Wednesday, August 16, 2006

New Planets Proposal

The International Astronomical Union (IAU) is considering a proposal that would finally define a planet. The proposal is:

- The object must be in orbit around a star, but must not itself be a star
- It must have enough mass for the body's own gravity to pull it into a nearly spherical shape

Ceres, Pluto (and Charon) and 2003 UB 313 will all qualify under the new definition.

Personally, (and the IAU didn't ask me!), I'd rather add a criterion like:

- It must have sufficient gravity to sustain an atmosphere if conditions would support one.

I think that would eliminate the silly little bodies and still offer some way forward. Do we really want a hundred "planets" in our solar system?


  1. Sustaining an atmosphere sounds good at face value, but it's fraught with technicalities.

    Over what period should such a body be able to sustain an atmosphere? Earth, Mars and Venus are eventually going to lose their respective atmospheres, but they will keep them for long enough that we consider them "sustainable". Of course, the Earth (and Venus?) has the added advantage of ongoing outgassing contributing to pressure, further extending this period.

    Mercury could have held an atmosphere, but its temperatures guaranteed that this didn't last for long. However, it easily met the original requirement of being a "Planet", or "Wanderer". (Just ask the ancient Greeks)

    Given the temperatures around the orbit of Neptune, I wonder if Pluto could have held onto a thin atmosphere for a while? It would be interesting comparing escape velocity of Pluto to the mean molecular velocity of gases at those temperatures.

    The new proposal has flaws, but I think it's a reasonable compromise. After all, a lot of people don't want to lose a planet they've had for years!

  2. That's why I suggested some form of criterion based on gravity. We really will end up with hundreds of "planets" in our system if the new proposal holds.

  3. I get the impression that the definition that would keep the most people happy would also be one of the finicky.

    The accepted models of solar system accretion yield simulations of several small rocky planets in the inner system, with large gas giants further out. The rest is a sort of "junk". I get the idea that many people want to accept the rocky inner bodies and the gas giants as planets, but that's it. Everything else can be a Pluton (even if you ended up with a rock in the Oort cloud as large as Mars).

    The problem with a "definition" like I mentioned, is that it's no definition at all. You'd be relying on emergent properties of your simulations. That might work fine in our solar system, but you can pretty much guarantee that it would break down somewhere. And the people arguing about it know this.

    So they argue points based on arbitrary definitions they created to match what they "feel" should be the right definition, even though there's no specific marker. Hence the funky compromises. :-)