New Planetary Puzzlers

The discovery of planets around distant stars has become like space-shuttle launches--newsworthy but just barely. With some 50 extrasolar planets under their belt, astronomers have to announce something really strange to get anyone's attention.

Last week they did just that. Standing in front of colleagues and reporters at the American Astronomical Society's semiannual meeting in San Diego, the world's premier planet-hunting team--astronomer Geoffrey Marcy of the University of California, Berkeley, and his colleagues--presented not one but two remarkable finds. The first is a pair of planets, each about the mass of Jupiter, that whirl around their home star 15 light-years from Earth in perfect lockstep. One takes 30 days to complete an orbit, the other exactly twice as long. Nobody has ever seen such a configuration.

But the second discovery is far stranger--a solar system 123 light-years away, in the constellation Serpens, that harbors one "ordinary" planet and another so huge--17 times as massive as Jupiter--that nobody can quite figure out what it can be. It is, says Marcy, "a bit frightening."

What's frightening is that these discoveries make it clear how little astronomers know about planets, and they add to the dawning realization that our solar system--and by implication Planet Earth--may be a cosmic oddball. For years theorists figured that other stars would have planets more or less like the ones going around the sun. But starting with the 1995 discovery of the first extrasolar planet--a gassy monster like Jupiter but orbiting seven times as close to its star as Mercury orbits around our sun--each new find has seemed stranger than the last. Searchers have found more "hot Jupiters" like that first discovery. These include huge planets that career around their stars not in circular orbits but in elongated ones; their gravity would send any Earthlike neighbors flying off into space. Says Princeton astronomer Scott Tremaine: "Not a single prediction for what we'd find in other systems has turned out to be correct."

Last week's giant was the most unexpected discovery yet. Conventional theory suggests that it must have formed like a star, from a collapsing cloud of interstellar gas. Its smaller companion, only seven times Jupiter's mass, is almost certainly a planet, formed by the buildup of gas and dust left over from a star's formation. Yet the fact that these two orbs are so close together suggests to some theorists that they must have formed together--so maybe the bigger one is a planet after all.

Or maybe astronomers will have to rethink their definition of "planet." Just because we put heavenly objects into categories doesn't mean the distinctions are necessarily valid. And as Tremaine puts it, "When your classification schemes start breaking down, you know you're learning something exciting. This is wonderful stuff."