Pluto and Planethood

Today NASA's "New Horizons" mission, launched just over 13 months ago, will fly past Jupiter on its way to Pluto. The giant planet will provide a gravitational boost to the spacecraft, helping it reach the edge of our solar system by 2015. Over the past few decades, NASA satellites have visited Mercury, Venus, Mars, Jupiter, Saturn, Uranus, and Neptune -- but this is the first mission ever to visit Pluto. Meanwhile, astronomers have decided that Pluto should no longer be considered a planet.

To the average person, it probably seemed ridiculous when the International Astronomical Union announced last August that Pluto would henceforth be known as a "dwarf planet" -- the prototype of a new class of objects in the outer solar system. It even led to a satirical headline reading "NASA Launches Probe To Inform Pluto Of Demotion". However, there were legitimate scientific developments that compelled astronomers to adopt a new definition of planet -- it was just unfortunate that this new definition removed Pluto from the list.

Starting in 1992, astronomers began to discover many small icy objects outside the orbit of Neptune that appeared similar to comets, but which never came close enough to the Sun to evaporate and develop tails. As time went by and the technologies for detecting these objects improved, surveys began to identify some larger examples. In the last few years, astronomers found several that are comparable in size to Pluto -- and even one that is larger! Theories suggested that there were likely to be hundreds or thousands of such objects in the outer solar system, so classifying them all as new planets could create real problems for school children trying to remember them all.

Since Pluto appeared to be just one of the larger members of this class of objects, it fell victim to the new classification scheme. Like many political decisions, the available choices were limited to bad (define planet in a way that excludes Pluto) or worse (bestow the title of planet on hundreds of new objects). Conspicuously absent was an alternative proposal to adopt a new definition of planet that would avoid such proliferation, while honoring the historical status of Pluto as an exception to the new rule. When the IAU meets again 3 years from now, I suspect that such an alternative will be considered by the astronomers -- with plenty of time to spare before "New Horizons" reaches its final destination.