Everything you know (about planets) is wrong

Right now the International Astronomical Union (IAU) is convening for their 26th General Assembly (or the XXVIth for you Romans).  Among their tasks is to formally define the idea of a planet and in turn, decide just how many planets are in this solar system, anyway. 

Today they've drafted a resolution which creates not only a definition for planets, but includes a definition for a new class of object called a "pluton" (including Pluto, its companion Charon, and the as-yet-unnamed 2003 UB313, an object similar to Pluto that's probably a bit larger but further away).  Should these definitions be accepted by the mass of eager astronomers, we'll suddenly have twelve planets! 


Image from here.

We're due for such a discussion, officially.  Modern observational techniques continue to find new examples to confound the generally accepted idea of a planet. 2003 UB313 (popularly called "Xena" but the IAU won't touch that one--anyway it's not "official" yet) is at least as big as Pluto but has a much more eccentric orbit.  And astronomers regularly find new objects beyond Pluto (an area with a great deal of dust and ice called the Kuiper belt).

Details can be found on the IAU's excellent question and answer page.  But I'll summarize the questions I had.

1. So what's a planet?
To be a planet, an object must satisfy two criteria.  First, the object has to orbit a star.  Second, it has to be massive enough such that it's mostly spherical.  (Size and shape are related.  If an object has a mass above 5 x 10^20 kg and diameter greater than 800 km, its own gravity will tend to form it into a spherical shape in time.  This is really pretty small if you think about it.  800 km is about 500 miles.  Pluto, the currently-accepted smallest planet, has a diameter of about 2300 km.)

2. There were nine.  Now twelve?  What the...?  Where did the extras come from?
I personally thought this was mainly about whether Pluto and 2003 UB313 were going to stay, or get into, the planetary club.  But the IAU proposal makes a point of not only letting them in, but further recognizing Charon and the largest asteroid, Ceres.  Not only that, but if the proposal gets approved, there are twelve more candidates for planethood (other plutons and asteroids), pending further observations and refinement of the definition.  And there will undoubtedly be other Kuiper belt objects found that will enter the planetary debate.

3. Where do they get off counting Charon?  Isn't it a moon?
The IAU considers Pluto and Charon a different breed altogether, calling them a "double planet" instead of a planet and satellite, because the center of mass of the system (the "barycenter") is above Pluto's surface (i.e., in the space between the two).  All moons orbit planets around the center of mass, but for the Earth's Moon and other large satellites around the outer planets, the barycenter is well below the planet's surface.  (I won't get into a big physics discussion about orbits and barycenters now, but have a look at the wikipedia definition here for an explanation and nice animations of the concept.)

Posted at 09:55AM Aug 16, 2006 by WILSON, JOSHUA in Bits of news  |  Comments[2]