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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.

Does all the observed chaos in the universe portend dire consequences for small rocky planets? Not at all, Laughlin says. The technique of measuring the back-and-forth wobbles of stars, sensitive as it is, would have to be about ten times finer to reveal objects the size of Earth. But satellite telescopes scheduled for launch in the next few years might be able to detect "shadows" of alien earths as the small planets pass in front of their stars. Laughlin predicts the satellites will find such bodies in droves, even around stars where no large planets have yet been seen. "It's very likely that [sun-like] stars are accompanied by terrestrial planets," he says. "My intuitive sense is that our solar system is not uncommon at all."

Berkeley's Geoff Marcy agrees, because he says every star is born with enough raw material around it to create many planets. Lots of solid planets like Earth should form, he says, as dust coalesces into pebbles, which collide again and again to make asteroids and moons and planets. "Maybe Jupiters are rare," he says, "but rocky planets almost certainly are common. I just don't see how making an Earth could be hard."

The small exoplanet recently detected by Marcy and Butler's team supports that view. They found it while monitoring the two resonant planets in the Gliese 876 system, which is 15 light-years away. Something was exerting subtle extra tugs on the planets' orbits, and the best explanation for that is a third planet perhaps 7.5 times as massive as Earth. Given its size, the planet is likely rocky, like Earth, rather than a gas giant. The discovery was a major step toward answering the question on everyone's mind: Can we find potential habitats for life elsewhere?

Astronomers were hoping that question would be answered by a NASA satellite mission called Terrestrial Planet Finder. It was supposed to go beyond detecting exoplanets: it would take images of the most tantalizing exoplanets and analyze their atmospheres. But early this year, NASA put the mission on hold, largely because of budget overruns from the space station and the space shuttle and the expected cost of the plan to send people to Mars.

In the meantime, the California-based team keeps looking for more exoplanets. In a few months, Marcy and co-worker Debra Fischer of SFSU will start working with a new telescope at Lick called the Automated Planet Finder that will feature the most sensitive light-analyzing instrument yet made for exoplanet searches. The robotic instrument will scan about 25 promising stars every clear night, with the potential to detect planets as small as three to five times bigger than Earth. "This will be the world's first telescope completely dedicated to planet hunting," Fischer says. "People thought it would take billion-dollar space missions to find other planets like Earth, but I think we have a shot at it from the ground."

Marcy says finding planets from earth is just the beginning. "Ultimately, we need to go, with robotic spacecraft and a small digital camera, and send that little puppy to Tau Ceti or Epsilon Eridani," Marcy says, naming two nearby stars with particular promise for hosting Earth-like planets. They are 12 and 10.5 light-years away, respectively. "Sure it will take 100 years [to develop the technology], but it is a wonderful goal for our species, and it is within our grasp. It is entirely technologically feasible to get the first pictures of the surface of a planet around another star. We can launch a global mission, an emissary from Earth. The effort we're doing now is simply reconnaissance for that mission, but it is a glorious reconnaissance to spot the first oases in the cosmic desert."

Robert Irion directs the Science Communication Program at the University of California at Santa Cruz. Photographer Peter Menzel co-authored Hungry Planet: What the World Eats.

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