Astronomers estimate the number of Earth-sized planets in the Milky Way

Science : Although astronomers have found nearly 500 extrasolar planets, or exoplanets, in our galaxy, the impressive sample lacks an exoplanet whose mass and orbit match our Earth’s. The latest advance in the quest to find an exo-Earth is a paper in today’s Science. Andrew Howard of the University of California, Berkeley, and his collaborators report the results of a spectroscopic survey of 166 sunlike stars. The survey, which was undertaken using the HIRES instrument at the W. M. Keck Observatory on Mauna Kea, Hawaii, was sensitive to exoplanets whose orbital periods are 50 days or less. Planets with longer orbits are harder to detect. Thanks to the quantity and quality of the survey data, Howard and his collaborators could determine the exoplanets’ mass distribution. Their conclusion: Contrary to certain models of planet formation, 23% of stars harbor a close-in Earth-mass planet. The survey also implies that NASA’s planet-hunting Kepler observatory, which was launched last March, should find 120–260 of those planets around sunlike stars.