition that in the end will be widely adopted," Binzel said by telephone from Prague today. "And we will move forward. I think all will agree that it's time to move forward."
It has been suggested that the definiton actually defines planets into three subtypes -- terrestrial/rocky (Mercury, Venus, Earth and Mars), gas giants (Jupiter, Saturn, Uranus, and Neptune) and icy dwarfs (Pluto, UB313/Xena, and perhaps as many as a dozen or more others).
Personally, I like the idea, if it comes to pass. If it has enough gravity to form into a round shape, and it is orbiting a star, then I think it should be a planet. Mercury and Jupiter really don't have that much in common, no more so than Pluto and Jupiter do. One strike against Pluto is an eccentric orbit, but it doesn't appear that unusual if you also take into account extra-solar planetary systems.
Historically, the asteroid Ceres was considered a planet for a number of years, so if the new definition fits it (at under 1000 km, it's about as small as you can get and still be round) or some other objects, I don't have a problem with it. From what I can gather there are about 9 or 10 KBOs that are around 900 km in diameter or larger, not including Pluto (2300 km) and its satellite Charon (1200 km). Call them icy dwarfs and end the controversy, I say.