Earth's Moon Could Become a Planet

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 Aug. 24 at the IAU meeting in Prague.

• Top 10 Ways to Destroy Earth
• Image Gallery: The 12 "Planets"
• Those Wild and Crazy Astronomers
• Astronomers Sharply Divided on New Planet Definition
• Adding Planets Means New Textbooks, Toys
• Top 10 Cool Moon Facts