Putting Pluto in its place
The other night while watching PBS, I heard astrophysicist Neil DeGrasse Tyson give the most cogent explanation yet of why Pluto is no longer considered a planet.
"If Pluto were in the orbit around the sun Earth has," Tyson explained, "it would grow a tail. And that's no way for a planet to behave!"
If Pluto were in the orbit around the sun Earth has, it- like any other Kuiper Belt Object Kuiper Belt Object (KBO)- look like the picture at the left. It would be a very large comet.
Mercury, Venus, Earth and Mars are rocky worlds. Jupiter, Saturn, Uranus and Neptune are huge balls of gas (if you had a big enough bathtub, Saturn would actually float in it!), usually with metalic and perhaps liquid cores. But Pluto, Charon, Eris and the other KBO's are essentially "dirty snowballs-" just economy-sized comets in more or less stable orbits.
KBOs become comets when their orbits are perturbed and they are sent inward toward the sun. The gasses which form the ice become heated and start to boil away- forming the tail.
PLuto was one of countless thousands of KBO's. As long as it was the largest, it was sort of grandfathered in to the category "planet." Several KBO's almost as large as Pluto were discovered, though. And when Eris was discovered by Dr. Michael Brown of Cal Tech a few years ago, suddenly Pluto was only the second largest KBO- with the likelihood that there are hundreds of others as large as Pluto or larger out there.
Ultimately, Pluto was demoted in order to save schoolchildren from the necessity of having to memorize the names of a couple of hundred planets instead of only eight or nine.
Comments
Pluto and Eris are considerably larger than any comet; Pluto is estimated to be 75 percent rock, and Eris, being of higher density, is even more rocky. Neither have comet like orbits; neither ever come into the inner solar system. Any planet brought close enough to its parent star would develop a tail due to outgassing or sublimation of its atmosphere. Mercury has a small tail, and Earth, if put in Mercury's orbit, would develop a tail as well. This does not make any of these objects comets.
Now, to respond to your point.
You are correct in saying that dwarf planets are nonetheless planets. But we've been through this once before. Following the discovery of Ceres and its brief career as a mainstream planet, astronomers realized that it was only an eccentric example of a new class of solar system objects, subsequently called by the rather strange collective name asteriods. But Ceres and other large asteroids were nevertheless also known well before 2006 as planetoids, and often quite plainly called "minor planets."
As of last year there were more than 535,000 recognized minor planets, with 251,651 having orbits sufficiently well-established to ahve been assigned numbers. 16,154 had official names. An average of 3,000 or so were being discovered each month. While technically "minor planets" are, by definition, planets,
before 2006 the term was generally understood in all but the most technical usage to refer exclusively to Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.
You are correct in saying that all the IAC actually decided was that Pluto was not one of the eight classical planets. To go a step beyond your point, it's fair to ask what constitutes a "classical planet," since there is a vast difference between the rocky terrestrial planets and the gas giants.
If neither Pluto nor Eris have comet-like orbits, neither have classically planet-like orbits, either. Neither of their very eccentric orbits is anywhere close to the ecliptic, and Pluto's is so eccentric that it passes inside the orbit of Neptune every so often! I think you would agree that the hypothetical outgassing from Pluto or Eris were it in our orbit would differ both in degree and kind from that of Mercury, or of any of the other terrestrial planets were it in Mercury's orbit.
While the behavior of Pluto, Haumea, Makemake, Eris, Sedna, Quoror, or other large KBO's in earth orbit would not be identical to that of a comet, it would be closer to that of a comet than that of a classical terrestrial planet would be.
Actually, I sympathize with your position. My purpose (and Dr. Tyson's) was not to argue against Pluto being regarded as a planet of any kind, but rather to make accessible to the average person at least a sense of the difference between Pluto and the classical planets. As things stand, the complex artificial-seeming definition of a planet (or classical planet) the IAU has come up with is one the average member of the public simply doesn't grasp.
I agree that the definition the IAU came up with is inadequate, for precisely that reason among others. But Pluto is more like a comet than either a terrestrial planet or a gas giant, and Dr. Tyson's statement was meant to point that out in order to help people get at least a sense of the reason for the controversy.