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Thegraphicshows optimistic and
conservative habitable zone boundaries around cool, low mass stars. The
numbers indicate the names of known Kepler planet candidates. Yellow
color represents candidates with less than 1.4 times Earth-radius. Green
color represents planet candidates between 1.4 and 2 Earth radius.
Planets with "+" are not in the habitable zone. Image: Penn State
PARK, Pa. -- The number of potentially habitable planets is greater
than previously thought, according to a new analysis by a Penn State
researcher, and some of those planets are likely lurking around nearby
"We now estimate that if we were to look at 10 of the nearest small
stars we would find about four potentially habitable planets, give or
take," said Ravi Kopparapu, a post-doctoral researcher in geosciences.
"That is a conservative estimate," he added. "There could be more."
Kopparapu detailed his findings in a paper accepted for publication
in Astrophysical Journal Letters. In it, he recalculated the commonness
of Earth-sized planets in the habitable zones of low-mass stars, also
known as cool stars or M-dwarfs.
Scientists focus on M-dwarfs for several reasons, he explained. The
orbit of planets around M-dwarfs is very short, which allows scientists
to gather data on a greater number of orbits in a shorter period of time
than can be gathered on Sun-like stars, which have larger habitable
zones. M-dwarfs are also more common than stars like the Earth's Sun,
which means more of them can be observed.
According to his findings, "The average distance to the nearest
potentially habitable planet is about seven light years. That is about
half the distance of previous estimates," Kopparapu said. "There are
about eight cool stars within 10 light-years, so conservatively, we
should expect to find about three Earth-size planets in the habitable
The work follows up on a recent study by researchers at the
Harvard-Smithsonian Center for Astrophysics which analyzed 3,987 M-dwarf
stars to calculate the number of Earth-sized planet candidates in cool
stars' habitable zones—a region around a star where rocky planets are
capable of sustaining liquid water and therefore life. That study used
habitable zone limits calculated in 1993 by Jim Kasting, now an Evan
Pugh Professor in Penn State's Department of Geosciences. Kopparapu
noticed that its findings, based on data from NASA's Kepler satellite,
didn't reflect the most recent estimates for determining whether planets
fall within a habitable zone.
These newer estimates are based on an updated model developed by
Kopparapu and collaborators, using information on water and carbon
dioxide absorption that was not available in 1993. Kopparapu applied
those findings to the Harvard team's study, using the same calculation
method, and found that there are additional planets in the newly
determined habitable zones.
"I used our new habitable zone calculations and found that there are
nearly three times as many Earth-sized planets in the habitable zones
around these low mass stars as in previous estimates," Kopparapu said.
"This means Earth-sized planets are more common than we thought, and
that is a good sign for detecting extraterrestrial life."