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.

[beezdotcom]

In effect, we have a double-planet system.

Yep. But YOU try getting everyone to stop calling it "The Moon".

In fact, I would argue that all other moons are incorrectly defined and named - our sister planet was FIRST to get the name "Moon", so we should probably use a different term other than "moon" to describe natural satellites.

[Sergio]

Clyde was a professor at New Mexico State University while I was a student there (80-83). He walked hunched over, as many older folks do, but very quickly, and really enjoyed talking to students. He always had a very cheery greeting for anyone he passed while walking on campus. Very cool guy.

[RonF]

Another criterion for calling a body a planet would be that it's primary should be the Sun. This would keep Titan, the Moon, and other natural satellites from being called planets.

[RonF]

Nick Fury, Agent of S.H.I.E.L.D.!

[RonF]

The point about which both the Moon and the Earth orbit about each other stays about 1000 miles beneath the Earth's surface (but obviously it's location changes as the two bodies orbit and the Earth spins on it's axis). In such a case, I'd hold that the Moon orbits the Earth. If the point about which two bodies orbit each other is not beneath the surface of one of them, but is between them, then I'd call that a binary system.

[GretchenM]

Next one to go:

[GretchenM]

Maybe you need to expand your definition and say that it's a bigger than Pluto and is the largest object within a million miles.

I was hoping we could keep Hillary's posterior out of this discussion.

[UnbelievingScumOnTheOtherSide]

Actually, if you look at the orbit, "our" moon actually orbits the sun, not Earth. The moon's orbit is always concave toward the sun.

It depends on what is meant by "directly orbiting". This geometric definition is not valid. The wave formed by the motion of the moon around the earth being carried around the sun is an accident of the location of the earth. If you moved the earth far enough from the sun so that the moon's orbital velocity around the earth were faster than the earth's orbital velocity around the sun, then the moon would no longer fit that definition. The definition of a planet should not depend on its distance from the sun.

If the sun were gone, earth would still be a planet but the moon would not because it would remain in orbit around the earth (and earth is not a star). The moon could only be a planet then if the earth were also gone. Therefore, the moon is in direct orbit around the earth, not the sun.

Almost every moon is bound in orbit to the sun in the sense that if the planet suddenly disappeared, that moon would remain in the solar system. The only exceptions might be moons very close to Jupiter whose velocity around Jupiter combined with Jupiter's orbital velocity would exceed escape velocity from the sun at certain points in its orbit.

But I submit that any smaller object which would remain gravitationally bound to a primary object other than the sun if the sun were gone has an intermediary and is not in "direct orbit" around the sun and is not eligible to be designated a planet. (Certain objects that are merely in orbital resonance with another body around the sun, which would escape that primary body if not for the sun's gravity, such as certain Apollo asteroids which come around one side of the earth and then the other forming an extremely elongated and unstable temporary "orbit" but would fly away if not for the sun bringing it back are in "direct orbit" around the sun and not the earth.)

[RightWhale]

Here's the thing to do: determine if the celestial body has a function in the astrological charts. If it does it is a planet, the moon, or the sun. If not, it is an asteroid, a moon (not the moon), or a comet. This would make everything beyond Saturn a non-planet unless an astrologer (Persian or Indian) of some repute has a use for it.

[RightWhale]

we should probably use a different term other than "moon" to describe natural satellites

Galileo's fault.

[steve-b]

(Certain objects that are merely in orbital resonance with another body around the sun, which would escape that primary body if not for the sun's gravity, such as certain Apollo asteroids which come around one side of the earth and then the other forming an extremely elongated and unstable temporary "orbit" but would fly away if not for the sun bringing it back are in "direct orbit" around the sun and not the earth.)

Example: Cruithne

[beezdotcom]

The point about which both the Moon and the Earth orbit about each other stays about 1000 miles beneath the Earth's surface

Oh YEAH? Say that to my FACE (in about ~10⁸ years or so, when I might actually be RIGHT...gotta remember to watch those decimals...)

[King Prout]

I have some arbitrary standards of my own

1. is the satellite's primary a star?

if no: it ain't a planet
if yes to 1 and 2:

2. Is the body essentially spherical?

if no: it ain't a planet
if yes:

3. does the body in question have sufficient gravity to maintain the stable orbit(s) of natural satellite(s) of its own?

if no: it ain't a planet
if yest to all the above: it IS a planet

[King Prout]

if no: it ain't a planet
if yes to 1 and 2:

2. Is the body essentially spherical?
if no: it ain't a planet
if yes:

need more coffee...

[Surtur]

Von Strucker must be happy

If the Baron isn't happy, he can always A.I.M. for something better.

[Tanniker Smith]

so we should probably use a different term other than "moon" to describe natural satellites.

In a story dating to the late sixties, one of my older brothers when he was of a single-digit age, asked my mother if the Moon was a planet. She replied, "No, it's a satellite." He looked a little surprised and then asked, "Really? Who launched it."

The funny (or maybe "strange") thing is that when I repeated this story to my girlfriend back in the late 80s, she that it was silly to refer to the Moon as a "satellite". She asked her younger sister to define what a satellite was, and apparently she didn't know that the Moon was a satellite. Sigh. Hello?? This story took place when there were more *natural* satellites in our Solar system than mad-made ones. The definition has kind of changed.

TS
(And this is coming from a guy who had a problem with rationalizing the term "satellite nations" when I first learned about the Soviet Union in 6th grade.)

[Alamo-Girl]

Thanks for the ping!

[Tanniker Smith]

So if the Moon is in the 7th house and Jupiter is aligned with Mars . . .

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

LOL!

[Surtur]

And RAM. And disk. And GHz. And bandwidth. And Pixels.

That group sounds like some of Von Strucker's henchmen.