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 The discovery, announced on Thursday, is the closest scientists
have come so far to finding a true Earth twin. The star, known as
Kepler-186 and located about 500 light years away in the
constellation Cygnus, is smaller and redder than the sun.
The star's outermost planet, designated Kepler-186f, receives about
one-third the radiation from its parent star as Earth gets from the
sun, meaning that high noon on this world would be roughly akin to
Earth an hour before sunset, said astronomer Thomas Barclay, with
NASA's Ames Research Center in Moffett Field, California.
The planet is the right distance from its host star for water — if
any exists — to be liquid on the surface, a condition that
scientists suspect is necessary for life.
"This planet is an Earth cousin, not an Earth twin," said Barclay,
who is among a team of scientists reporting on the discovery in the
journal Science this week.
NASA launched its Kepler space telescope in 2009 to search about
150,000 target stars for signs of any planets passing by, or
transiting, relative to the telescope's point of view. Kepler was
sidelined by a positioning system failure last year.
Analysis of archived Kepler data continues. From Kepler's
observational perch, a planet about the size and location of Earth
orbiting a sun-like star would blot out only about 80 to 100 photons
out of every million as it transits.
The pattern is repeated every 365 days and at least three transits
would be needed to rule out other possibilities, so the search takes
time.
"It's very challenging to find Earth analogs," Barclay said. "Most
candidates don't pan out, but things change as we get more
measurements."
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Scientists don't know anything about the atmosphere of Kepler-186f,
but it will be a target for future telescopes that can scan for
telltale chemicals that may be linked to life.
"This planet is in the habitable zone, but that's doesn't mean it is
habitable," Barclay said.
So far, scientists have found nearly 1,800 planets beyond the solar
system.
"The past year has seen a lot of progress in the search for
Earth-like planets. Kepler-168f is significant because it is the
first exoplanet that is the same temperature and is (almost) the
same size as Earth," astronomer David Charbonneau, with the
Harvard-Smithsonian Center for Astrophysics, wrote in an email.
"For me the impact is to prove that yes, such planets really do
exist," Charbonneau said. "Now we can point to a star and say,
"There lies an Earth-like planet.'"
(Editing by Leslie Adler)
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