Political Cortex: Brain Food for the Body Politic

Most of those who wanted to throw Pluto back into the camp of "minor planets" -- another vague definition that's been used as a blanket term for both asteroids and comets -- have done so because Pluto hasn't met their size limits .  At only 18% the diameter of Earth, Pluto is the smallest of the planets by far (Mercury, the next smallest, is over twice as large at 38% of Earth).  There's also that pesky tilt of Pluto's orbit that indicates that it might not have formed from the same disk of dust that gave rise to its larger siblings.  Finally, there's the eccentricity of Pluto's orbit making it at times the eighth, not the ninth, most distant planet from the sun.

The debate over Pluto's fate has scrambled back and forth for decades, but at last it looks like the pro-Pluto forces have scored a decisive victory.  But don't go thinking this means your foam projects and wall posters are safe from change.

Because what the International Astronomical Union did was more than just decide to bury the hatchet over Pluto.  They decided, for the first time, to put a rule around the word "planet" that weren't just based on an arbitrary size limit.

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 shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.

It's a brief definition, and it really breaks down into two part test of planethood:

1. A planet has to be large enough that gravity forces it to be nearly round (that's the "hydrostatic equilibrium bit)

2. A planet has to orbit a star, not another planet.  Something that orbits a body other than a star is a moon.

By this rule, Pluto is most definitely a planet.  Earth's moon, which passes the round test, fails on step two because it orbits the Earth rather than going around the sun on it's own.  So... cool.  The trouble is, there are already at least three other objects that fit these rules, and a good dozen more that make decent candidates.  The new rules are not yet official, but if the AIU votes in the rule on August 24th, it'll be a bonanza for textbook companies and sellers of Styrofoam.

New planets under the revised rules

The nearest of our newly anointed planets is Ceres.  When it was discovered in 1801, Ceres was considered a planet from the outset.  It was only the discovery of may other objects that caused Ceres to be demoted to the largest of the asteroids.

The candidate planet Vesta and new planet Ceres shown next to the Earth's moon for a size comparison.

The next of the new trio is Charon.  Charon is commonly called a moon of Pluto, and you might think it would fail on step 2 of the new rules.  However, Pluto and Charon actually orbit a point in space that lies between the pair, so they would become something unique in the solar system (so far): a double planetary system.

Pluto and Charon

To get to the next new planet, which still carries the label 2003 UB313 (the discoverer nicknamed it "Xena," as in "warrior princess," but the stogy AIU doesn't want to accept that name) you have to go up to twice as far as the already hugely distant Pluto -- though like Pluto, 2003 UB313 has a very elliptical orbit that at very rare times can actually make it closer than Pluto.  It also appears that "Xena" is actually a bit larger than Pluto, so any system that accepts Pluto as a planet would have a hard time keeping the warrior princess on the sidelines.

Despite the consistency of the new rule, it's still possible that the AIU could reject the new rules on the 24th, leaving us with and arbitrary definition for planethood, but protecting a million sixth grade science projects. I'm hoping for a yes vote.  Right now, I'm getting out a tiny little crumb of foam -- appropriate to Ceres less than 1000km diameter -- and getting ready to stick it between the red pea-sized Mars and that big basketball-sized Jupiter.