Monday, August 14, 2006

How many planets in the solar system?

The New York Times has had a few articles of late on a meeting of astronomers to attempt to define what makes a planet with Pluto's (and Xena's plus a few others) fate hanging in the balance. Since we have a few astrophysicists around, what are their opinions on the matter?


Blogger Eric said...

This debate has been intensifying since they started finding lots of Kuiper belt objects, brown dwarfs, extrasolar planets, and other objects often given colorful names like "planemos". It's just gotten more public attention since the new Rose center in NYC, and the discoery of Xena. I beleive there's been an IAU subcomittee trying to come up with some decision for at least a year now. But from the rumors I've heard, the committee is split fairly evenly.

IMHO, Pluto is one of the relatively large and close members of the Kuiper Belt. The reason that many people call it a planet is largely historical. My prediction is that eventually (but not sure when that will be) Pluto will be demoted. There's historical precedence for this, as back in the old days (I want to say 1800s), they found one asteriod and called it a planet, then another, then a whole bunch more. And eventually, they realized that it made more sense to call those objects large memebers of the Asteroid belt.

8/14/2006 08:37:00 PM  
Blogger Eric said...

For a story with a little more meat see... this at After reading this, I do remember hearing about the new subcommittee having a proposal. Academics prize their freedsom. So, like Alan, I won't be convinced that astronomers in the field would accept this definition, until it actually comes to pass.

8/14/2006 08:52:00 PM  
Blogger Vincent said...

I don't really care. Eight, nine, a gazillion, whatever. If pressed on the issue, I'd probably go with eight, since it's not clear where to draw the line between KBOs and planets. But if you actually tried to define a planet in a way that excludes Pluto, all the Pluto-lovers would throw a fit. It doesn't matter one way or another to astronomers.

8/15/2006 12:36:00 AM  
Blogger Eric said...

here's the result of the IAU subcommittee

8/16/2006 11:21:00 AM  
Anonymous Anonymous said...

And here's what the Onion has to say.

-Qian (forgot my login)

8/17/2006 12:55:00 PM  
Blogger Vincent said...

Er... there are a couple of obvious astronomy errors in the quotes. But perhaps that's just The Onion being more real than reality. Not many people really know what a light year is.

8/17/2006 07:37:00 PM  
Blogger Eric said...

Good news... Rumor has it that Julio Fernandez (a good dynamisist from Uruguay) is trying to offer an alternative proposal that includes the clause that a planet also has to be "by far the largest body in its local population".

8/19/2006 03:39:00 PM  
Blogger Justin said...

Why is that good news? Why should we declare that there cannot possibly be 2- or 3-planet systems? There are plenty of binary star systems, after all.

8/19/2006 09:58:00 PM  
Blogger Eric said...

Yes, astronomers have written papers proposing such possibilities. And I agree that there's a good chance that there are some very interesting and unexpected things out there, that at some point we may decide should be considered planets. However, it's hard enough to come up with a definition that provides a good classification system for the objects we've already found, without also trying to write a definition that will nicely work for all exotic things that might be discovered in the future. Once we find one, it will be a puzzle that we'll refer to as something like HD123456cd-like objects for a while. Then once we find a few more, we'll have a better idea of what are the characteristics of such objects and to what category they most logically belong.

provides more details on the counter proposal. and under "Further considerations" it explicitly discusses this possibility...
"Is may be possible that in the near future cases of objects not foreseen at present could appear beyond our solar system, as for instance free-floating planets, stray planets, or double planets. We think that we should not advance definitions at this point for these exotic cases and leave their discussion when/if they became a part of the observed world."

I think that's a sensible approach. Personally, I'm not thrilled that the alternative still has clause b, about the object needs to be round. whether something assumes hydrostatic equilibrium is a composition dependant statement. For example Mimas (a satellite of Saturn) is round, while Vesa is not, because Mimas is mostly ice (easily deformable), while Vesta is mostly rock (hard to deform). Yet Vesta is more massive and larger in volume. So I'm not a proponent of the roundness criteria. Further, the shape of an object is one of the most difficult things to determine observationally, especially for extrasolar planets (or planet candidates). It's even quite dificult for some objects in our solar system. So if this is a criteria for a planet, then there will be a lot of objects that we assume are probably round based on theory, but we have no direct observational evidence. So we'll be stuck in limbo thinking that they probably are planets for a very long time, before we ever can determine whether they satisfy one of the criteria.

8/20/2006 09:22:00 PM  
Blogger Eric said...

You can watch some of the deb^H^H^Hdiscussion at the IAU.

8/23/2006 09:33:00 AM  
Blogger Eric said...

Update on the last page of the pdf file

8/24/2006 03:05:00 AM  
Blogger Eric said...

It's official... we now have 8 planets in our solar systems.

From the the IAU newletter...
"The XXVIth General Assembly was formally closed yesterday,
during its Second session and Closing Ceremony in the main
Congress Hall. First, a few pieces of Medieval music were performed
and introductory remarks by the outgoing IAU President
Ron Ekers were made. This session was more exciting
than expected, particularly because of the voting on new IAU
Resolutions. There were last minute changes (most of them
dutifully reported in previous issues of the Nuncius Sidereus
III), not only in the controversial Resolution 5 on the "Definition
of a Planet in the Solar System", but also in some other
Resolutions. As a consequence, the voting lasted longer than
anticipated and the final result is:
– Resolution 1 (Adoption of the precession theory and definition
of the ecliptics): adopted.
– Resolution 2 (Supplement to the IAU 2000 Resolutions
on reference systems): adopted.
– Resolution 3 (Re-definition of Barycentric Dynamical
Time): adopted.
– Resolution 4 (Endorsement of the Washington Charter
for Communicating Astronomy with the Public): adopted.
– Resolution 5A (Principal definition for the IAU usage of
"planet") - adopted with ammendments that the definition
does not include satellites and in the definition the words
"dwarf planet" should be used with inverted commas .
– Resolution 5B (Adding the word "classical" for the eight
planets) - rejected by a large majority.
– Resolution 6A (Definition of Pluto as a "dwarf planet" and prototype
of a new category): adopted (237 votes yes, 157 no with 30
– Resolution 6B (The name of the new category of objects should
be “plutonian objects”): rejected (183 votes for, 186 no). The name
will be selected by standard IAU procedure."

8/25/2006 03:08:00 AM  
Blogger finou said...

Noooooooooooooo! I am still calling Pluto a planet. It's the cutest planet in the solar system. Dwarf planet sounds cute but you might make Pluto angry and, if I know my mythology, you don't want Pluto to be angry... ;)
This is so silly; it's going to cost a bunch to change lots of elementary to high school textbooks and stuff. It's been a planet for ~70 years; leave it the way it is and move on to doing useful stuff instead of spending time on this. I think it is just a marketing ploy to make astronomy hip again...

8/25/2006 02:16:00 PM  
Blogger Qian said...

And there was much rejoicing in the halls of science textbook publishers. :)

Actually I think they might have been even happier if the other Kuiper Belt objects made it as planets, one at a time, with about 2 years in between each one.

8/25/2006 06:16:00 PM  
Blogger Eric said...

The textbook publishers publish new versions regardless. (I remember when I was teaching differential equations, the textbook publisher came out with a new version that had nothing changed except they removed two chapters (and maybe fixed some typeos).) If anything, this was a nuissance to them, since it was high profile and they would look outdated if they had the wrong thing in their 2007 edition, I heard that it delayed at least one encyclopedia from going to press.

Sorry finou, but now that we know of hundreds of objects with orbits very similar to Pluto's, including some of comparable size, it no longer makes sense for Pluto to be a planet.

I'd like to say that public opinion didn't enter in. But I'm afraid that it did (due to letter writing from school children), and if anything it pushed astronomers towards leaving Pluto as a planet. We came dangerously close to doing that. But in the end, the scientists did not pander to popular opinion, but rather chose something that makes more sense.

8/25/2006 06:24:00 PM  
Anonymous zana said...

What I want to know is when they'll decide there are really only 25 letters in the English alphabet. After all 'W' is really just two 'U's. There is no reason for it to be its own letter...

(for some reason, the password I set with my blogger account isn't working. oh well.)

8/30/2006 11:28:00 AM  
Blogger Vincent said...

Unsurprisingly, the reaction in Tombaughville was in support of retaining planetary status for Pluto.

8/31/2006 01:09:00 AM  
Blogger Vincent said...

This is hilarious.

9/02/2006 03:31:00 PM  

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