The newest exo-apples of the
planet-hunting Kepler space telescope’s unblinking eye are two rocky,
Earth-sized planets hovering around Kepler-20, a sunlike star 950
light-years away.
Though snuggled too close to their star to be habitable, these
first Earth-sized worlds confirmed by the Kepler team are another big
step forward for the planet hunters, who recently found a planet
somewhat
larger than Earth orbiting a sunlike star at a distance
hospitable to life. Finding habitable distant worlds — Earth-sized
planets at the right distance from their stars to allow the presence of
liquid water — is the team’s ultimate goal.
"The hunt is on to find a planet that combines the best of both
of these worlds — a true Earth twin," says David Charbonneau, an
astronomer at the Harvard-Smithsonian Center for Astrophysics in
Cambridge, Mass., and a coauthor of a study describing the small planets
that appears online December 20 in Nature.
One of the planets, the pragmatically named Kepler-20e, is a bit
smaller than Venus — 0.87 times as wide as Earth — and completes a trip
around the star every 6.1 days. The other, Kepler-20f, is 1.03 times as
wide as Earth, and a year on that planet would last just 19.6 days.
Because the planets are so small, they’re probably made of ingredients
similar to Earth’s.
Depending on where and how it formed, Kepler-20f could even have
developed a water vapor atmosphere, says planetary scientist Jonathan
Fortney of the University of California, Santa Cruz. “If it started out
with the amount of water we had on Earth and Venus, it’s probably long
gone — just like it is on Venus,” he says. “But if that planet had a
tremendous amount more water, then it might have some left over.”
The Kepler-20 system is a quintet comprising three large planets
(Kepler-20b, c and d) and the two Earth-sized ones, all tucked in nearer
to their star than Mercury is to the sun. Moving out from Kepler-20,
the five spheres alternate in size, with the runts of the planetary
litter bracketed on either side by their bigger siblings.
“It’s one of the most shocking architectures we’ve seen,”
Charbonneau says. “Exoplanets have had a lot of surprises, but this is
going to be very difficult to explain.”
The strange — but stable — configuration is encoded in the blips
and blinks the planets produce as they pass in front of their sun, which
is one of more than 150,000 in a field of stars the telescope stares
at. Different-sized blips correspond to different-sized planets, and
watching the star for long enough reveals how frequently each planet
completes its journey.
Currently set to wrap up at the end of 2012, the mission could be
extended for several more years if limited budgets allow. More
observing time will let scientists monitor Kepler’s starry patch for
long enough to detect Earth-sized planets in longer, habitable orbits.
“The spacecraft doesn't know about politics and financial
difficulties — it will continue to beam data back to Earth until at
least 2015, even if no one is listening,” says astronomer Debra Fischer
of Yale University. “You just have to keep the lights on, and keep the
science team intact. The next three years are where they’re going to
detect the Earths at habitable distances.”
Source : http://www.sciencenews.org/view/generic/id/337052/description/First_Earth-sized_planets_netted