LOS ANGELES (AP) --
Astronomers have discovered what they say is the most Earth-like planet
yet detected - a distant, rocky world that's similar in size to our own
and exists in the Goldilocks zone where it's not too hot and not too
cold for life.
The find, announced
Thursday, excited planet hunters who have been scouring the Milky Way
galaxy for years for potentially habitable places outside our solar
system.
"This is the best case
for a habitable planet yet found. The results are absolutely rock
solid," University of California, Berkeley astronomer Geoff Marcy, who
had no role in the discovery, said in an email.
The planet was
detected by NASA's orbiting Kepler telescope, which examines the heavens
for subtle changes in brightness that indicate an orbiting planet is
crossing in front of a star. From those changes, scientists can
calculate a planet's size and make certain inferences about its makeup.
The newfound object,
dubbed Kepler-186f, circles a red dwarf star 500 light-years from Earth
in the constellation Cygnus. A light-year is almost 6 trillion miles.
The planet is about 10
percent larger than Earth and may very well have liquid water - a key
ingredient for life - on its surface, scientists said. That is because
it resides at the outer edge of the habitable temperature zone around
its star - the sweet spot where lakes, rivers or oceans can exist
without freezing solid or boiling away.
The planet probably
basks in an orange-red glow from its star and is most likely cooler than
Earth, with an average temperature slightly above freezing, "similar to
dawn or dusk on a spring day," Marcy said.
The discovery was detailed in Friday's issue of the journal Science.
Lead researcher Elisa
Quintana at NASA's Ames Research Center said she considers the planet to
be more of an "Earth cousin" than a twin because it circles a star that
is smaller and dimmer than our sun. While Earth revolves around the sun
in 365 days, this planet completes an orbit of its star every 130 days.
"You have a birthday every 130 days on this planet," she said.
Scientists cannot say
for certain whether it has an atmosphere, but if it does, it probably
contains a lot of carbon dioxide, outside experts said.
"Don't take off your
breathing mask if you ever land there," said Lisa Kaltenegger, a Harvard
and Max Planck Institute astronomer who had no connection to the
research.
Despite the
differences, "now we can point to a star and know that there really is a
planet very similar to the Earth, at least in size and temperature,"
Harvard scientist David Charbonneau, who was not part of the team, said
in an email.
Since its launch in
2009, Kepler has confirmed 961 planets, but only a few dozen are in the
habitable zone. Most are giant gas balls like Jupiter and Saturn, and
not ideal places for life. Scientists in recent years have also found
planets slightly larger than Earth in the Goldilocks zone called "super
Earths," but it is unclear if they are rocky.
The latest discovery is the closest in size to Earth than any other known world in the habitable region.
Kepler-186f is part of
a system of five planets, all of which are roughly Earth's size.
However, the other planets are too close to their star to support life.
Astronomers may never
know for certain whether Kepler-186f can sustain life. The planet is too
far away even for next-generation space telescopes like NASA's
overbudget James Webb, set for launch in 2018, to study in detail.
Kepler completed its
prime mission and was in overtime when one of the wheels that keep its
gaze steady failed last year. NASA has not yet decided whether to keep
using the telescope to hunt for planets on a scaled-back basis.
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AP Science Writer Seth Borenstein contributed to this report.
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