Pluto's Brave New Worlds
Pluto has become the butt of jokes lately, replacing Uranus as the solar system's laughingstock -- and all because scientists find themselves forced, at last, to come to terms with the meaning of the word "planet."
Tacit definitions have existed since ancient times, when planetai, meaning wanderers, applied to seven moving lights in the sky: the sun, the moon, Mercury, Venus, Mars, Jupiter and Saturn. But telescopes have revealed more objects in the solar system than were dreamt of in ancient philosophy, and new discoveries demand strict, useful terminology that will help astronomers categorize a host of newfound worlds.
Pluto, discovered in 1930, was hailed as a planet before its true nature came to light. In time, Pluto proved to be far smaller than any of the other planets, and very unlike them in the way it orbits the Sun at an exaggerated tilt. Even so, there seemed no need to coin a new designation for Pluto, and it held on to its planet classification. But in 1992 astronomers made the first of what now amount to several hundred sightings of other solar system bodies at the distance of Pluto and beyond. Suddenly there was reason to reclassify Pluto as a member of this new society, which quickly became known as "trans-Neptunian objects" or "Kuiper Belt objects," in honor of Gerard Kuiper (1905-1973), who had predicted a vast zone of small bodies in the environs of Pluto.
As astronomers began to debate the issue, it spilled over into popular awareness and ignited considerable heat, for the planets are held in common, in awe, by all humankind. What might have constituted a purely scientific discussion, akin to deciding whether a particular tree was coniferous or deciduous, instead became public discourse.
On one side were major-planet purists who felt that bodies smaller than 1,500 miles in diameter (a size calculated to eliminate Pluto) should be dropped from the planet list. On the other side were Plutophiles who objected to arbitrary size discrimination. One Pluto specialist asked pointedly, "Is a dachshund not a dog?"
The lack of consensus on the "planet" definition struck people both within and outside the planetary science community as ludicrous, though several everyday terms we all think we understand are similarly vague. "Life," for example, poses semantic problems for biologists -- as well as for exobiologists, who hope to identify it if and when they find it on Mars or Europa.
In 2005, after a Kuiper Belt object tentatively named "Xena" turned out to be larger than Pluto, the question changed from "Should Pluto continue to be called the ninth planet?" to "Are there 10 planets in all?" Also in 2005, Ceres, a small body discovered more than 200 years ago between Mars and Jupiter and long dismissed as a "minor planet" or "asteroid," was observed by the Hubble telescope to be more or less round. Scientifically, a roundish object carries more weight than a potato-shaped one, because roundness signifies the greater mass required to pull itself into a ball, or "hydrostatic equilibrium." Round Ceres raised the question, "Are there perhaps 11 planets?"
Not for the first time, but with new urgency in 2006, the International Astronomical Union (IAU) impaneled a committee to define both the word "planet" and the status of Pluto.
Our committee -- seven in number, like the planets of old -- met at the Paris Observatory in late June and reached a unanimous agreement. In short: A planet is a body in orbit around a star (as opposed to orbiting another planet) and big enough for gravity to make it round. The full text of our proposed definition is being released today, to be discussed by astronomers from around the world, now in Prague at the IAU General Assembly, and voted upon next week. If approved, our resolution will not only leave Pluto in place but will also add "Xena" (2003 UB313) and Ceres to the current census of planets -- with room for additions as future discoveries warrant.
What's more, Pluto will lend its name to a newly defined category of planets -- the "plutons" -- which differ from the other planets by virtue of their highly inclined, elongated orbits, which take more than two centuries to complete and which suggest a different origin. As the prototype of this class, Pluto may still attract funny remarks, but it will have gained new significance.
Dava Sobel, author of "Longitude," "Galileo's Daughter," and "The Planets," served as the sole non-scientist on the Planet Definition Committee.