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.

[Gorjus]

If the point about which two bodies orbit each other is not beneath the surface of one of them,...then I'd call that...

Well, as you said, it's what you would call it, and you're free to do whatever you want. It's a logical distinction, and there are worse.

However, I think a better one is to say that if the gravitational attraction of some other body (like a nearby planet) is greater than the gravitational attraction of the star, then it's a moon. Otherwise, it's a planet (assuming it's also big enough to be round).

By that definition, our Luna is still a planet, since if you work the math the gravitational attraction of the Sun on Luna is greater than the gravitational attraction of the Earth on Luna.

[Gorjus]

This geometric definition is not valid.

I guess we'll just disagree. While you can make the case that being or not being a 'wanderer' is independent of distance from the sun (so that 'loose' planets far from any star are still planets, and not rocks or whatever), the issue is whether Luna is a planet or a moon. If you took the earth away, then Luna would be a planet at any distance from the sun, so is your definition that distance from the sun doesn't make a body a planet, but distance from another body does?

My definition of the difference between moon and planet would be based on whether the gravitational attraction of the nearby 'planet' (in this case Earth) on the smaller body is greater or less than the gravitational attraction of the sun on that smaller body. In the case of Luna, the sun exerts a greater force than the Earth does.

Obviously, this sort of distinction - whether a particular body is considered a moon or a planet - depends exactly on whether there is another nearby planet which exerts a greater force on the body than the sun itself.

By that definition, though the moons of Jupiter and Saturn would continue to orbit the sun if the planet were removed, they are nonetheless moons because the force on them from their planet is greater than the force on them from the sun.

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.

[Thoro]

They should kick Uranus out.

Farnsworth: "I'm sorry, Fry, but astronomers renamed Uranus in 2620 to end that stupid joke once and for all."

Fry: "Oh. What's it called now?"

Farnsworth: "Urectum."

[OrioleFan]

In a Constitutional Republic Pluto would be entitled to 2 Senators and one Representative. That would qualify it for red state status.

[Ichneumon]

Sheesh! You're right, but there's a character limit on the tagline! ;o)

..."and tagline space".

[Ichneumon]

What the bleep does this have to do with EVOLUTION.

Perhaps if you clicked the link in his ping post which was helpfully labeled "See the list's explanation", you could actually find the explanation which answers your question.

[operation clinton cleanup]

Why do all the other planet's moons get to have names, but Earth's moon does not?

[PatrickHenry]

Why do all the other planet's moons get to have names, but Earth's moon does not?

Tradition, going back to the time (before Galileo's discoveries) when our moon was assumed to be the only moon. Similarly, the sun has no name either, unlike all the other prominent stars.

For those who seem to need a name for the moon, it's often called Luna, which is "moon" in Latin. When your grandchildren are living on Mars, presumably they won't call Earth's moon "the moon." Until then, there's not much room for confusion.

[Raycpa]

Change the defintion to exlude smaller bodies and grandfather in Pluto.

[longshadow]

Similarly, the sun has no name either, unlike all the other prominent stars.

"Sol."

[Ichneumon]

I like it when scientists argue in Aristotelian/Scholastic terms.

Me too, because it shows the lack of validity of such attempts, and thus the failure of Aristotelian/Scholastic philosophical views in the first place.

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.

While it's convenient to say that "the Solar System has nine planets" (or some other number), it's based on an arbitrary and, ultimately, unrealistic way of classifying things. The reality is closer to a description along the lines of, "the Solar system has vast numbers of objects moving around in it, of various sizes (ranging from the vast down to the microscopic), with differing compositions, rotational axes, shapes, and trajectories, acted upon by the gravity of the Sun and other bodies in varying amounts." Cataloging the specific properties of various objects is much more informative, useful, and reflective of reality than any attempt to break them into arbitrary categories like "planet", "moon", "asteroid", etc.

The attempt to describe reality in Aristotelian/Scholastic terms all too often leads to occurences of the continuum fallacy and the paradox of the heap, which are indications that Aristotelian terms are fallacious ways of conceptualizing.

[PatrickHenry]

"Sol."

Yeah, same deal as Luna. Unless you're talking about your ne'er-do-well uncle.

[operation clinton cleanup]

I could probably google it, but when did our planet start commonly being referred to as the Earth?

[PatrickHenry]

I could probably google it, but when did our planet start commonly being referred to as the Earth?

In lots of SF stories, where people are living on lots of planets, Earth is Terra, Latin like Luna and Sol. As for when "Earth" was first employed as the name of our planet, I'd be guessing. I suppose it also dates from after Galileo, when it was realized that what had been "the world" was just one of several planets. Interesting question. Maybe someone has a more authoritative answer than my speculations.

[Ichneumon]

I could probably google it, but when did our planet start commonly being referred to as the Earth?

It's also interesting to note that throughout history, the most common name that various groups of humans (i.e. various tribes, nations, etc.) would apply to themselves in their native language was, "the People".

[operation clinton cleanup]

It was probably coined by the same guy who invented the wheel.

[stands2reason]

I misread your last sentence.

I thought you wrote that "the universe is named Dave."

Strange name for the universe.

[stands2reason]

You forgot Terra, which would be ? in Greek?

[stands2reason]

They realized that what they thought were brontosaurs were actually another dinasour that was named first.

[Bernard]

Little stars twinkle, Big Stars TWINKLE.