Friday, September 01, 2006

Pluto 2.0

I hadn't intended to say any more about the Pluto debate, but unfortunately, it doesn't seem to want to die. Public interest has waned apparently, but according to Yahoo news scientiests are still squabbling.
"The IAU created a definition which is technically flawed, linguistically flawed and scientifically embarrassing," [Alan] Stern [planetary scientist at the Southwest Research Institute in Boulder, Colorado] said in a phone interview.

The 300 astronomers and planetary scientists who signed the petition said they would not use the IAU's definition.
While I still stick by my earlier statements that ultimately, the title Pluto itself is given is unimportant, I have the same reservations the Bad Astronomer has: namely that the defition is overly vague.

One of the requirements is that a planet be able to "clear" its orbit. However, what this truly means is not explicitly defined. Pluto is obviously disqualified for lurking near the edge of the Kuiper belt. But what about all the Near Earth Objects that cross our orbit? There's a lot of 'em, so do we not qualify?

I'd argue that we do. Earth's done a great job clearing out its orbit, but it's like having just dusted a room, there's more that's bound to settle immediately.

But what about other Planets? Jupiter has two groups of asteroids (one preceeding it and one trailing) known as the Trojan asteroids that follow in its orbit. I think the "cleared" is meant to imply that objects must orbit with the body in question.

Mercury's orbital area is pretty clear. But we also have to stop to consider whether or not Mercury really did this on its own. It's quite possible that its had help from a rather massive neighbor (ie, the Sun). So, if placed elsewhere in the solar system, would Mercury still be able to make the cut?

Mars too has done a good job, but its two moons (Phobos and Deimos) aren't in stable orbits. One is slowly spiraling outward while the other is slowly heading inwards to crash into the planet. Thus, do these disqualify Mars?

Another requirement is that the planet be massive enough that it be able to make itself spherical. As the Bad Astronomer points out, there's no clarification on how spherical is "spherical". While most planets are pretty round, Earth is slighly pear shaped. Not noticeable visually, but still not spherical.

Saturn meanwhile, is very noticibly oblong. Due to a rapid rate of rotation, Saturn in about 10% wider along the equator. Thus is Saturn disqualified for this reason?

So while the public may not be up in arms as much as they were when the decision was still front page news, it's obviously still a very hot button issue in the scientific realm. As petulant as those are that are refusing to use the now ratified definition are, I'm forced to agree that the new definition is inadequate in every regard (although slightly less so than the former lack of a definition).

Thus, while I wasn't planning on discussing the issue any further, I would also have to call upon the IAU to adopt a definition that is quantitative rather than ambiguous and qualitative.

6 comments:

The Science Pundit said...

If it's in a stable orbit around a star, then it very well can't be a wanderer, now, can it?

ruidh said...

I saw a paper on the web not long ago in which someone calculated the ratio of the mass of a planet to the total mass of the objects which cross its orbit and could potentially could hit it. For example, the NROs v. the mass of the Earth.

The 8 classical planets all had ratios that were seperated by several orders of magnitude from the other objects in orbit around the sun. He's provided a quantative measure for the concept of "cleaing an orbit".

I wish I could find that paper now.

valhar2000 said...

Asimov suggested in the instoduction to an essay (I don't have the book here, so I can't tell you the name of the essay) that there should be three categories of planets:

1) The Macro-Planets: the eight or nine previously accepted planets.

2) The Meso-planets: objects that orbit the sun that are smaller than pluto but larger than some other arbitrarily chosen example.

3) The Micro-planets: everything else that orbits the sun, down to dust particles.

He also suggested calling moons moons, regardless of their size, and he may have suggested that comets are also clearly distinguishable form other bodies.

I am amazed that the IAU is considering all this non-sense when such a superior ideaalready exists. At the very least they could have simply admitted that the name "planet" is arbitrary (which it was) and left it at that, but no, they have to waste time and effort debating irrelevancies...

Stephen said...

I still say it would be of some value to have a definition of 'planet' that covers extra-solar planets as well. This definition needs an upper bound. The 'having ever fused anything, including lithium' definition allows one to check spectrographically. This puts the upper limit around 13 Jupiter masses, i think. The definition needs a lower bound. Some sort of hydrostatic equilibrium (spherical) inspired minimum diameter makes sense to me. I'd go with 800 km. When Dawn visits Vesta, the arbitrary limit could be slightly reduced, if it turns out that Vesta is in pretty good hydrostatic equilibrium after all. It's quite OK if we have 20, or 200 planets in our own solar system. Sub categories of planets like terrestrial planets, gas giant planets, ice giant planets, ice planets and minor planets are all fine. All except minor planets would be planets. Minor planets would be anything smaller than the cut off. I'm quite OK that you have to learn a fair amount about an object before it's called a planet, especially if it is near a boundary. Double planets need to be addressed. I'd drop the barycenter thing. One might have a double planet in the same solar orbit that are 60 degrees apart with similar masses. One might have two objects co-orbit each other. But instead of barycenter, i'd go with some simple ratio of masses. If one is more than pi times the mass of the other, then it's planet and moon, or some such.

The dynamic sweeping thing is very odd. The Sun has something like 95% of the mass of the solar system. Jupiter has something like 95% of the rest of the mass of the solar system. Basically everything resonates with Jupiter. So, our solar system has one planet. It just doesn't say very much. And stars are stars even if they orbit other things. What matters is the thing itself. I'd drop this whole dynamic nonsense. In order to figure out what something is, you have to figure out everything else? Feh.

We also need a lower limit on the size of moons. We're finding small rocks around Jupiter. They don't need names. Perhaps a simple ratio of planet size to moon size makes sense. Anything smaller would be a minor satellite.

So, Sunday night it was clear weather after all. I saw Ceres - once a planet, and nearly a planet again. From my front yard. Over the glare of the supermarket parking lot lights. In a 10 inch scope, using a computer program that showed field stars down to 10th mag.

Paper Hand said...

And stars are stars even if they orbit other things. What matters is the thing itself. I'd drop this whole dynamic nonsense.

Well, as long as you insist on a difference between "moon" and "planet", that is a relevant issue. Location is all that keeps the largest moons from being called planets, and the smaller ones from being called asteroids.

valhar2000 said...

Asimov suggested in the instoduction to an essay (I don't have the book here, so I can't tell you the name of the essay) that there should be three categories of planets:

1) The Macro-Planets: the eight or nine previously accepted planets.

2) The Meso-planets: objects that orbit the sun that are smaller than pluto but larger than some other arbitrarily chosen example.

3) The Micro-planets: everything else that orbits the sun, down to dust particles.

He also suggested calling moons moons, regardless of their size, and he may have suggested that comets are also clearly distinguishable form other bodies.

I am amazed that the IAU is considering all this non-sense when such a superior ideaalready exists. At the very least they could have simply admitted that the name "planet" is arbitrary (which it was) and left it at that, but no, they have to waste time and effort debating irrelevancies...