One of Our Planets is Missing
No, it's not the action of a neutronium-clad doomsday machine, nor a huge, sperical space station terrorizing the galaxy. Rather, a humble, but sometimes raucus group of astronomers in Prague decided yesterday to downgrade Pluto's status leaving our Solar System with only eight planets.
This is directly in contradiction to earlier reports of a new definition for a planet that would have included not just Pluto, but three other known objects and the potential for dozens of large objects that have yet to be discovered.
According to the International Astronomical Union:
...a "planet" is defined as a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
Pluto is now considered a "Dwarf Planet", which has a definition similar to the above except for the end which now reads "...(c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite." Pluto is joined by Ceres and 2003 UB313 in this somewhat dubious category.
History of the Problem
Pluto was discovered in 1930, and Clyde Tombaugh, the astronomer who found the object wanted to name it after Percival Lowell, so he found a Roman name that included Lowell's initials. At the time, Pluto was thought to be roughly the same size as the Earth, but eventually the object was slimmed down to a mere 1,413 miles across -- smaller than the Earth's moon.
Since the 1990s, astronomers have battled over Pluto's status. Many correctly point out that its unusual orbit and small size identify it as likely just a large piece of debris from the Kuiper belt. Ultimately every planet in this Solar System was made from such debris and the Kuiper belt beyond Neptune and the asteroid belt between Mars and Jupiter are just left over material. Public support and tradition have long been on Pluto's side, however. Textbooks and years of material have identified Pluto as a planet. When New York's Hayden Planetarium left Pluto off a model of the Solar System, it drew an enormous outcry, not just from the public, but from other astronomers who felt the museum had overreached its authority.
In 1999, the IAU had its first brush with this issue when a proposal was made to consider Pluto a planet associated with the Kuiper belt. Astronomers then were strongly opposed to reducing the planet's status. But the discovery of 2003 UB313 (unofficially called "Xena" and it's satellite is "Gabrielle") might have caused this issue to be revisited. Slightly larger than Pluto, "Xena" was surely going to become our Solar System's tenth planet. But it's position in the Kuiper belt and the possibility for more similarly-sized objects caused some to worry that the number of official planets to grow dramatically. Might the Sun be home to 15, 20, 30 planets ultimately? Clearly, the matter needed to be pinned down. What is a planet after all?
Amazingly, until yesterday, there was no official astronomical definition for a planet. The IAU settled the problem by arguing over several different definitions, ultimately selecting one that kicked Pluto out of the club after two years of frustrating efforts. Indeed, the first committee to look at the problem, a group of astronomers, could not agree on a scientifically accurate definition. A second committee comprised of historians and educators was formed a few months ago to look at the problem from a fresh perspective.
Alan Stern, the head of NASA's New Horizons mission to Pluto, is outraged that less than five percent of the world's astronomers had a vote on the issue. Others think that the new category for Pluto is awkward when it says "A dwarf planet isn't a planet." Ultimately, I think the definition needs to be refined. Read it again. Then look at this chart. By the official definition currently accepted by the IAU, the Earth itself is not a planet. The existing definition not only places Earth on the questionable list, it also will present problems when we begin to identify planets in younger solar systems orbiting other stars. Inevitably, the issue will have to be revisited, and at that time Pluto may be restored to planetary status.
Clyde Tombaugh, may you rest in peace.