Bulldoze Pluto? I Don’t Think So

By Jeffrey Bennett
University of Colorado
posted: 31 August 2006
10:05 am ET

The International Astronomical Union (IAU) has spoken on the status of Pluto. The only thing missing when they announced the decision at their press conference was the “Mission Accomplished” banner. Yes, I’m afraid this matter is about as settled as the Iraq war in 2003.

You’ve probably heard the basics: Pluto is no longer to be considered a “real” planet, but will instead be part of a new class of objects called “dwarf planets.” These midgets may number anywhere between a handful and hundreds in our own solar system, depending on how you count them and how hard we search for them with more powerful telescopes. But no matter how you cut it, this new definition takes away any pretense of Pluto being a member of the same elite, planetary club as Earth and Jupiter.

As a scientist, I think this was a pretty good outcome, though some of the justifications used to achieve it are dubious. But as a teacher, textbook writer, and builder of scale model solar systems, I have some reservations. In particular, for the model solar systems I’ve helped develop on the University of Colorado Boulder campus and on the National Mall in Washington, DC, I have to ask: Should we now bulldoze Pluto?

A little background: Pluto was discovered in 1930, at a time when astronomers were searching for an object thought to be causing slight perturbations to the orbits of other planets around the Sun. Neptune itself had been discovered in just this way in 1846, after scientists used perturbations in Uranus’s orbit to predict the existence and location of an “eighth planet.” Neptune was found as soon as astronomers pointed telescopes to the calculated position, which is I like to say that Neptune was discovered with physics and mathematics, and only confirmed with a telescope.

Over the ensuing decades, a few scientists claimed to see ongoing orbital discrepancies and embarked on a search for a “ninth planet” that might be causing them. Pluto was found during this search, though about 12 full-moon-widths away from the predicted position. And though hailed as a planet upon discovery, its status gradually became suspect, as we learned that its orbit is much more tilted and elongated than that of any of the other planets, and that it has a mass much less than 1% that of Earth. Worse, reanalysis of past observations suggested that the claimed orbital discrepancies had simply been errors in measurement, making Pluto a solution to a nonexistent problem.