Pluto Could Lose Planet Status

PhysOrg.com ^ | 21 June 2006 | Staff

[PatrickHenry]

At its conference this August, the International Astronomical Union (IAU) will make a decision that could see Pluto lose its status as a planet.

For the first time, the organisation will be officially defining the word "planet", and it is causing much debate in the world of astronomy.

There is only one thing that everyone seems to agree on: there are no longer nine planets in the Solar System.

The debate has been brought to a head by the discovery of a potential 10th planet, temporarily named 2003 UB313 in January 2005. This new candidate planet is bigger than Pluto.

The question now facing the IAU is whether to make this new discovery a planet.

Pluto is an unusual planet as it is made predominantly of ice and is smaller even than the Earth's Moon.

There is a group of astronomers that are arguing for an eight-planet SolarSystem, with neither Pluto or 2003 UB313 making the grade as a planet; but a number of astronomers are arguing for a more specific definition of a planet.

One of these; Kuiper Belt researcher Dr Marc Buie, of the Lowell Observatory in Arizona, has come up with a clear planetary definition he would like to see the IAU adopt.

I believe the definition of planet should be as simple as possible, so I've come up with two criteria," he said.

"One is that it can't be big enough to burn its own matter - that's what a star does. On the small end, I think the boundary between a planet and not a planet should be, is the gravity of the object stronger than the strength of the material of the object? That's a fancy way of saying is it round?"

This definition could lead to our Solar System having as many as 20 planets, including Pluto, 2003 UB313, and many objects that were previously classified as moons or asteroids.

One possible resolution to the debate is for new categories of planet to be introduced. Mercury, Venus, the Earth and Mars would be "rocky planets". The gas-giants Jupiter, Saturn, Uranus and Neptune would be a second category.

Whatever the outcome of this debate there is only one thing that we can be certain of; by September 2006 there will no longer be just nine planets in our Solar System.

[stands2reason]

No offense meant, but your screenname is an anagram for "bays crudely."

[lightman]

Hmmm...an excellent example of the ongoing ancient game of space pool.... :D

Do you play space pool with spaceballs?

[UnbelievingScumOnTheOtherSide]

And this can be recognized by whether their motion is ever retrograde with respect to the sun or whether their motion is ever convex toward the sun.

You're welcome to your own definition any way you want to make it, but I think the gravitational force definition makes more sense. You are, of course, free to disagree.

More or less gravity and whether or not the moon moves slower around a planet than the planet moves arond the sun (thus never being retrograde with respect to the sun) is not a good definition of being a moon. A moon is an object that would continue circling its primary in the absence of the sun. Thus, Saturn, which cirles around Jupiter, would not be a moon of Jupiter because it would go off on its own if the sun were gone. But a rock circling Jupiter slowly tens of millions of miles away would still be a moon of Jupiter if it would continue doing so in the absence of the sun. It is a satellite if it is gravitationally bound (not merely influenced or in resonance with a solar orbit) AT ALL to a primary other than the sun. What's more, scientists would call it a satellite of Jupiter just as they call the moon a satellite of earth except in the context of your silly contention.

[freedumb2003]

Mercury
Venus
Earth
Jose de CosasGratis
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto

[UnbelievingScumOnTheOtherSide]

And this can be recognized by whether their motion is ever retrograde with respect to the sun or whether their motion is ever convex toward the sun.

You're welcome to your own definition any way you want to make it, but I think the gravitational force definition makes more sense. You are, of course, free to disagree.

More or less gravity and whether or not the moon moves slower around a planet than the planet moves around the sun (thus never being retrograde with respect to the sun) is not a good definition of being a moon. A moon is an object that would continue circling its primary in the absence of the sun. Thus, Saturn, which circles around Jupiter, would not be a moon of Jupiter because it would go off on its own if the sun were gone. But a rock circling Jupiter slowly tens of millions of miles away would still be a moon of Jupiter if it would continue doing so in the absence of the sun. It is a satellite if it is gravitationally bound (not merely influenced or in resonance with a solar orbit) AT ALL to a primary other than the sun. What's more, scientists would call it a satellite of Jupiter just as they call the moon a satellite of earth except in the context of your silly contention.

[Republican Wildcat]

Anti-Plutites!!!!!

[FreedomCalls]

I like the idea of eight major planets and any number of minor planets that would include Pluto. You can choose where the dividing line goes between "not a planet" and a "minor planet." But leave the eight fixed major planets.

[FreedomCalls]

Re: star vs planet
if we think it's big enough to burn it's on matter, but isn't doing so? What if it's doing so but only on a very limited basis and doesn't look like a star?

As Jupiter currently does. It emits more radiation than it receives from the Sun. None of it is in visible wavelengths though. Is Jupiter therefore a star?

[UnbelievingScumOnTheOtherSide]

As Jupiter currently does. It emits more radiation than it receives from the Sun. None of it is in visible wavelengths though. Is Jupiter therefore a star?

So did the earth when it was in its molten state and still would if not for its insulating crust. The point is that Jupiter does not have enough mass to sustain a 4H fusion chain reaction. Of course, matter could accrete slowly enough in a larger object that fusion would never ignite, or so fast in a smaller object that fusion would occur briefly in a smaller object, or that an object would be so hydrogen-poor that fusion would not occur. And many stars no longer have a chain reaction going because their fuel is exhausted. These stars will continue to cool forever until they resemble terresatrial planets except for their mass and density. Some of the lightest of these may even resemble gas giant planets like Jupiter because there is not enough gravity to reduce its outer layer to white dwarf electron degeneracy. So the dividing line between the largest planets and smallest stars is a mathematical calculation of the smallest mass that will create the nuclear confinement to sustain a continuous fusion process, regardless of whether such process is, will or ever had taken place.

[DannyTN]

"As Jupiter currently does. It emits more radiation than it receives from the Sun. None of it is in visible wavelengths though. Is Jupiter therefore a star?"

Yeah, I thought there was something like that about Jupiter. Under his definitions I think Jupiter is too big to be a planet. There will be ceaseless arguments about what size, what mass, what makeup constitutes a planet.

But it's the small end that really gets me. How do you know when a planet's gravity is stronger than it's material.

His definitions are about the worse I can imagine, and yet he thinks they are concise.

[Gorjus]

Okay, you lost it. Changing from a rational discussion to an ad hominem attack (calling my suggested definition "silly") is the sign of failure on your part.

Of course, like any other high-handed declaration you choose to make, you don't have to agree with me, but if you think using a liberal approach to debate - i.e. ad homimen attacks - contributes to a discussion, you'll have to find someone else because I won't play your silly game.

[ffusco]

Sol and Luna are Latin.

[Aquinasfan]

Nature doesn't recognize the category "planet", nor is there any "Aristotelian essence of planethood", it's simply a manmade category, a shorthand descriptive term which, as the argument in the article makes clear, runs into more and more problems as one attempts to delineate its (imaginary) limits.

If the term is man-made, and refers to no objective and universal reality, then how can you know that we're both speaking of the same thing when we use the term "planet," or any other term?

The fact is that scientists, like all people, can't help but use terms when speaking. If terms do not refer to universals or essences, then we're left with nominalism and ultimately solipsism or absolute skepticism.

Nominalism

I realize that you have no interest in Aristotelianism and Thomism, but I post this info for the benefit of others.

[<1/1,000,000th%]

I believe it's Gaea in modern Greek.

Antichthon was the counter-earth in an earlier scheme.

[UnbelievingScumOnTheOtherSide]

Okay, you lost it. Changing from a rational discussion to an ad hominem attack (calling my suggested definition "silly") is the sign of failure on your part.

"Ad hominem" means "to the man". An ad hominem is an attempt to discredit an argument by discrediting you. Every characterization of an argument is not an ad hominem on the person making it. And frankly, your argument was ad hoc and therefore silly. Its purpose (and I know that you did not originate it so this imputation of bad motive, which would be an ad hominem attack, is not an ad hominem on you) is to confuse the debate over what is a planet by dragging some long-recognized satellites into it.

Of course, like any other high-handed declaration you choose to make, you don't have to agree with me, but if you think using a liberal approach to debate - i.e. ad homimen attacks - contributes to a discussion, you'll have to find someone else because I won't play your silly game.

You are free to be the pot calling the kettle black, but declaring victory and leaving smacks of the same liberalism. I can only assume you are being intentionally ironic :-)

[PatrickHenry]

Antichthon was the counter-earth in an earlier scheme.

Teach the controversy!

[<1/1,000,000th%]

LOL!

[dfwgator]

What about Planet Claire?

[<1/1,000,000th%]

Yeah, I thought there was something like that about Jupiter. Under his definitions I think Jupiter is too big to be a planet.

What if we call it a tiny brown dwarf star?

[DannyTN]

"What if we call it a tiny brown dwarf star?"

That would be consistent with the link you posted where they named a Jupiter sized object incapable of fusion orbiting another star a brown dwarf. But I don't like it for two reasons.

Jupiter is in our back yard and has been a planet all this time. I don't really want to be in a two star system, especially if one star is a dud.

Which brings me to the second reason. Stars ought to glow. I think one of the defining characteristics for a star should be that it produces visibile light. Or at least produced light at one time and burned out.