How and Why

Stephen Hawking may be right about marauding space aliens

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Brian Palmer
Monday, September 27, 2010; 6:34 PM

Aliens have been in the news this year. In April, cosmic oracle Stephen Hawking, the legendary theoretical physicist, proclaimed that extraterrestrial life is almost certain to exist. He also mentioned, by the way, that we should stay as far away from aliens as possible, since they're probably scavenging the universe for resources after destroying their own homes.

Then, in August, two separate teams of astronomers discovered that distant solar systems had as many as 10 planets, one of which may be capable of hosting life because of its near-Earth size. That brings the total count of confirmed planets outside our solar system to more than 400.

How do astrophysicists find these far-off planets? What makes a planet a good candidate for hosting life? And when can we expect to be Skyping with E.T.? Let's pull on our alien-hunting boots.

The first step is defining our quarry. Hawking could be right - maybe there are fearsome extraterrestrials zipping around the universe at hyper-speed. Or maybe silicon-based life forms are riding across a distant galaxy on an asteroid as you read this. But most astrophysicists aren't betting on that.

Because current technology and resources limit our search to the nearest few hundred stars, our best odds lie in searching for the only kind of life we know to exist: carbon-based cells that consume oxygen and carbon dioxide. That means we're looking for a planet a lot like Earth.

The problem is that it's really hard to find planets more than 20 trillion miles away, because the bright light of their nearby star obscures them from view. It's like trying to find a needle in a haystack, but with someone shining a floodlight in your eyes.

Over the last couple of decades, scientists have gotten around this problem through indirect observation: detecting the planets' effects on other objects.

The first method they use involves gravitational wobble.

Think of a drum major at the head of a marching band spinning his baton. He holds the staff at the center, because the weight is equally distributed. But if one end weighed more than the other, he'd have to grasp the baton closer to the heavy end.

The same thing happens with a planet and its star, which have vastly unequal masses. (Our sun is 333,000 times as massive as Earth.) The greater the difference, the closer the rotational center moves toward the star.

Most stars are so much heavier than their planets that the point of rotation is inside the star itself, just off its geometric center. That makes the star wobble a little bit as it rotates. Scientists used wobble to identify the first extrasolar planets in the 1990s.

Transiting is another technique.

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