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Planet Hunters Find Eccentric Titans

AUSTIN, TEXAS--Astronomers have thus far found 17 planets circling other sunlike stars--enough for an early census of their properties. The results are confounding: All planets beyond a certain distance from their parent stars have weird orbits with egglike shapes, making them dangerous companions for any smaller, Earth-like planets that might harbor life, researchers said here last week at a meeting of the American Astronomical Society.

For years, astronomers expected to see elsewhere what they saw in our own orderly solar system: rocky planets close to a star and gas giants farther away, all in neat, nearly circular orbits. The discoveries, in 1995, of the first Jupiter-sized planets whirling in very close orbits dispelled the first part of that notion. These planets had relatively circular orbits, but a few others found soon thereafter had highly elliptical orbits. Now, planet hunters have realized that all nine planets revolving more than 30 million kilometers from their stars are following such orbits, says astronomer Geoffrey Marcy of San Francisco State University. "That's nine more than we expected," Marcy says.

Astronomers now think that passing stars or interactions between two large planets may spawn these wacky orbits. "If Saturn had grown to be as large as Jupiter, our own solar system's house of cards probably would have fallen apart," says Marcy. Perturbations would eject one such planet from the system, leaving the other behind in an oval orbit.

Such planets probably act as gravitational bullies; any Earth-like planet in the same system would eventually be slung into space. That's bad news for the prospects of intelligent life in those systems, which exobiologists think could arise only on rocky planets or moons. The good news, Marcy is quick to add, is that just 5% of the hundreds of stars regularly watched by his group are orbited by such eccentric titans. If "normal" planets circle other stars at greater distances, his team's techniques should reveal some of them within several years.

Astronomer William Cochran of the University of Texas, Austin, says the new findings are "removing our ethnocentric blinders," because no planetary system discovered so far even closely resembles our own. "We can no longer pull out ad hoc explanations for each planet with a weird orbit," he says. "It's a general phenomenon."