No Lack of Exo-Earths Out There

In May 2009, NASA took a big gamble: It launched a spacecraft on a $600 million mission to hunt for Earth-like planets around other stars that could harbor life. Astronomers are now more hopeful than ever that the gamble will pay off. A new study published in tomorrow's issue of Science estimates that in the coming months, the spacecraft, called Kepler, should find a few hundred Earth-sized exoplanets orbiting uncomfortably close to their stars. As many or more should turn up in the next year or two, farther from their stars where life could thrive.

How tough the search for habitable worlds would be wasn't at all clear when NASA gave the Kepler team the go-ahead almost 10 years ago. Only huge, scorching-hot exoplanets larger than Jupiter had been found by then. To get a better handle on the challenge, astronomers Andrew Howard and Geoffrey Marcy of the University of California, Berkeley, selected 166 stars similar to the sun and within 80 light-years of Earth, though not Kepler targets. For 5 years, they and colleagues monitored the frequency of light from those stars for signs that orbiting planets were gravitationally tugging on their stars and producing a detectable Doppler shift in the starlight's frequency.

From their survey, Howard and his colleagues found that smaller exoplanets are much more abundant than larger ones. The smallest ones they could detect were at least three times the mass of Earth, and all their detections were orbiting close enough to their stars to have a "year" of 50 days or less. Extrapolating down to Earth-sized exoplanets—those one-half to two times the mass of Earth—the group predicted that 23% of sunlike stars have Earth-sized planets in close orbits. Applying these statistics to Kepler's search, the group estimates that Kepler will find 120 to 260 planets orbiting close in to sunlike stars.

The new results support the "general trend that there are a lot more smaller exoplanets than bigger ones," says astrophysicist Alan Boss of the Carnegie Institution for Science in Washington, D.C. That makes some sense, he says. Rocky planet formation requires smaller bits of primordial stuff to agglomerate into bigger bits, and those must somehow form ever bigger chunks until a rocky planet emerges. In one planet-forming scheme, at least, to make Jupiter-like, gas-giant planets, a planet-sized rocky body must go on to grab a huge mass of gas using its gravity. Because every chunk has some chance of not reaching the next level, smaller leftover pieces would outnumber the largest final products.

Although the new survey strictly applies only to close-in and therefore lethally hot exoplanets, Earth-sized bodies probably also abound at orbital distances where life would be comfortable, says Boss. "Earth-like planets are probably going to be common," he says. "Kepler will give us that number."