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Pluto lives!
2006-08-16 08:44
by Bob Timmermann

Part I of our story.

The International Astronomical Union, meeting in Prague, has decided to keep Pluto as a planet.

For the time being. The general membership could change the decision.

To be a planet, IAU says:

Q: What is the exact wording of the official IAU proposed definition of "planet" in "Resolution 5 for GA-XXVI"?


"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."

In addition to Pluto, three other objects have now been upgraded to planetarty status: Ceres, an asteroid between Mars and Jupiter; Charon, a moon of Pluto; and UB313, which is nicknamed "Xena", but will now get a more fitting name.

Charon qualifies as a planet because its center of gravity is between it and Pluto. The Earth's moon has its center of gravity inside the Earth.

Please debate this politely.

IAU press release explaining the proposal more completely.

2006-08-16 09:13:13
1.   Humma Kavula
My wife's an astronomer, so we talked a bit about the topic this morning.

First -- has this been adopted? The last I read, it was a proposal, but maybe things changed this morning.

Points 1 and 2 are redundant. Size is what makes objects round; a body's own gravity will pull in the edges. That said, it's just easier to tell how big something is than if it's round or not. The 800 km rule is about how big scientists think a body has to be before its own gravity makes it round.

The scientists who wrote the rule decided to use gravity as the determining factor in what makes a planet. That's fine, I guess, but it opens the door for dozens of objects. To me -- and granted, my vote counts for zero -- size should also be a determining factor. And elliptical orbits...

Things get nutty when we start talking about planets outside our solar system. The rule is going to have to change again -- there are planets that do not orbit anything, and other objects that are currently classified as planets but are really brown dwarf stars. I don't really understand all the details -- but it seems to me that if you're going to make a rule, it's best to make one that works for the very largest planets as well as the very smallest.

2006-08-16 09:14:19
2.   Humma Kavula
I meant to say "concentric elliptical orbits." Left out that important word...
2006-08-16 09:16:29
3.   Bob Timmermann
I did insert a line hedging my bets. It's the third one.

I will put in a link to the press release.

2006-08-16 09:23:40
4.   Humma Kavula
So you did.
2006-08-16 09:25:18
5.   Bob Timmermann
Now there is a tiny town in the Central Valley of California named after a planet!

2006-08-16 09:32:07
6.   the OZ
5 The residents of UB313, Oregon will also be pleased that a planet is being named after their town!
2006-08-16 09:45:39
7.   mehmattski
Most importantly, how does this effect the standard mnemonic device?

Right now its:
My very eager mother just served us nine pizzas. (or something similar)

Looks like we're going to have to insert two C words and hopefully the other planet isn't UB313 or Xena, as it would make mnemonic reconstruction difficult

2006-08-16 09:46:16
8.   Ken Arneson
I'm still not buying into the idea of Charon as a planet. Does it orbit Pluto, or not?
2006-08-16 09:48:13
9.   Humma Kavula
8 According to my wife, Pluto and Charon orbit each other -- it's a binary planet. Both orbit the sun.
2006-08-16 09:51:16
10.   Cliff Corcoran
I don't understand why you need a mnemonic to remember the planets any way. They're all so different (save Uranis and Neptune) that it's like needing a mnemonic to remember the starting line-up of your favorite team, except the planets have remained the same since they added Pluto. Why is this hard?

The notes in the spaces and on the lines of the bass and treble clef, now that's where mnemonics come in handy, but the planets?

Anyway, just insert "could" in that sentence.

My Very Eager Mother Could Just Serve Us Nine (Crusty) Pizzas. Of course the order of the last three depends on the time.

2006-08-16 09:54:09
11.   Cliff Corcoran
9 In that case the name should be a hyphenate, like Pluto-Charon or Charon-Pluto or Zeta-Jones or Newton-John or Romijn-Stamos (though now that I think about it, I'm not sure any of those last three were hyphenated).
2006-08-16 09:54:41
12.   StolenMonkey86
FWIW, I would amend #3 to "must orbit the sun and not any other planet."
2006-08-16 09:55:53
13.   Cliff Corcoran
12 Then per 9 Pluto would also not be a planet, and we'd still have nine of them, but with Ceres taking Pluto's place.
2006-08-16 10:01:56
14.   Humma Kavula
13 Don't take this as gospel, but if I understand my wife correctly, you can't think of Ceres and Pluto orbiting each other like the moon orbiting earth.

The Earth is a host planet to the moon. Pluto and Charon are roughly of equal size -- as both pointed out, the center of mass that the two objects orbit around is not inside either planet. Therefore, they qualify as a double planet.

I'm pretty sure (but could be very, very wrong) that there are some very large objects outside our solar system that are double planets. Would you say they aren't planets?

What's more, according to the wife, there are some planet-like objects that don't orbit anything. Are they not planets?

2006-08-16 10:02:30
15.   Humma Kavula
Second paragraph should read, "As Bob pointed out..."
2006-08-16 10:02:38
16.   Bob Timmermann
2006-08-16 10:05:13
17.   Bob Timmermann
Approved and then modified to make it sound meaningless!
2006-08-16 10:09:47
18.   Cliff Corcoran
14 I would say a planet-sized object that does not orbit anything is indeed not a planet. I think part of the definition of planet is that it has to be part of a solar system and orbit a star.

As for the double-planet thing, I'm cool with that, but the new version of #3 above sounded like it was designed to rule out Charon (you meant Charon, not Ceres). But what you said in 14 confirms my assumptions in 11 and 13 that either both Pluto and Charon are planets (preferably referred to together as a double or binary planet) or neither of them are.

2006-08-16 10:10:09
19.   Humma Kavula
14 Gah. Not Ceres and Pluto. Charon and Pluto. Forgive me.
2006-08-16 10:10:23
20.   Cliff Corcoran
18 second sentence should read "should be" not "is"
2006-08-16 10:13:19
21.   Bob Timmermann
I have modified #3 again to show my lack of astronomical chops.
2006-08-16 10:14:59
22.   Cliff Corcoran
My Very Eager Mother Could Just Serve Us Nine Peanutbutter-Cups?


My Very Eager Mother Could Just Serve Us Nothing?

2006-08-16 10:17:53
23.   Kayaker7
I got a ribbon in second grade for making a model of the solar system with construction paper. I gotta find it so that I can revise it.
2006-08-16 10:18:21
24.   Humma Kavula
18 Right, either both Charon and Pluto, or neither.

I'm about to tread dangerously outside my breadth of knowledge, but bear with me:

Planet-like objects that don't orbit anything happen like this: they were orbiting a star when another star came along and pulled the object out of orbit.

Would you say that the object was a planet when it was orbiting its star and ceased to be a planet when pulled out of orbit? What do you call the new object?

Ultimately, I'm with you. I think there are eight planets in our solar system. At the same time, my wife brought up issues that make it pretty clear to me that people who know more than I do think this isn't as cut-and-dried as I'd like it to be. I like thinking about this stuff, but quickly, I get into areas that I don't understand.

2006-08-16 10:20:09
25.   Humma Kavula
24 "the new object" should read "it now."
2006-08-16 10:29:21
26.   Greg Brock
For the record (that does not exist), I object to the IAU proposal.
2006-08-16 10:34:08
27.   Bob Timmermann
You better get on a plane to Prague now!

Drink plenty of fluids.

2006-08-16 10:38:57
28.   Cliff Corcoran
24 Yes, I would argue that a planet that is pulled out of orbit and is floating in space is no longer a planet and should be reclassified with a new term. To me the term "planet" implies orbit (in astronomical terms) or something that's an offshoot of a central hub (in more general terms). Once an object is no longer part of that system, it is no longer a planet.

I'm not sure what terms exist for objects not in orbit, however.

2006-08-16 10:40:26
29.   Greg Brock
I'm just a man. What can a single man do against the awesome power of the International Astronomical Union? They can make planets. I can barely make eggs benedict.

I just have to accept it and move on.

2006-08-16 10:41:41
30.   Bob Timmermann
Didn't UB313 perform "Red Red Wine"?
2006-08-16 10:53:48
31.   Cliff Corcoran
30 No, that was Neil Diamond.
2006-08-16 10:55:26
32.   Linkmeister
Did anyone consult Van Allen (he of the asteroid belt -- I think he died last week) before fiddling with the lineup?
2006-08-16 10:57:01
33.   Jacob L
As long as Mars is still batting cleanup, I'm cool.
2006-08-16 10:58:52
34.   Bob Timmermann
Earth keeps its spot in the #3 spot. It's expected to produce and it seems to do that, but Al Gore is worried about its C02/0 ratio.
2006-08-16 11:01:21
35.   Cliff Corcoran
33 God of War in the four-hole, how can you complain. Seriously, the old 9 do kind of resemble a baseball lineup:

Mercury - small, hot leadoff hitter
Venus - a bad version of the #3 hitter
Earth - the best player on the team
Mars - God of War clean-up
Jupiter - the biggest bopper fifth, but lacks the solid surface to hit fourth
Saturn - Lesser version of #5
Uranis, Neptune - none descript 7 & 8 hitters
Pluto - the small pathetic one, that in the NL isn't even an every day planet

2006-08-16 11:02:14
36.   Humma Kavula
Venus hits like a girl.
2006-08-16 11:05:30
37.   Cliff Corcoran
Pluto's a total dog.
2006-08-16 11:06:25
38.   Cliff Corcoran
35 And I totally skipped the fact that Mercury is named for the fleet-footed god and that it rounds the bases faster than any of the other planets.
2006-08-16 11:16:31
39.   Bob Timmermann
But Ceres is now going to be in the #5 slot. It's like they went and put Neifi Perez right in the heart of the order!
2006-08-16 11:18:47
40.   Greg Brock
The IAU is makes more bad choices than the BBWAA.
2006-08-16 11:27:49
41.   Ali Nagib
40 comments so far and not ONE Kornheiser-style Uranus joke. What is this world (or all the other ones, for that matter) coming to?
2006-08-16 11:34:28
42.   Linkmeister
35 You forgot that big red spot in Jupiter's swing.
2006-08-16 11:48:08
43.   Cliff Corcoran
41 I was going to say something about the lack of Uranus jokes, but I figured I'd wait to see how long the thread could go without mention of it. Since Ali and I were struck by the lack at the same time, I think we have our answer.
2006-08-16 12:10:36
44.   BruceR
28 With apologies to HK's wife,, term for objects not in orbit = "thingies floating around in space".
2006-08-16 12:12:14
45.   Greg Brock
The head of astrophysics at the American Natural History museum is on MSNBC, and he says that Pluto is not a planet, and neither is Ceres.

The man is obviously a genius. And yes, I'm that one guy that watches MSNBC.

2006-08-16 12:21:37
46.   the OZ
46 It's OK, I watch Book TV on C-SPAN2.

24 28 Since the term "Planet" means "Traveler", representing how the planets didn't seem to hold a fixed position like other stars, I'm assuming that a large, round, mass without a traceable orbit doesn't technically qualify as a "Planet", but you've already covered that, seemingly. So, cool.

2006-08-16 13:25:06
47.   Xeifrank
So with the planetary roster shakeup, who now bats leadoff? Do we go with Mars and drop Venus to the #8 hole, moving Uranus up to the #2 hole? After all girls are from Venus and probably don't hit too well.
vr, Xei
2006-08-16 13:35:52
48.   Linkmeister
Here's a post chock-full of links. If you don't click anything else in there, look at the link to John Scalzi's daughter's feelings about Pluto. Caution: excessive cuteness.

2006-08-16 13:41:58
49.   Greg Brock
46 Book TV is awesome!

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