What is a planet?

[LunarOrbit]

The International Astronomical Union is meeting this week to decide on a definition of "planet". The good news is that Pluto is officially a planet, and two other recently discovered Pluto-like objects will likely be labelled planets also. The problem is that there could be billions of objects like Pluto, so where do you draw the line?

Here is the criteria the IAU is using:

    1) 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.

    (2) We distinguish between the eight classical planets discovered before 1900, which move in nearly circular orbits close to the ecliptic plane, and other planetary objects in orbit around the Sun. All of these other objects are smaller than Mercury. We recognize that Ceres is a planet by the above scientific definition. For historical reasons, one may choose to distinguish Ceres from the classical planets by referring to it as a “dwarf planet.”

    (3) We recognize Pluto to be a planet by the above scientific definition, as are one or more recently discovered large Trans-Neptunian Objects. In contrast to the classical planets, these objects typically have highly inclined orbits with large eccentricities and orbital periods in excess of 200 years. We designate this category of planetary objects, of which Pluto is the prototype, as a new class that we call “plutons”.

    (4) All non-planet objects orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”.

Phil Plait, the Bad Astronomer, wrote an interesting blog about this. He points out the problems astronomers have had trying to define exactly what a planet is.

[DonPMitchell]

Seems reasonable.  The definition is based on a significant physical criteria (gravitational rounding) instead of an arbitrary choice ("I don't think Pluto is big enough...but Mercury is").  I think people would have been unhappy if Pluto was downgraded, and it might have adversely effected funding of research.

Best of all, we can stop arguing about it now!

[Ottawan]

Looks like "Xena" will probably be renamed Persephone.

[LunarOrbit]

What a difference a few days can make... it looks like things have turned around completely because now the IAU is saying Pluto is not a planet.

Astronomers say Pluto is not a planet

[sparkmaster]

Poor Pluto... : (

[Ottawan]

I guess they are going to have to recall the New Horizons mission then

[Satanic Mechanic]

The IAU is pissed off because that one guy found that Pluto has 3 moons.  The new IAU definition of Pluto being a dwarf planet is pretty weak in my opinion, especailly about its orbit and clearing an orbital path.

SM

[DonPMitchell]

Here is what Stern had to say, the project leader of the New Horizion mission:

"It's patently clear that Earth's zone is not cleared," Stern told Space.com. "Jupiter has 50,000 Trojan asteroids," which orbit in lockstep with the planet.

Stern called it "absurd" that only 424 astronomers were allowed to vote, out of about 10,000 professional astronomers around the globe.

"It won't stand," he said. "It's a farce."

[sparkmaster]

Plus, the Plutonians didn't get to vote. After all, it's their planet.

[DonPMitchell]

There's an interesting article on the BBC website, about the politics of the IAU vote.  The anti-Pluto contigent engineered the vote to take place after most delegates had gone home.  The chairman of the Pluto committee called the vote "hijacked".

http://news.bbc.co.uk/2/hi/science/nature/5283956.stm

Doesn't look like this is case closed.  And the IAU is not looking good in the public media.

[spacecat27]

I'm going to have to coin a good word for this phenomena- but it seems like a classic case of science guys who are usually hidden and unknown getting a shot at publicity in mainstream media and so they'll do anything (controversial) for the recognition.  The Stanley & Pons "Cold Fusion" debacle comes to mind- as well as a story I think I've told here before.

A coastal survey company that I'd done work for in the past hired me to assemble and wire an underwater lighting grid.  Guys in the corporation I never even knew existed were coming out of the woodwork... checking progress.... offering opinions.... acting more important than they actually were.  Finally the 'secret' came out; the company had been hired by that miserable Leonard Nimoy "In Search Of" TV show to get pictures of a plane wreck on the sea floor that was 'believed' to be one of the Bermuda Triangle 'lost squadron.'  The possibility of getting their faces and names on television made all these quiet, unknown guys absolutely nuts!

[DonPMitchell]

Yes, a lot of this is about publicity.  Neil deGrasse Tyson certainly began his anti-Pluto compaign for that reason, he is a total publicity hound.

I have to respect Mike Brown, since he has been done some good astronomy and finding KBOs, but I think he is also angling to be famous for killing Pluto.  He basically said as much in the BBC article.

[spacecat27]

Exactly- I read an interview with Brown a few months ago- it may even have been in a (yuk) "Discover" magazine- and he comes across as an enthusiastic, open-minded, progressive fellow...... but several times in the interview he makes a statement along the lines of-
....'and people want to argue over whether Pluto is a planet;  of COURSE it's NOT a planet'.... yet he never elaborates on WHY he thinks this is so, and never offers an alternative (if it's not a planet it's a________)

I also have to wonder... with the 'clearing their own space' definition- if there is really enough data on the far out bodies to say for sure?  That's alot of 'unknown real estate' to extrapolate over. 

It would just be nice if everybody quits arguing over definitions- and gets into the core science implications.  In grade school we were taught most of our planets were ejected from the sun- but some may have been 'captures' from outside the solar system.  With Pluto's oblong orbit- it probably was a capture and could be the first body to tell us about space 'out there.'

[DonPMitchell]

These graphs of the inner and outer solar system so the situation pretty well.



In the inner solar system, the Astroid Belt is a very dense and pronounced feature.



Pluto is clearly part of the Kuiper Belt, and lucky to be in an orbit that has not collided with Neptune.  An interesting question is, what will they do if they find a KBO that is the size of Earth?  The "dynamicists" are the ones who hijacked the vote at IAU, because they feel the "clearing the orbit" definition is important, even though it is imprecise.

Scientists crave publicity.  Brown wins either way since he is a major discoverer of large KBOs, but he seems to have decided to play the card of "I am not going to claim to be a planet discoverer".  As for Tyson, I have much less respect.  He's an extremely marginal scientist who has made himself famous purely from publicity instead of real scientific achievement.

[Johno]

Who's we, white man?