August 24, 2006 10:35

About Pluto....Oops.

I'm sure nobody remembers this far back (yeah, I wish), but last week I prematurely shouted that "Pluto's In!" A new definition of the term "planet"--well, actually, the only actual definition since the Greeks first used the word to describe objects that wandered through the sky against the fixed backdrop of stars--meant Pluto, subject of much debate, would retain its formal status as the ninth planet of the Solar System.

I should have said this was a proposed new definition. A committee of the International Astronomical Union had agreed that "planet" should refer to any object that is massive enough to be squeezed into spherical shape because of its gravity and which doesn't orbit another planet. That would have added three new planets to the existing nine.

But astronomers at the IAU's meeting in Prague reacted  vehemently against this idea--it would have made Charon, Pluto's moon, a planet (why, you ask? because the center of mass of the Pluto-Charon system lies out in space, whereas the center of the Earth-Moon system is below the surface of the Earth--a loophole so ridiculous it turned everyone off). And it would have opened the door for dozens of objects not named in the first cut to be planets--asteroids and Pluto-like objects that are just a bit smaller)--as soon as the paperwork could be arranged.

So the committee hurriedly went back into session to come up with a new definition and it was just adopted an hour or so ago by majority vote of the thousands of scientists in attendance. A planet, says the IAU (and they're the ones who get to decide), is "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."

This last clause, meaning a planet's gravity has slingshotted everything else out of the neighborhood, knocks Pluto off the list: its orbit overlaps with Neptune's (Pluto's orbit is so elongated that it's sometimes actually closer to the Sun than Neptune is), and it sure hasn't cleared Neptune away. Of course Neptune hasn't cleared Pluto away either--but if they ever had a close encounter it would, trust me.

That leaves the asteroids out too, since thousands of them share an orbit, more or less.

So what is Pluto? The IAU has created a new category, called "dwarf planet" (sort of like an honorable mention) for objects that meet criteria a) and b) above, but not c). Pluto, the asteroid Ceres and 2003 UB313, the object discovered last year, similar to Pluto, but bigger, will all go into the new category.

So Pluto is not in. Pluto is out (why do I feel like Heidi Klum on Project Runway?) or at least half-out.

I do have to needle my colleague at the Associated Press for this one very strange line in his otherwise terrific report from the conference. "It was unclear how Pluto's demotion might affect the mission of NASA's New Horizons spacecraft, which earlier this year began a.... journey to the oddball object to unearth more of its secrets," writes William J. Cole. I'm just trying to imagine what he thinks could possibly change--NASA will tell it to turn around and come back, or what?