Pluto is saved! [NOT]

By now, you’ve probably heard about the proposal to add planets to the solar system. Pluto would remain, and at least initially, three others would be added: the asteroid Ceres, Pluto’s moon Charon, and a new thing called 2003 UB313. I like the idea of having a definition of a planet (which astronomers have never had before) but I’m not thrilled with the idea of having 12 or more of them. Some scientists say there may be hundreds of them! Here’s the proposed definition:

A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.

But wait, you say, how can Pluto’s satellite, Charon, be a planet? Well, it turns out that both Pluto and Charon orbit around a point in space that is outside either object. Therefore, they will technically be “double planets.”

The key question is how the astrologers will take it. Now they’ll have to account for the influences of all these extra planets, and keep revising their predictions whenever a new planet is named. If they don’t, then they’ll be open to serious criticism that what they purvey is pseudo-scientific crap. If I were on the IAU committee deciding this issue, I’d consider messing up the astrologers as reason enough to vote yes on the new definition.

DadObservations08/16/06 8 comments


David • 08/18/06 10:21 PM:

So I just said to Dad, “yeah, I’m going to comment on Pluto” (referring to this post, and the fact that I had read about it this morning). Dad then said, “What poodles?” We then laughed really hard for a while.

Dad • 08/19/06 11:52 AM:

That’s it? That’s your comment?

David • 08/24/06 5:33 PM:

I guess Pluto is no longer a planet. It got demoted. I don’t really understand all this.

Yes, that was my comment.

Dad • 08/25/06 1:03 PM:

Yeah, Pluto’s gone. Now the LSU observatory (which only shows 8 planets in the stonework on the entrance) is right after all!

Dad • 08/26/06 11:13 AM:

Naturally enough, reading about Pluto and Charon locked in that celestial dance of the double dwarfs got me thinking. Obviously, the centripetal force holding them in orbit can be determined by Newton’s universal Law of gravity. But given two masses, how does one determine the distance between them and their speed of rotation once equilibrium is reached? And how do they get together in the first place?

I haven’t solved the problem, but I’ve thought about it enough to see how interesting it is. Maybe David could consider using it in his physics class.

David • 08/26/06 1:03 PM:

Mom, Dad, I guess you’ll now have to relabel Pluto as a “dwarf planet” on my solar system plate. I’m going to cry.

I am going to use a NYT article on this topic in class on Monday or Tuesday. And hopefully revisit it when we get to Newton’s laws and Universal Gravitation.

Dad • 08/26/06 4:35 PM:

I forgot to mention that booting Pluto from the classical planets seems unscientific to me. Granted, there has historically been no definition of what a planet is. However, to get rid of Pluto because it doesn’t “dominate” its orbit or have “cleared the neighborhood around it” is pretty arbitrary.

Go to this link and click on “Watch why some think planet size doesn’t matter” near the bottom of the article for a good explanation of what’s going on.

Dad • 10/04/09 11:25 AM:

Here’s a follow-up on Planet Pluto.

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