theengineer

The Engineer

The Life and Times of Donald F. Simmons


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theengineer

Our New Planets

As you all have heard by now, the International Astronomical Union has finally put forth its new definition of "planet" (basically an object big enough that its own gravity pulls it into a more-or-less spherical shape, it doesn't orbit around another planet, and it's not a star). This gives us right away three new planets: the former asteroid Ceres, Pluto's former moon Charon (Pluto and Charon get classified as a "double planet" in the fine print), and 2003 UB313 (better known as "Xena").

This is just about the worst of all worlds. My readers know I don't think Pluto should be called a planet, and this entire mess started just because it was thought at discovery that Pluto was much larger than it actually is. If they'd had an idea of its true size, planet status would never have come up and we'd be spared all this.

What should have happened is that  the new Trans-Neptunian objects (even the ones larger than Pluto) get their own classification (which has practically happen anyway) and Pluto gets grandfathered in as a planet for historical reasons.

But my real problem here is that they would declare Ceres to be a planet, and that's really flying in the face of history. Since people have been aware of planets everyone's known that there wasn't a planet between Mars and Jupiter. When Ceres was discovered (January 1, 1801) there was some debate about what it should be called, but once more asteroids were found everyone quickly agreed on "asteroid" (minor planet). It hasn't been a planet for over 150 years, and no one outside the IAU committee would have thought of calling it one. But if you want Pluto to be an "official" planet, this is what you get stuck with.

Mike Brown, who discovered 2003 UB313, says that the new definition could result in 53 planets currently, with more being discovered all the time, as it's all but certain there's a lot of stuff in the outer solar system still to find. And Phil Plait, over at the Bad Astronomy Blog, has an excellent discussion of the whole business.

From the Wikipedia, below is a comparison of our new planets to Earth.


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doesn't orbit around another planet or star
I think that should be "it doesn't orbit around another planet, but it does orbit a star."

Thanks of the proofreading, it is now corrected (and updated).

Hiho,

But my real problem here is that they would declare Ceres to be a planet, and that's really flying in the face of history. Since people have been aware of planets everyone's known that there wasn't a planet between Mars and Jupiter. When Ceres was discovered (January 1, 1801) there was some debate about what it should be called, but once more asteroids were found everyone quickly agreed on "asteroid" (minor planet).

What you wrote here isn't quite accurate - particularly the "quickly agreed" part. Ceres in particular was an official member of the list of planets from 1801 to the 1850s or 1860s (depending on which "official" reference you go with), at which time it was demoted along with several other larger asteroids to the term "minor planet" and then finally down to "asteroid". See http://aa.usno.navy.mil/hilton/AsteroidHistory/minorplanets.html for a good summary of this history.

I think it's definitely counter-intuitive, now, to consider Ceres a planet. But in the end, so long as there's finally an official definition that is applied consistently, I don't have problem with it. After all, change of definition is nothing new in science! :)

Cu,
Andrew

You're right about Ceres' history, curse my faulty memory!

I think the real problem with this definition is that it won't be applied consistantly, there's already the "Pluto-Charon" loophole, and I predict that as "planets" start piling up in the outer solar system we'll be back to hemming and hawing.

Hiho,

Yeah, you're probably right... I can see it now: "Ok, kids, who can name all 596 planets in our solar system?" :)

Cu,
Andrew

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