Thursday, February 23, 2006
Pluto Makes a Case for Planethood
Not even the sun's inner planets have rings. It seems Pluto may have them, according to a research team from the Southwest Research Institute. 
Not even the solar system's inner planets (all much larger than Pluto) have rings.
The controversy in astronomical circles is whether or not Pluto qualifies as a true planet. It seems there's not such a cut-and-dried boundary, especially since an object larger than Pluto has been discovered orbiting the sun a bit farther out. If Pluto is a planet, then this other body certainly deserves the classification. But what if dozens of Pluto-sized orbs are found in the cold orbits beyond Neptune? Do they all get the designation?
Rings or not, I still think the better case is to demote Pluto from 
planethood. Look at the size over to the left. The moon is about one-half bigger than Pluto. If we do find dozens of buddies for Pluto, where would we draw the line between planet status and big comet? Pluto is about 1300 miles across. Is the dividing line 1000? 500?
Charon, named for the boat captain who ferried the dead across the river Styx to Pluto's underworld realm, is a bit bigger than Texas. The new moons are about the size of a county: 36 and 30 miles across each, as far as we can tell. It will be interesting to see what they are named. Proserpina, the consort of the underworld god, will probably be reserved for that larger body. Moons are always named for people, and Pluto 
has some associates: Sisyphus (condemned to roll that stone up the hill), Cerberus, the three-headed hound. Maybe Orpheus and Eurydice. Any would be better than P1 and P2.
More of a concern will be the aim of the New Horizons probe when it arrive at Pluto in 2015. The moons P1 and P2 are farther out from Pluto than the large moon Charon. The four rings systems of the outer planets are all contained around and inside the orbits of the largest moons. If Pluto has rings they might be in the path set for New Horizons.
