Right from the moment of its discovery Pluto was proving itself to be something of a puzzle. Outside of having a very eccentric orbit around the Sun there was little we could say with any certainty about it. It was round and, being so far from the Sun, frozen but its size and composition was a mystery. Plus, the fact that Pluto seemed to exist out of context with the rest of the solar system only served to make it more of an oddball object.
Pluto remained a lonely anomaly until 1978 when astronomers discovered that it had a moon. Coincidentally, this find was made at the U.S. Naval Observatory in Flagstaff, AZ, just across town from where Clyde Tombaugh first discovered Pluto at the Lowell Observatory back in 1930. This companion to Pluto was named Charon and the two are gravitationally locked together in synchronous rotation. By watching the two orbit around one another astronomers were able to work out their mass and size. Pluto is some 1,433 miles in diameter while Charon is a little more than 750 miles across; both worlds could easily fit across the continental United States. Today there are a total of five known moons, all tightly packed together, and it’s thought that the system may be the debris left behind after something big collided with Pluto long ago.
In the 1990’s astronomers began to find even more objects within Pluto’s neighborhood; in fact, there are some 1,300 known icy objects in this outer zone of the solar system we now call the Kuiper Belt (in honor of astronomer Gerard Kuiper, one of the big proponents of its existence in the 1950’s). Some of these objects are icy boulders while others are nearly as big as Pluto and we suspect that there could be billions more out there.
All of these discoveries begged the question as to whether or not Pluto and other Kuiper Belt objects could rightfully be called planets or whether they should belong to a special category all their own. Much to the embarrassment of the astronomical community came the realization that there wasn’t a formal definition of what a planet actually is. The International Astronomical Union, the organization responsible for officially naming celestial objects, decided to rectify that. The IAU came up with three criteria an object must meet to be defined as a planet. First, the object must orbit the Sun. No problem there, Pluto certainly does that. Second, the object must be massive enough to have pulled itself into a spherical shape. Pluto meets this requirement too. And third, the object must be able to have cleared the neighborhood of its orbit. This means that a planet must be the dominant gravitational presence in its orbit. Like all the other classical planets it must have consumed smaller bodies or hurled them out of its way. Clearly, Pluto was not massive enough to have done that and, so, it was deemed not a planet. Instead, it belongs to the new category known as a dwarf planet. Rather than Pluto being the solar system’s ugly stepchild it had now found its rightful place as one of the largest members of its kind in a newly discovered realm of the solar system. Unfortunately the public and many in the astronomical community didn’t see it this way. There were organized protests and many an angry letter written to various scientists deemed as being “Pluto Killers”.
Perhaps in the end it doesn’t really matter what we call Pluto, what really matters is that Pluto is a world worth knowing and this month, on July 14th, after a nine and half year journey, NASA’s New Horizons spacecraft will make a flyby to image, map, study, and reconnoiter this distant world and its busy moon system. Up until now all of our images of Pluto have been nothing more than fuzzy blobs of pixels but now we should be getting high definition images of this mysterious world. After its flyby of Pluto the spacecraft will head deeper into the Kuiper Belt to explore. New Horizons is literally sailing into unknown waters and who knows what wonders await us there. Exploration and discovery is what makes the human adventure worth being a part of and I for one eagerly await what we will learn about Pluto and the other denizens of this remote corner of our solar system.
Also, in a fitting side note, New Horizons is carrying some of Clyde Tombaugh’s ashes out to the little world he discovered all those years ago in 1930. The inscription on the memorial reads:
Interned herein are remains of American Clyde W. Tombaugh, discoverer of Pluto and the solar system’s ‘third zone’. Adelle and Muron’s boy, Patricia’s husband, Annette and Alden’s father, astronomer, teacher, punster, and friend: Clyde W. Tombaugh (1906-1997).”
Darrell Heath works at the University of Arkansas at Little Rock and is a producer and host for the UALR Television show “The Night Sky.”