Earth's moon could become a planet
By Robert Roy Britt
SPACE.com


(SPACE.com) -- If astronomers approve a newly proposed planet definition
next week, things could get really strange. Sure, asteroid Ceres will
become a planet. Pluto's moon Charon will become a planet.

But we're talking really strange.

Eventually, if Earth and its moon survive long enough, the moon will have
to be reclassified as a planet, said Gregory Laughlin, an extrasolar planet
researcher at the University of California, Santa Cruz.

The new definition, proposed this week by the International Astronomical
Union (IAU), basically says every round object orbiting the sun is a
planet, unless it orbits another planet. But there is a big caveat: If the
center of gravity, called the barycenter, is outside the larger object,
then the smaller object is a planet. That wording elevates Pluto's moon
Charon to planethood, an idea some astronomers have criticized.

But here's the thing. Earth's moon was born in a catastrophic collision
more than 4 billion years ago. It started out very close to the planet but
has been moving away ever since. It's currently drifting away about 1.5
inches (3.74 centimeters) every year.

For now, the system's barycenter is inside Earth. But that will change.

"If the Earth and moon do survive, then the barycenter will eventually
move outside the Earth as the moon recedes," Laughlin told SPACE.com.
"At that point the Moon would be promoted to planetary status."
[What would we call it?]

None of this would occur for a few billion years. And Earth and the moon
would have to survive a host of remote catastrophe scenarios along with the
predicted swelling of the sun into a red giant, which Laughlin and others
have previously said might engulf and vaporize our planet (unless we can
figure out a way to move it).

Other astronomers have noted that it is possible there are three-object
systems yet to be found in the outer solar system. If they are all round
and have that certain barycenter thing happening, then they'd be called
triple planets under the new definition.

It gets stranger.

Astronomers expect to find hundreds of Pluto-sized objects in the outer
solar system. If one has a satellite that is round, and which has a certain
eccentric orbit -- meaning the two objects come very close together at one
point and then diverge greatly -- then the barycenter could dip inside the
larger object during part of the orbit, Laughlin explained.

In such a case, the smaller object would be defined as a moon part of the
time and a planet the rest.

A vote on the new definition is scheduled for August 24 at the IAU meeting in Prague.

Find this article at:
http://edition.cnn.com/2006/TECH/space/08/18/moon.planet

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