The Meaning of Pluto

Pluto was always a little different. It was smaller and slower than the other planets, and its orbit was awkward too. Little Pluto never did conform to the tidy rules of the Victorian orrery. The discovery of its partner in 1978 did little to help; Pluto proved smaller than anyone had imagined and as Charon's diameter was over half that of Pluto, their relationship was scandalously non-traditional. That an atmosphere was found in 1988 seemed to matter little, and the two moonlets found in 2005, Nyx and Hydra, merely added to the head-scratching. Even the most tolerant astronomers had to admit Pluto was a special planet, with special needs.

But what caused Pluto to be cast out was not that it was special, but rather that it turned out to be not so special after all. Starting with 1992 QB1, astronomers discovered thousands of bodies in Pluto's neighborhood. Astronomers began to wonder if Pluto, like Ceres before it, should be demoted to the status of merely the largest asteroid in its belt. But it wasn't until 2003 UB313 ("Xena") came knocking at the door that astronomers realized they had to face the question they had avoided for 75 years: did Pluto really belong in the planetary club? One aberration might be overlooked, but must they now admit all of Pluto's strange brothers and sisters too?

The issue aroused passion in the general public, and it is not hard to see why. Some people saw Pluto as the outsider that just wanted to belong; others saw Pluto as a runt that had no business sitting at the table with its betters. Some welcomed the notion of co-orbital and double planetary systems, celebrating a new-found diversity in stellar systems; others saw the notion of honoring such queer arrangements as a threat to the sacred institution of planethood. In the end, it was the backlash against the bold decision of a few unelected astronomers to recognize at least three new planets, and potentially scores more, that led to the expulsion of Pluto altogether.

Unlike the treatment given to Ceres 150 years ago, however, astronomers did not demote Pluto completely. In place of true planethood, the IAU has created a new status of "dwarf planet" for Pluto. Separate, but not equal. There's a lesson in there somewhere.