What is a planet?

This week's episode of Nova on PBS, "The Pluto Files", reminded me of a story.  It was around the beginning of a new century, and an astronomer was predicting that a planet existed which had yet to be discovered, and he organized a search for that planet.  An object was observed and declared to be a new planet.  However, the new planet was quite small, and other objects were later found to orbit in roughly the same part of the solar system, so after half a century or so, a large number of astronomers stopped calling it a planet.  Then, in 2006, a proposal was made to the International Astronomical Union that would have included it as a planet, but this was rejected because it shared its region of space with other objects, so it became reclassified as a "dwarf planet".

No, I'm not talking about the subject of the Nova episode, Pluto.  I am talking about the asteroid Ceres, which as discovered by Giuseppe Piazzi on the first day of the 19th century, 1801 January 1 (JD 2378862), after a missing planet between Mars and Jupiter was predicted by Johann Bode.  The Kuiper belt object (KBO) Pluto was discovered by Clyde Tombaugh on 1930 February 18 (JD 2426026) after a missing planet was predicted beyond the orbit of Neptune by Percival Lowell, who had begun the search in 1906.

However, Lowell's Planet X was supposed to be massive enough to perturb the orbits of the gas giants Uranus and Neptune, and Pluto was much dimmer than expected for such a large planet.  We now know that the prediction was based on inaccurate observations.  Over time, estimates of its size continued to shrink, until the discovery of a moon in 1978 (JD 2443682) allowed for a precise calculation of its size, being only 18% the mass of earth's moon or 0.2% the mass of earth, itself, although it's still at least ten times heavier than Ceres, and twice as wide.  But what really did Pluto in was the discovery of numerous other objects in the Kuiper belt, some nearly as large, and at least one, Eris, that was larger.  It was the discovery of Eris in 2005 (JD 2453376) which forced the issue, since either Eris had to be considered a planet or Pluto not one, as well as the possibility of other new planets being discovered, and the question of where to draw the line between Pluto and its slightly smaller neighbors.

So in the end, as described in the Nova episode, it was voted (JD 2453972) that not only must a planet be round and orbit the sun, but it must also contain most of the mass in its vicinity.  This excludes Pluto and its new-found neighbors, and also continues to exclude Ceres, the only asteroid large enough to be round.  These round objects are now called "dwarf planets".  This new distinction between "planet" and "dwarf planet" is still controversial, because Pluto still has its fans, especially those astronomers whose careers are based on it, but I do not see how any other definition would be better.  As I described above, this is not the first time a planet has been demoted, and for the same reasons.  Either we have one less planet, or we must allow for new ones.

But it is really just a matter of semantics.  Pluto is no more or less important because we call it a planet or the first of a different kind of object, not the biggest but perhaps the closest and certainly the brightest.  I am excited that in five years both Ceres and Pluto will be visited by unmanned spacecraft for the first time, allowing us to see what these dwarf planets look like in detail.

JD 2455260.033