Ceres, Charon and UB313

If you've been feeling a little cosmically cramped lately, relax. This part of the Milky Way, as of today, is 11% less crowded. Officially, at least.

International Astronomical Union members are meeting in Prague this month and, despite the rarity of this event traditionally grabbing lots of headline space, have repeatedly made front-page news this month with the announcement of redefining the official definition of "planet." It seemed three new planets would be added to our old familiar solar-system model: Ceres, Charon and UB313, but in the end the International Astronomical Union decided it was not to be.

Though not seen as such at the time, the problem of distinguishing bodies such as "planets," the en vogue "plutons" and even "satellites" began back in 1930 when Clyde Tombaugh discovered Pluto. Pluto accounted for some of the "missing" gravitational forces then extant in astronomers' calculations. Much to astronomers' chagrin, however, subsequent calculations showed that Pluto was too small to account for the hole in mathematics.

California Institute of Technology astronomer Michael Brown reopened the seventy-year-old can of worms in 2003 when he discovered a large cold heavenly body beyond the orbit of Pluto that was in fact larger than the ninth planet: a Pluto-sized object provisionally called UB313.

Realizing that, in fact, no definition of "planet" officially existed, the 9000-plus members of the IAU started working on the problem. A new definition, which appeared as though it will pass in the 24 August vote, stated that a planet is "a celestial body that has sufficient mass for gravity to make the body roughly spherical, and orbits a star, while not being a star or a planet's satellite. Sizewise, a planet must be 500 miles in diameter and have a mass of 1/12000th that of Earth or greater. A seemingly less popular proposal had "planet" defined as closer, larger bodies and would actually eliminate Pluto from the planetary roster and give the solar system eight planets. What was ultimately adopted was a compromise between the two claims: Added was the requirement that the body in question must "clear the neighborhood around its orbit." Since Pluto's orbit drunkenly encroaches upon Neptune's at times in its long path around the sun, Pluto is no longer considerd a planet.

And in one fell swoop, a solar system that might have become larger shrank. Not included in 2007-edition textbooks will be the status of "planet" for Pluto, Ceres, Charon and UB313 (a.k.a. Xena).

Ceres has been known as our solar system's largest asteroid since its discovery in 1801 by Guiseppe Piazzi and was actually called a planet for part of the 19th century. Originally known as Ceres Ferdinandea, Ceres is some 950 kilometers across, making it about half as large as Pluto. Ceres is an atypical asteroid not only due to its size (indeed, Ceres is large enough to sometimes allow naked-eye visibility), but also its composition. Ceres is thought to reach a max temperature of -38°C (warm by solar system standards) and may actually sport an atmosphere. Cornell University's Peter Thomas recently postulated that Ceres may have a mantle containing 200 million cubic kilometers of water, making Ceres infinitely wetter than the wettest dreams about Mars.

Charon was originally thought to be a moon of distant Pluto, but may gain bragging rights as a full-on planet in the new IAU definition. Charon was discovered in 1978 and was originally thought to be a simple moon of Pluto; cheerily enough, Charon was named for the mythological figure who ferried dead souls to Hell. Charon is imagined as essentially a huge iceball which may have formed due to collision with Pluto or another body billions of years ago. Astronomers recently discovered that Charon is about half the size of Pluto and the two bodies actually orbit one another with a gravity well somewhere close to Pluto.

Finally, there's UB313, currently known as Xena at present, a name which will perhaps become fodder for future IAU conventions to rechristen. According to IAU rules, the name "Xena" cannot stand because all "trans-Neptunian objects" such as UB313 must be named after deities of creation. Some think UB313 will be known as Persephone, though an asteroid currently holds this name. UB313 went undiscovered for so long mainly because of its distance and its extremely slow orbital period of 550-plus years. Little is known about UB313 physically, though astronomers figure there is methane or methane ice there.

Citizens of Earth, let's give Pluto Ceres, Charon and UB313 a big hand as they exit our planetary club. Though perhaps a sad occasion, at least some of us here are happy. Most believe that the more liberal definition of planet. means some twelve more bodies are destined to become honest-to-Lowell "planets," and some postulate a solar system of 200 planets within ten years. One can only wonder what effect this might have had on high school science students and teachers. Not to mention the daily horoscope…