There are only eight planets!
Published on Aug 24, 2006 at 10:13 am.
13 Comments.
Filed under planets.
Last week, I did a four part series on planets, and the debate over the definition of “planet.” My first posting presented the proposed definition set forth by a sub-committee that was looking at the issue. My second and third postings looked at the issues created by the idea of binary planets and the larger moons of the Solar System, which are larger that some planets. Then, finally, I had a posting explaining just why the original definition was bad. In this fourth posting, I also pointed out that this wasn’t a done deal, because most astronomers didn’t like the definition being proposed, and I offered a definition of my own.
Well, the debate is over (for a while). The International Astonomical Union turned down the original proposal, as I figured that they would. They came up with an alternate definition. Under the new definition, a planet has to meet several criteria. (1) It must orbit a star. (2) It must be large enought for gravity to overcome rigid forces and pull it into a nearly spherical shape (if it were not rotating, which flattens it). (3) It must not be large enough to sustain nuclear fusion in its core. (4) It must be by far the largest body in its local population (It has to be larger than anything else who’s orbit comes near it.).
These are not the actual words of the definition, but they capture the meaning. Under this definition, there are only eight planets. Pluto does not fit the criteria, as it is not significantly larger than other bodies with similar orbits. Also, 2003 UB313, while larger than other nearby bodies, is not significantly larger. Ceres is also larger than other asteroids nearby, but again, not significantly so. Bodies such as Pluto, 2003 UB313, and Ceres meet some of the criteria, but not all, so they are to be called “dwarf planets.” Everything else are “dwarf Solar System objects.”
Not everyone is happy with this definition, but I think that it was the correct one. Pluto doesn’t fit. When discussing planets in class, we can talk about it in a historical context when we discuss planets, but it really doesn’t fit with the rest of the planets. Basically, that is what I’ve said for a long time.
Interestingly enough, the sub-committee that worked so hard to come up with a definition that left Pluto on the list may have doomed it by coming up with a definition that was so far fetched as to include so many objects that nobody had previously thought of as planets. Astronomers tend to be pretty conservative, so Pluto having been on the list of planets made it likely that it would stay on the list, even as an accident. But, the orginially proposed definition was so unpopular that the response was to kick Pluto off of the list of planets. Really, it shouldn’t have been on the list in the first place, so this doesn’t upset me.
-Astroprof
Seeking Solace on August 24, 2006 at 5:23 pm: 1
Ok, the reasoning makes sense to me now. If Pluto really should not have been there in the first place, then I guess a correction must be made. But is there a chance of new planets taking over the ninth spot or losing another like Pluto?
Astroprof on August 24, 2006 at 5:35 pm: 2
There is the possibility that there is something very large out there, but it is unlikely. Probably we just have 8 planets in our Solar System. It is unlikely that any of the 8 left will ever be demoted. Of course, they can always form a new committee that comes up with a new definition …
Aviatrix on August 24, 2006 at 9:39 pm: 3
Congratulations. I thought of you immediately when I heard the news.
I need a new mnemonic. “Mother Very Easily Made Jam Sandwiches Under No Protest.”
Astroprof on August 24, 2006 at 10:15 pm: 4
Hmm. Maybe we should have a contest for a new mnemonic. Any takers?
Mary Jo on August 24, 2006 at 10:19 pm: 5
I’m still mourning Pluto!
Bunny on August 25, 2006 at 9:17 pm: 6
I saw this, and throught you might get a kick out of it.
http://www.baltimoresun.com/news/health/bal-plutoside0825,0,6367343.story?coll=bal-home-headlines
Astroprof on August 25, 2006 at 10:24 pm: 7
It’s still not a planet!
Bunny on August 27, 2006 at 1:06 am: 8
I just feel bad for Pluto. One day it’s in, the next it’s out. I think it should at least be grandfathered in, poor thing.
vanessa on October 10, 2007 at 2:21 pm: 9
so pluto isnt a planet way cant it be
vanessa on October 10, 2007 at 2:21 pm: 10
so pluto isnt a planet way cant it be
Astroprof on October 10, 2007 at 11:01 pm: 11
Pluto is too small, and it doesn’t fit with the other planets. It is like Ceres, Pallas, Vesta, and Juno, all of which were once listed as planets, and now are not.
Natalie on May 5, 2008 at 6:03 am: 12
I know there’s been debate in the past on whether or not Pluto is a planet, but has there ever been another instance of Pluto being demoted?
Astroprof on May 5, 2008 at 10:00 am: 13
Pluto, itself, has not been previously demoted. However, Ceres, Vesta, Pallas, and Juno all used to be classified as planets and were later demoted to asteroid status. So, Pluto is not the first thing to be demoted, only the latest.