What’s a Planet?

Tuesday afternoon The Space Telescope Science Institute, which operates the Hubble Telescope, reports that it has taken its first image of the putative tenth planet, the one identified last summer, far beyond the orbit of Pluto, by Caltech astronomer Mike Brown.  He nicknamed the object "Xena," after the warrior princess of syndicated-TV fame, but for now, the International Astronomical Union is sticking to the official designation 2003 UB313. Brown used to argue that Pluto didn’t qualify as a planet–too small, too unlike the other planets–but last year he said he’d changed his mind because, well, every preschooler thinks of Pluto as a planet.  Later, when followup observations suggested that 2003 UB313 was considerably larger than Pluto, Brown said the case was clear: it deserved to be a planet, for which he’d submitted a serious (as yet unannounced) name. The Hubble image muddies the water.  In it, 2003 UB313 is only 1.5 pixels across (the rest is glare).  It’s probably only slightly larger than Pluto–1,490 miles in diameter, vs. 1,422 miles for Pluto.  Maybe both of them ought only to be regarded as "Kuiper Belt Objects," part of the vast band of icy, rocky planetoids, asteroids and comets that orbit far beyond Neptune. (More information from NASA is HERE.  The Space Telescope Science Institute has more HERE.) What does this matter?  Well, science looks for order, and if not for these distant interlopers, the Solar System would be very orderly.  The inner planets–Mercury, Venus, Earth, Mars–are small and rocky; the outer planets–Jupiter, Saturn, Uranus, Neptune–are gas giants.  A noisy little movement in the 1990s said Pluto, at best, qualified as "other." So what makes a planet?  Anything spherical that orbits the Sun?  In that case there are a bunch of asteroids that qualify (the astrophysicist Neil Tyson smiles about the discovery of "the planet Ceres" between Mars and Jupiter in 1801).  Anything larger than Pluto?  In that case you could make an argument for several moons of Jupiter, Saturn and–oh, forgive me–Earth, since their size puts Pluto to shame, and, strictly speaking, they do circle the Sun.  Anyone got a better idea?  Science is waiting for one. —– (A quick note of thanks to those of you who caught our editing error from Monday.  Yes, I know the Moon isn’t a planet, and I’m pretty sure our editors do too–but one of them, in a rush, apparently changed the first line of the piece I wrote from "…an early step to send the first astronauts there…." to "…an early step to delivering the first astronauts to the planet…."  Correction made.)