Almost Paradise

It’s been in the newspapers this week: the International Astronomical Union has decided that it’s time to welcome a few new members into the solar system’s roster of nine planets. And while they were at it, the astronomers’ union decided to clarify the definition of exactly what counts as a planet.

In the last few years, astronomers have gotten really good at finding things in the far reaches of the solar system out beyond the orbit of Neptune. There’s Pluto, of course — but there are a lot of other rocks too, some almost as big. There’s even one that appears to be a little bigger than Pluto, a rock called 2003 UB₃₁₃ (nicknamed “Xena” until they assign it an official name) that has a moon of its own (nicknamed, of course, “Gabrielle”).

So how do you decide what’s just a rock, and what’s a planet? The IAU came up with two criteria: (1) it has to be big enough that its gravity has pulled it into a round shape; and (2) it has to be orbiting the sun itself, rather than as the satellite of another body (which would make it a moon). Given those criteria, it turns out that there are three really big rocks floating out there that now qualify as planets:

• Ceres, the biggest of the asteroids, between the orbits of Mars and Jupiter
• Charon, the object formerly known as Pluto’s moon
• 2003 UB₃₁₃, interim name “Xena”

Hey hold on, how come Charon gets to be a planet, if it’s Pluto’s moon? Well, the IAU also decided that Charon isn’t really a moon, because Charon is big enough (about 12% the mass of Pluto) that the center of mass of the Pluto-Charon system is a point in space between the two of them. That makes Pluto and Charon a double planet. By contrast, the next biggest moon in comparison to its planet is our own Moon, which is only 1% the mass of Earth. The center of mass of the Earth-Moon system is about 1700 km below the Earth’s surface, so our Moon is indeed still a moon.

The new planetary lineup now looks like:

(from the IAU Q&A page)

The IAU came up with a new category, “plutons”, to describe Pluto, Charon, Xena, and other planets like them out in trans-Neptunian space. And there may even be more planet-class rocks out there. There are a dozen more objects that have been sighted, but not observed thoroughly enough yet, that astronomers suspect will also meet the new criteria. Five are asteroids, and the other seven are plutons:

Time to rewrite the textbooks!

This entry was posted on Friday, August 18th, 2006 at 10:27 pm and is filed under Sci/Tech, General. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.