Pluto - A Planet No Longer

At the IAU conference in Prague, Czechoslovakia this week, astronomers voted to reclassify Pluto into a new category of solar system object -- a dwarf planet. This class of objects would include Pluto, Sedna, Quaoar, and the recently discovered 2003 UB313 (Xena).

While I understand why it happened, I am sorry that astronomers reached this particular decision. The reaction that I most like is the bumper sticker from Cafe Press that says "Honk If Pluto Is Still A Planet".

Pluto was discovered by Clyde Tombaugh in 1930, and for the last 75 years, has been part of the cultural consciousness of what defines the Solar System. It will be a while before popular culture catches up to the new definition.

Something similar happened in the nineteenth century. Ceres was discovered on January 1, 1801, and was declared to be a new planet. But it was soon realized that Ceres was the first of many similar objects. By the 1850s, astronomers had defined and named that new class of solar system objects -- asteroids -- and relegated Ceres, Pallas, Vesta, and all the other small objects discovered between the orbits of Mars and Jupiter to be members of this new group.

In the current view, Pluto was the first trans-Neptunian dwarf planet discovered. As technology -- both optical and sensor -- improved after the launch of the Hubble Space Telescope, more of these trans-Neptunian Kuiper Belt objects were found, and the current view is that there may well be thousands. Hence, the need to redefine and recategorize.

Regardless of the arguments and quibbling of astronomers, the New Horizons space probe continues on its way to Pluto. Launched on 19 January 2006 from Cape Canaveral, New Horizons crossed the Moon's orbit before midnight that day, aimed for a Jupiter flyby in February 2007 and ultimately, a flyby of the Pluto/Charon dwarf planet system in July of 2015. When that is completed, the initial reconnaissance of the major bodies in the Solar System, as known in the twentieth century, will be completed.