Survey Says... Earth-Sized Planets Common
NASA Survey Suggests Earth-Sized Planets are Common
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 for planets of various sizes, ranging from three to 1,000 times the mass of Earth. 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.
“We studied planets of many masses -- like counting boulders, rocks and pebbles in a canyon -- and found more rocks than boulders, and more pebbles than rocks. Our ground-based technology can’t see the grains of sand, the Earth-size planets, but we can estimate their numbers,” said Andrew Howard of the University of California, Berkeley, 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 study appears in the Oct. 29 issue of the journal Science.
Howard and his planet-hunting team, which includes principal investigator Geoff Marcy, also of the University of California, Berkeley, 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. The search was confined to planets orbiting close to their stars -- within 0.25 astronomical unit, or a quarter of the distance between our Sun and 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. That includes the three highest-mass planet groups in the study, or planets comparable to Saturn and Jupiter. 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 findings challenge a key prediction of some theories of planet formation. Models predict a planet “desert” in the hot-zone region close to stars, or a drop in the numbers of planets with masses less than 30 times that of Earth. This desert was thought to arise because most planets form in the cool, outer region of solar systems, and only the giant planets were thought to migrate in significant numbers into the hot inner region. 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 celestial bodies in the solar neighborhood,” said Mario R. Perez, Keck program scientist at NASA Headquarters in Washington. “This work is part of a key NASA science program and will stimulate new theories to explain the significance and impact of these findings.”