A planet like ours is apparently hard to come by. Only 23% of Sun-like stars harbor a close-in planet between 0.5 and 2.0 times the mass of Earth, researchers report 28 October in Science.
“Earths are not that common, at least close-in ones,” says Andrew Howard, an astronomer at the University of California, Berkeley who is first author on the paper.
Howard and his team obtained their result by looking for a “wobble” in 166 Sun-like stars that denoted the tug of a companion planet orbiting at a distance less than one-quarter that of the Earth to the Sun. Though their methods were only sensitive to planets greater than 3 Earth masses, the team discovered more small planets than large ones and extrapolated this trend downward to estimate the occurrence of even smaller planets, says Howard.
Being so close to their host star, these planets are probably too hot to be inhabited, he adds. But finding Earth-mass planets in this location makes it more reasonable to find them farther out, in habitable orbits, so the results are a key step in discovering potential sites for life, he says.
The findings also suggest that the picture of planet formation need to be revised. Current models predict that most planets develop by accumulating dust and ice, says Howard. “It’s the ice that provides the glue to stick everything together,” he says. Since ice can’t stay frozen under the glaring heat of a star, there must be a mechanism for planets to drift from a system’s outer reaches inward without getting lost into the parent star, he adds.
The occurrence of close-in, Earth-like planets is roughly half that of a previous estimate from the Geneva Observatory in Switzerland, says Greg Laughlin of the University of California, Santa Cruz, who was not involved with the work. This discrepancy will be solved when data from Kepler, a space-based telescope that can detect exoplanet with high precision, becomes available early next year, he says. “I think this is an advance clue to what the Kepler results will be,” he says, since it is consistent with the small amount of Kepler data already released.
Despite their variation, the different numbers point to astronomers’ ever-increasing knowledge about exoplanet populations, says Laughlin. “We can now try putting everything into context, talking about many planets rather than individual odd worlds,” he says.
Image: ESO/L. Calçada