Just in case you don't know that Pluto isn't a Planet anymore

It was on the radio and news a few days ago and it's on MSN Today.

Thought this was pretty interesting.

This knob of rock and frozen nitrogen dwells so far out in the solar system—averaging about 3.6 billion miles, or almost 40 times the distance from Earth to the Sun—that in the 76 years since its discovery it didn't get to complete even a third of its orbit.

"You grow up and there are nine planets, and now there aren't. Imagine all those dorm rooms where they have to scrape Pluto off the ceiling."

But as more was known, astronomers began to question whether it belonged in the same category as the other eight planets. Some even wanted to call it a "comet"—a class of generally much smaller balls of ice and dust that swoop in and out of the vicinity of the Sun on highly eccentric orbits.

What forced the issue of Pluto's status was a discovery last year by a Caltech astronomer named Mike Brown. Since 1998, Brown and his collaborators, David Rabinowitz at Yale and Chad Trujillo of the Gemini Observatory in Hawaii, have been doing more or less what Tombaugh did back in 1930: taking repeated pictures of the sky and looking for things that move. The pictures are now digital, of course, and computers do the initial screening, flagging as many as 100 objects a night for Brown to look at the next morning. The great majority are meaningless, but every now and then he turns up a new Kuiper Belt object, glinting in the light from the distant Sun. The one he spotted one morning last January, in an image originally recorded in October 2003, was unusual, uncommonly bright and in an orbit that took it far beyond Pluto. No Earth-based telescope can measure the size of a planet at that distance, so Brown had to wait until he could get time on the Hubble Space Telescope. By April of this year, he had his answer: the pinpoint of light—officially designated 2003 UB313, but temporarily nicknamed Xena—was about 2,400 kilometers across, around 5 percent bigger than Pluto.

Suddenly, the cozy certainties about the solar system with which an entire generation had grown up were called into doubt. Was Brown's discovery the 10th planet? It certainly looked that way—and if it weren't, then why should Pluto be one? But would the solar system stop at 10? There was no reason to think that the Kuiper Belt didn't hold more objects that are larger. Would they be planets, too? What if one were just a little smaller than Pluto? The whole episode called attention to the fact that astronomers had never formalized exactly what they meant by "planet." It had always seemed obvious, but the definition that seemed to be emerging by default—"something orbiting the Sun, about the size of Pluto, or bigger"—seemed embarrassingly ad hoc.

So the IAU proposed, at first, to set a threshold for planethood defined by shape. Planets form out of an accumulation of dust and rocks. Below a certain diameter, roughly 1,000 kilometers, they tend to stay in whatever random shape emerges from that process; above that size, their own gravity molds them into a sphere. So the first criterion was that a planet had to be round. But many moons are large enough to be round, so the second part of the definition was that a planet had to orbit the Sun, and not another planet.

That at least had the virtue of intellectual consistency. But it led to a complicated, 12-planet solar system, not counting all the Kuiper Belt objects that could qualify once their sizes are determined. The definition would include Pluto and UB313, but also Ceres, an asteroid just under 1,000 kilometers across, which in fact had been considered a planet when it was first discovered, in 1801. And, confusingly, it would also include Pluto's own moon, Charon. All other moons in the solar system are much smaller than the planets they orbit, so there's no question about which is which. But Charon is almost half the size of Pluto itself, so in the mathematical description of their orbits, the two objects actually appear to be circling each other. If you wanted to keep Pluto, you'd get all these others as well.

So the IAU—perhaps anticipating endless future wrangling over which objects would make the cut, who would get credit for the discovery and what to name them—decided that the solar system had enough planets already. More than enough: it added a third condition that a planet had to dominate its own orbit, clearing the immediate region of smaller objects, that in effect draws the line at eight. No asteroids can qualify, no Kuiper Belt objects. And no Pluto. They are now "dwarf planets."


via http://msnbc.msn.com/id/14529757/site/newsweek/