In fact, the number of planets has never been fixed: ancient astronomers knew of five, and European astronomers added to this count incrementally. And then there are tens of thousands of asteroids, comets or other minor planets. What defines a planet: size, orbit, composition or some combinations thereof? How can we discover distant planets when we can’t agree on what defines our own planetary neighborhood?

Labels matter. And unless you were on Pluto, you probably heard that in August, the IAU attempted to settle the matter with a tripartite definition. They specified that: a planet must orbit the Sun, be spherical (resembling a tomato rather than a potato) and have cleared its orbit of other bodies. Eight planets meet these criteria. The third criterion disqualifies Pluto since many so-called “plutinos” have similar orbits. Pluto, along with asteroid 1 Ceres (discovered in 1801) and Eris (formerly Xena) are henceforth “dwarf planets.” 

Roundness is a gravitational necessity of any object of reasonable size, so there is some agreement there. Clearing the orbit is less precise, but quantitative arguments can be made concerning the relative masses. But why not just define the “Big-8” as planets? For my part, I think the requirement that a planet orbit the Sun remains problematic: what are we to call large, round and massive objects found orbiting other stars? Aren’t they planets as well?

Alas: Pluto was no longer a planet, and many considered this a demotion an insult. While reasonable people can disagree, they were few in number at the General Assembly, and there is today no clear consensus among amateurs, professional or planet lovers. So it seem the matter is far from settled, and the IAU will have a “do-over” in about three years. What Pluto or Clyde Tombaugh thought of all this hubbub is not known. 

But ideas matter much more than labels, especially when they challenge us. I believe that January’s launch of New Horizons was a far more significant event. This space probe is well on its way: the gravity assist from Jupiter will occur just three months from now. The mission objectives are to map Pluto and its moons Charon, Nix and Hydra and to characterize their temperature, composition and atmospheres. Then and for the next five years, New Horizons will explore icy Kuiper Belt Objects, the likely source of comets and representing materials of the primordial solar system as far aways as 50 AU. Rather than debatable definitions, New Horizons will stream us new data and images and inspire novel concepts to discuss. The true meaning and utility of the label “planet” are sure to become much better resolved.

We have walked on the Moon and visited each of the other eight “true” planets, even 6-wheeled around Mars, and encountered many comets and asteroids. Yet, for now, Pluto remains unexplored. In 2015, New Horizons with Clyde Tombaugh aboard, will arrive and explore Pluto, whatever you call it.

Moondark is written by Doug Miller, published at the Moondark web site, and printed in the Delmarva Star Gazers' Star Gazer News. This document was last revised on 26 November 2006. Text and images copyright © 2006 by Douglas C. Miller, All Rights Reserved. This material may not be reproduced in any form without prior permission.