As many as one in four Earthlike extrasolar planets should have a moon-like moon, new supercomputer simulations show.

Having a massive moon may be needed for complex life to evolve on a wildly tilting planet. Earth’s moon raised ocean tides and stabilized our planet’s chaotic spin, allowing life’s baby steps to occur in relative peace.

“Without a large moon, our Earth would spin around randomly,” said astronomer Ben Moore of the University of Zurich in Switzerland, coauthor of a paper to appear in the journal Icarus. “Maybe it’s unlikely that a planet would have a massive moon, and that makes the Earth sort of special.”

At one-quarter of Earth’s diameter and more than 1 percent of its mass, Earth’s moon is proportionately one of the largest satellites in the solar system. It holds Earth’s rotation axis at 23.3 degrees, with a deviation of just 1.3 degrees every 41,000 years.

Mars, by contrast, oscillates by 10 degrees every few hundred thousand years. It’s even thought to have flipped on its axis at least once, which may have switched it from the warm wet world it once was to the dusty wasteland it is now.

If the collision that formed Earth’s big moon is a one-off event, however, the chances for alien life — at least as we know it — may become even slimmer. No matter how many exo-Earths are found, they may all be too unstable for life.

To find out how rare moon-like moons are, Moore ran 64 supercomputer simulations of planets forming via collisions between smaller clumps of rock.

Earlier studies began their simulations at the point in the system’s evolution where only 20 or 30 proto-planets remained. But Moore and colleagues, who usually study galaxy formation, were accustomed to tracking thousands of particles at once. They turned back their simulations’ clock to when thousands of small rocky bodies jostled about the sun, making their simulations more realistic than previous attempts.

The team found that most sun-like stars should have a rocky planet in the habitable zone, the region where temperatures are right for planets to have liquid water on the surface. Of those planets, one in 12 experienced an impact that formed a moon capable of holding it steady.

‘It looks like habitable exoplanets are quite common.’

“It looks like habitable exoplanets are quite common,” Moore said.

Some uncertainties remain in the simulations’ raw data, and details of moon formation are still under debate, he added. As many as one in four Earth-like planets, or as few as one in 45, should have stabilizing moons.

“This is good quality stuff. These guys know what they are doing,” said astronomer Sean Raymond of the Astrophysics Laboratory of Bordeaux in France, who was not involved in the new study.

However, Raymond said a large moon may not be as important for life as Moore suggested. If Earth spun 2.5 times faster, it wouldn’t need a big moon, he said.

According to Don Brownlee of the University of Washington in Seattle, co-author of the book Rare Earth, the question isn’t whether life could survive on a moonless Earth, but what kind of life it would be.

“Microbes might not care at all, but you’re probably not going to have elephants on planets without big moons,” said Brownlee. “So the big question is, what fraction of these planets are truly Earth-like? And how Earth-like does a planet have to be to have something like us on it?”

Image: The famous “Earthrise” image taken by Apollo 8 astronaut Bill Anders. Credit: NASA.

Citation:
S. Elser, B. Moore, J. Stadel and R. Morishima. “How common are Earth-moon planetary systems?” Icarus, in press. DOI: 10.1016/j.icarus.2011.05.025

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