The finding is the first quantitative measurement of the frequency of planets of various masses in the galaxy.

“It’s a landmark paper,” said exoplanet expert Josh Winn of MIT, who was not involved in the new study. “There’s been all this talk, that low-mass planets like the Earth are very common. But this is the first time it’s been documented.”

The study, published in the Oct. 29 Science, also found plenty of planets in a mass range that astronomers expected to be empty, which may prompt an overhaul of planet-formation models.

Using the Keck Observatory in Hawaii, astronomer Andrew Howard of the University of California, Berkeley and colleagues watched 166 sun-like stars for the telltale wobbles induced by a planet’s pull.

Unlike previous surveys, Howard and colleagues were just as interested in stars that lack planets as stars that host them. To avoid biasing the study toward planet-bearing stars, the team selected the nearest and brightest stars in the 120,000-star Hipparcos Catalog.

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