Pluto Fallout - Alan Stern

There is understandably still a lot of anger and division on the Pluto/Planet definition issue. A lot of it rightly has to do with ambiguity issues relating to the definitions. Phil Plait on Bad Astronomy has already weighed in on some of the ambiguities, namely how round is round, and what about planets hurled out of orbit of their stars. The new clause that determines Pluto's fate is that of the "cleared out their neighbourhood" variety. He misses out on some of the arguments here though, which are further explained on this MSNBC post.

The post outlines how part of the problem is that, while Pluto has other Kuiper Belt Objects in its neighbourhood, other planets, namely Jupiter, Mars, and even Earth have significant asteroids in their orbits as well. Jupiter, in particular, has the Trojan Asteroids, a partial belt of around 50000 asteroids in locked orbit. This would need to be cleared up for the definition to work. Interestingly, some studies suggest that the Trojan asteroids may be captured Kuiper Belt objects.

In it, the primary interviewees are Mike Brown, who discovered large Kuiper Belt object Xena, and Alan Stern, who is leading the NASA New Horizons mission to Pluto. Their views are interesting, as both have something to lose with the new definition. Brown is for it, and thinks that it makes sense that the large Kuiper Belt objects not be classified as planets. Stern, on the other hand, engages in a lot of angry hand-waving about why the ruling is a "farce," implying that he's letting his own vested interest get the better of him:

First, note that he called it a "farce" instead of the more sensible "silly" or "flawed." He also said, "the definition stinks, for technical reasons," which is more accurate. Still emotionally charged, though.

Second, let's look at his arguments.

1) Only 424 astronomers voted. That's about 5% of all astronomers, he claimed. That may be true, but it should give a good representative sample none the less, and you can't ignore that the majority voted to demote Pluto. There's probably a reason for that. And it's certainly not a "technical reason."

2) "It's patently clear that Earth's zone is not cleared," Stern told Space.com. "Jupiter has 50,000 Trojan asteroids," which orbit in lockstep with the planet.

He's clearly angry at this, but he has a point here. But looking at the distribution of the Trojan asteroids, there is certainly a large effect that Jupiter is having on them. I don't know enough about the Earth asteroids. Another astronomer stated it more sensibly: "confusing and unfortunate" and "not at all pleased with the language about clearing the neighborhood."

Intuitively, this is the right definition. Pluto and Xena are in a field of millions of similar objects, of which they just happen to represent the dozens or hundreds that are really quite big. They are observably different from either the rocky planets or the gas giant planets. If it was only Pluto and Xena, it might make sense to denote them as planets, but it turns out that they are not so special (what with the dozens, probably hundreds of other similar-sized KBOs out there). They certainly deserve their own category (I like Plutlets), but not that of planet.

While Stern is angry about his mission being downgraded in importance in the public eye, Brown is much more enthusiastic:

"As of today I have no longer discovered a planet," he said. But Brown called the result scientifically a good decision.

"For astronomers, this doesn't matter one bit. We'll go out and do exactly what we did," Brown said. "For teaching this is a very interesting moment. I think you can describe science much better now" by explaining why Pluto was once thought to be a planet and why it isn't now. "I'm actually very excited."