The Grand New Planet Definition

Posted by Mike Merritt in Science on

I had meant to post this entry yesterday, but didn’t find the time to do so.

So, the definition of planets has, for the first time, been explicitly defined, and Pluto has lost its status as a planet. I’ll give a short explanation for those who didn’t follow the progress of the past week in the astronomy world.

As you might know, the debate has been raging on for years as to whether or not Pluto is or is not a planet. It’s eccentric orbit, more oval than round, on a different plane than the other planets, and passing through the orbit of another planet (Neptune), things none of the other planets have in common. Then there is the fact that is so much unlike the other planets, with a very thin atmosphere. That, and not much seems to be known about its innards, whether or not it has a mantle or a core. It shares much more with the other objects in the Kuiper Belt than with the planets.

I thought I was always so right. Then I found out that Mercury is much closer to the Moon or Pluto than I had originally though. However, at least with Mercury, we know it has the features of a planet.

So, after the discovery of objects such as 2003 UB313, astronomers apparently decided that now was the time to decide how to define a planet, and whether or not Pluto should retain that status. The first definition they came up with was very controversial, and as I’ve already made an entire entry about it, I won’t go into it anymore here.

Well, it would appear that the scientists who study planet composition were responsible for the first definition. The people who study planety movement didn’t like it, so they were somehow able to get their own definition on the table. You can see the Wikipedia article for more on the second draft of the resolution, but here is the definition in its final form:

The IAU…resolves that planets and other bodies in our Solar System be defined into three distinct categories in the following way:

(1) A “planet” [1] 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.

(2) A “dwarf planet” 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 [2], (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

(3) All other objects [3] except satellites orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”.

Well, this effectively clears out Pluto, Sedna, 2003 UB313, and any others that were considered and puts them in a different status called “dwarf planets.” I’m happy about that, however, I do agree with those who say the whole “clearing the neighborhood” is a bit vague. It also may compel scientists to disqualify a planet from another solar system that is perfectly capable of supporting humans, if something happens to be in the way of its orbit. So, I would agree that the definition still needs some fleshing out.

Now, I know I said this was going to be a more philosophical entry, but Stargate SG-1 rings, and I’m off to New York City this weekend. So, this is an entry in progress, but I put it up now for you to see. Have a good weekend. Many pictures when I return!

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