Beyond the Stars

From Nature, by Eugenie Samuel Reich

Launched in 2009 to seek out worlds beyond the Solar System, the Kepler mission is exceeding expectations. Is it closing in on another Earth?

Sitting for an interview in his office at the Harvard-Smithsonian Center for Astrophysics (CFA) in Cambridge, Massachusetts, the normally voluble astronomer Dimitar Sasselov looks nervous. Asked for his favourite among the many potential planets discovered by NASA’s Kepler planet-finding mission, for which he is a co-investigator, he hesitates, then sidesteps the question entirely. “Personally, I’m already beyond that point. It’s not one. It’s not a single planet. It’s a whole family.”

Sasselov has good reason to be wary: his public lecture last July at the Technology, Entertainment and Design 2010 conference in Oxford, UK, earned him a stern rebuke from his colleagues on the mission. Not only had he presented numbers for possible planets greater than those released officially by the team, they said, but he had also used a careless phrasing that resulted in a raft of headlines proclaiming — incorrectly — the discovery of hundreds of other Earths.

That hullabaloo became a distant memory this week, as the mission released 400 candidate systems, adding to the 306 released last June. Along with the candidates came a bunch of confirmed planets. The latest finds, posted on NASA’s website last month (see and published in Nature this week1, include a rocky planet orbiting so closely to its star that its starlit side must be a seething sea of lava; and a planetary system containing several large, rocky or icy planets in orbits of tens of days, just one order of magnitude faster than Earth’s 365-day cycle. “It’s very exciting. It’s a type of system we haven’t seen before,” says Jack Lissauer, a space scientist and Kepler co-investigator based at the NASA Ames Research Center in Moffett Field, California, and a lead author on the paper in Nature.

Nonetheless, most of the Kepler scientists continue to be cautious. By watching the light from some 150,000 stars for the dimming that could signal a planet crossing in front of them, Kepler is extraordinarily efficient at finding possible planets. But Kepler has yet to find another Earth — a small, rocky planet with an orbit of a few hundred days and well inside the habitable zone in which water can exist and life can arise. That is for a fundamental reason; the blips that Kepler detects show only the radius, and not the mass, of an observed planet, which means that the density and composition generally remain unknown.

Moreover, the scientific objective of the Kepler mission is not to discover Earth-like planets. Instead, it is to estimate the fraction of Sun-like stars that have Earth-like planets — statistics that could greatly enhance astronomers’ understanding of how planetary systems form. Determining which of the blips correspond to planets — rather than systems of stars in which one is eclipsed, causing a similar dimming — is what the researchers spend most of their time on, says William Borucki, a space scientist and Kepler principal investigator at NASA Ames. The only way to do that, he says, is the hard way: painstakingly sorting the real signals from the false positives.

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