NASA Survey Suggests Earth-Sized Planets Are Common
WASHINGTON -- 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 study. "Earth-size planets in our galaxy
are like grains of sand sprinkled on a beach -- they are everywhere,"
Howard said.
The study is in the Oct. 29 issue of the journal Science.
The research provides a tantalizing clue that potentially habitable
planets also could 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 also is 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."
For information about exoplanets and NASA's planet-finding program, visit:
http://planetquest.jpl.nasa.
Source: NASA
