10th planet?

Astromers have finally nailed down the size of a distant object dubbed "Xena." First announced in July of 2005, the icy ball was announced as being possibly 30% larger than Pluto, the ninth planet. More precise measurements led by Michael Brown of Caltech (see CNN article here) have constrained the diameter to 2,397 km (1,490 miles), plus or minus 100 km. This compares to the diameter estimate of Pluto at 2,288 km (1422 miles). So Xena is only slightly (5%) larger than Pluto.

Is Xena a planet? That's a good question. The body of people who make that designation, the International Astronomer's Union (IAU) has been debating the issue but no decision has been made. It's part of the Kuiper Belt, which is a zone that extends from the orbit of Nepture at about 30 AU to 50 AU (AU = Astronmical Unit, by the way, where 1 AU is the mean distance from the Earth to the Sun). There are a lot of bodies in the Kuiper Belt - according to a dusty into textbook on my shelf from 1999, there were 60 objects greater than 100 km in diameter found by early 1998. If you assume the small slice of the sky that was searched is representative of the whole belt, you get a number around 70,000 objects larger than 100 km. I don't know what the most current estimates are, but it's still going to be a big number.

Deciding a cutoff diameter for KBOs (Kuiper Belt Objects) that represents a planet is tricky. An analogous situation would be trying to decide when a hill is a mountain? Something that is a small mountain on the east cost may only be a big hill in Colorado, for example, so context matters. Here's the silliest arguement for why Xena shouldn't be considered a planet: it will mess up the mnemonic device for remembering the order of the planets.

My Very Earnest Mother Just Served Us Nine Pickles.

Which stands for, of course, "Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto." It's going to be pretty hard to work in a planet that starts with the letter X into this scheme...