Earth Size Planets Common in Space

PASADENA, Calif. -- Nearly one in four stars similar to the sun may host planets as small as Earth, according to a new study funded by NASA and the University of California. The study is the most extensive and sensitive planetary census of its kind. Astronomers used the W.M. Keck Observatory in Hawaii for five years to search 166 sun-like stars near our solar system. All of the planets in the study orbit close to their stars. The results show more small planets than large ones, indicating small planets are more prevalent in our Milky Way galaxy.

Our ground-based technology can't see Earth-size planets, but we can estimate their numbers," said Andrew Howard of the University of California, lead author of the new study. "Earth-size planets in our galaxy are like grains of sand sprinkled on a beach -- they are everywhere."

The research provides a tantalizing clue that potentially habitable planets could also be common. These hypothesized Earth-size worlds would orbit farther away from their stars, where conditions could be favorable for life. NASA's Kepler spacecraft is also surveying sun-like stars for planets and is expected to find the first true Earth-like planets in the next few years.

Howard and his planet-hunting team, which includes principal investigator Geoff Marcy, looked for planets within 80-light-years of Earth, using the radial velocity, or "wobble," technique. They measured the numbers of planets falling into five groups, ranging from 1,000 times the mass of Earth, or about three times the mass of Jupiter, down to three times the mass of Earth. A distinct trend jumped out of the data: smaller planets outnumber larger ones. Only 1.6 percent of stars were found to host giant planets orbiting close in. About 6.5 percent of stars were found to have intermediate-mass planets, with 10 to 30 times the mass of Earth -- planets the size of Neptune and Uranus. And 11.8 percent had the so-called "super-Earths," weighing in at only three to 10 times the mass of Earth.

"During planet formation, small bodies similar to asteroids and comets stick together, eventually growing to Earth-size and beyond. Not all of the planets grow large enough to become giant planets like Saturn and Jupiter," Howard said. "It's natural for lots of these building blocks, the small planets, to be left over in this process." The astronomers extrapolated from these survey data to estimate that 23 percent of sun-like stars in our galaxy host even smaller planets, the Earth-sized ones, orbiting in the hot zone close to a star. "This is the statistical fruit of years of planet-hunting work," said Marcy. "The data tell us that our galaxy, with its roughly 200 billion stars, has at least 46 billion Earth-size planets, and that's not counting Earth-size planets that orbit farther away from their stars in the habitable zone." The new study finds a surplus of close-in, small planets where theories had predicted a scarcity.

"We are at the cusp of understanding the frequency of Earth-sized planets among planetary systems in the solar neighborhood," said Mario R. Perez, Keck program scientist at NASA. More information about exoplanets is at http://planetquest.jpl.nasa.gov . Thanks to Whitney Clavin Jet Propulsion Laboratory.