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Though the exoplanets found to date are in our galaxy, most are about 100 light-years away. (NASA, ESA, and A. Nota (STScI/ESA))

The Planet Hunters

Nevermind the demotion of Pluto to a dwarf planet. Astronomers have found about 200 planets orbiting other stars, and they say it's only a matter of time before they discover another Earth.

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It's a challenging night for astronomy at Lick Observatory near San Jose, California. The lights of Silicon Valley shimmer below the 4,200-foot summit of Mount Hamilton, washing out the faintest stars. Clouds drift closer from the north with a threat of rain. On the mountaintop are ten telescope domes, and I walk up a steep driveway to the biggest one. There's an eerie sound, like a loose shutter groaning in the wind. It's the dome itself, creaking as it rotates to keep its opening centered above the slowly moving telescope inside.

Chris McCarthy, an astronomer at San Francisco State University (SFSU), greets me at a side door. Wearing a small light strapped to his head, he leads me up metal stairways through the dome's interior, kept inky dark for nighttime observations, and into the warm control room. There, Howard Isaacson, an SFSU senior, and Keith Baker, a telescope technician, sit at computer screens amid thick bundles of cables and racks of antiquated electronics. McCarthy and Isaacson chat and sip hot tea while Baker uses his computer mouse to adjust the telescope. Before dawn, the astronomers will gather light from dozens of stars. Some of the stars, they hope, harbor new worlds.

In an era of rovers exploring Mars and space-based telescopes taking dazzling pictures of the cosmos from above the murk of Earth's atmosphere, the routine at Lick—pointing a 47-year-old telescope at star after star for hours on end—feels rather quaint. Yet these astronomers are members of a team that is the best in the planet-hunting business. Using telescopes in Hawaii, Chile and Australia, as well as those at the Lick Observatory, to monitor about 2,000 stars—most of them quiet and middle-aged like our sun and close enough to Earth for big telescopes to get a clear view of their movements—the team has found about two-thirds of the 200 or so planets discovered outside our solar system so far. (The recent hubbub about how to define a planet in our solar system hasn't threatened the planetary status of these distant objects.)

Some of the newfound extrasolar planets, or exoplanets, as they are called, are giant worlds the size of Jupiter that circle their stars on tight, roasting orbits, far closer than Mercury's orbit around the sun. Others swoop close to their stars and then swing far out on egg-shaped paths, scattering smaller bodies as they go. Some newborn planets fling their sibling planets to a fiery doom or into the depths of space.

Nowhere to be seen—at least not yet—is a solar system like ours, with solid planets near the sun and gas-giant planets on orderly processions farther out. Such a system is the likeliest place for a rocky planet like Earth to survive in a stable orbit for billions of years. Perhaps it's parochial, but astronomers seeking signs of life elsewhere in the cosmos—a quest that animates the search for exoplanets—are looking for planets and solar systems rather like ours, with a planet that is neither too far from nor too close to a star, and perhaps with water on its surface. The California team says finding Earth-like planets is just a matter of time.

The study of exoplanets is still very new, after all. Earlier than a decade ago, astronomers figured it would be impossible to see them against the brilliant glare of their stars. So a few astronomers tried to find exoplanets by looking for stars that seemed to wobble, tugged by the gravity of unseen bodies orbiting around them. But most experts doubted the approach would work. "People thought looking for planets was worthless," McCarthy says. "It was one step above the search for extraterrestrial intelligence, and that was one step above being abducted by aliens. Now, it's one of the greatest scientific advances of the 20th century."

The first exoplanet, discovered in 1995 by Michel Mayor and Didier Queloz of the University of Geneva, in Switzerland, was a giant object half the size of Jupiter, whirling around a star similar to our sun in a frenzied orbit every four days. The star, in the constellation Pegasus, is about 50 light-years away. More "hot Jupiters," or giant gaseous planets orbiting close to stars, quickly surfaced, if only because those big bodies impose the most pronounced wobbles on their parent stars.

Though astronomers haven't observed those planets directly, they infer that they're gaseous from their sheer size and what’s known about planet formation. A planet coalesces out of the debris in the great disks of dust and gas surrounding stars. If it reaches a certain size—10 to 15 times the size of Earth—it exerts such a gravitational pull and sucks in so much gas that it becomes a gas giant.

As measuring techniques improved, astronomers discerned gradually smaller planets—first the size of Saturn, then down to Neptune and Uranus. After a few years of spotting exoplanets, scientists saw a promising trend: as the sizes they could detect got smaller, there were more and more of them. The process that builds planets seems to favor the little ones, not the titans.

In the last year and a half, the California team and a group led by researchers in Paris discovered the smallest exoplanets yet seen around sunlike stars: the two planets were just five to eight times the mass of Earth. Astronomers say such worlds may consist mostly of metal and rock, perhaps with thick atmospheres. The exoplanet found by astronomer Geoff Marcy of the University of California at Berkeley and colleagues is close to its star and probably too hot for liquid to exist on its surface. The other planet orbits far from a faint star and may be as cold as Pluto. Still, learning that not all exoplanets are giant balls of gas was a landmark for the field. "These are the first plausibly rocky worlds," says Marcy. "For the first time, we are beginning to discover our planetary kin among the stars."

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