Research Finds Smaller Planets Likely To Vastly Outnumber
Larger Ones
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.
W.M. Keck Observatory
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.
Chart Courtesy NASA
"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."