NASA Survey Suggests Earth-Sized Planets are Common

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Trent Perrotto 202-358-0321
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News release: 2010-357 Oct. 28, 2010

NASA Survey Suggests Earth-Sized Planets are Common

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2010-357&cid=release_2010-357

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 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.

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, 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 units, 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 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 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 planetary
systems 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."

NASA's Exoplanet Science Institute at the California Institute of Technology, Pasadena, Calif.,
manages time allocation on the Keck telescope for NASA. NASA's Jet Propulsion Laboratory,
also in Pasadena, manages NASA's Exoplanet Exploration program office. More information
about exoplanets and NASA's planet-finding program is at http://planetquest.jpl.nasa.gov .

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