Reprieve for Pluto
Revisiting one of its favorite debates, the International Astronomical Union, meeting in the Prague, is trying to resolve whether Pluto is a planet or not. The problem is that there has never been an internationally accepted definition for planet. Until now defined planets using the Justice Potter Stewart definition of pornography, "I know it when I see it." The nine planets that we all learned in school do not easily fit into a single category that is very meaningful to science.
The word "planet" comes from an old Greek word for wanderer--"planeten." It described the way certain bodies appeared to move across the night sky. The stars all appear to stay in the same place, compared to each other, with the whole starry canopy circling the pole. The five known planets appeared detached from the night sky moving in paths that were independent and mysterious. The name planet has nothing to do with the origin or structure of the planets and isn't that useful for modern astronomy. The inner planets are small, rocky, and have very few moons. The outer planets are gigantic, gaseous, and have elaborate systems of moons. And then there is Pluto.
Ever since Clyde Tombaugh discovered Pluto in 1930, it has been an astronomical oddball. Astronomers at the time believed that we needed a large ninth planet to account for certain irregularities in the orbits of Uranus and Neptune. While they were excited about the discovery of Pluto, it was clear from the beginning that it was too small to be the longed for Planet X. As time went by, better observations showed that Pluto was even smaller than at first believed--smaller than the Earth's Moon--and that it had an irregular orbit far different that that of any other planet. That didn't stop generations of little kids from memorizing Pluto as one of the planets.
In the early nineties, astronomers began to confirm the existence of Kuiper Belt objects beyond the orbit of Neptune. These objects are various sized icy bodies that orbit in a disc extending from the orbit of Neptune to a distance about twice that far. While they might be the source of some of the moons of the outer planets and short-term comets, they are separate from the Oort cloud, which extends much further out and is the source of most comets. There was an immediate move by some astronomers to downgrade Pluto from planet to Kuiper object. My own feeling at the time was that Pluto probably is a Kuiper object but that it was extremely insensitive to tackle the subject while Clyde Tombaugh was still alive (he died in 1997 at the age of 91).
The International Astronomical Union proposed a definition for planet yesterday, but has still to vote on it. The new definition is that a planet must be big enough for its gravity to pull it into a spherical shape. This definition would include the inner planets, the outer planets, Pluto, about four of the asteroids between Mars and Jupiter, about four other Kuiper objects, and the list will be sure to grow as other large Kuiper objects are discovered. It also requires a planet to orbit a star and not another planet; that excludes all of the moons in the solar system.
This looks to be a compromise that will satisfy nobody. The definition, including four distinct types of objects is too broad to be of much scientific use. By doubling the number of planets, it’s too abrupt of a change to be embraced by popular culture. Basically, it's the type of decision that will mostly appeal to junior high nerds who will now get to tell their family and friends that they are wrong about a piece of common knowledge. “Ha ha, you can’t name the planets.” The family and friends will then knock the nerds down and steal their pants as they have done to generations of nerds in the past. And thus the cycle of life continues.