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| This artist's depiction shows Kepler-186f, an Earth-size world in the "habitable zone" of a red dwarf star. |
Red sunshine, seas, and maybe aliens? Scientists analyzing data from NASA's Kepler Space Telescope today report the closest thing yet to another Earth, a world in a habitable orbit around a red dwarf star some 493 light-years away.
Launched in 2009 with the goal of finding another Earth, the $600-million Kepler spacecraft has discovered more than 960 planets
orbiting nearby stars. Half a dozen of those seem to be rocky, like
Earth, and have orbits in the habitable zone around their star—but the
newly discovered world, named Kepler-186f, is the closest in size to
Earth.
"This is a first, validated Earth-size planet in the habitable zone of another star," says study lead author Elisa Quintana
of the SETI Institute and NASA's Ames Research Center in Mountain View,
California. The discovery of the planet was reported today in the
journal Science and in a space agency press briefing. (Related: "Motherlode of Alien Worlds Revealed by Space Telescope.")
One of five planets orbiting a red dwarf star (called
Kepler 186), Kepler-186f is 1.1 times wider than Earth. That means it's
almost certainly a rocky planet too. The researchers estimate its mass
is 1.5 times that of Earth's.
The new planet's orbit, meanwhile, places it at the "Goldilocks"
distance from its star—not too hot or too cold for liquid water to exist
on its surface. The origin of life on Earth required liquid water,
notes study co-author Stephen Kane of San Francisco State University.
"This is an historic discovery—the first Earth-size planet
found in the habitable zone around its star," says pioneering planet
hunter Geoff Marcy
of the University of California, Berkeley, who was not on the discovery
team. "This is the best case for a habitable planet yet found."
The planet's red dwarf star is only about half as big as
the sun, making it cooler and dimmer. But Kepler-186f is on a tighter
orbit than Earth is, taking only 130 days to circle its star. Though it
receives less warmth from its sun than Earth does from its own, the
discovery team says, it would still be warm enough to prevent seas from
freezing—provided it has an atmosphere that provides a substantial
greenhouse effect.
"This planet basks in an orange-red glow from that star,
much as we enjoy at sunset," Marcy says, by email. "The temperature on
the planet is likely cool, similar to dawn or dusk on a spring day."
Crowded Claims:
"Sounds like a great planet to visit, if we could figure
out how to travel there," says MIT astronomer Sarah Seager, by email.
But amid the excitement, she and planetary scientist Alan Boss, author of The Crowded Universe: The Search for Living Planets, caution that other discoveries have led to similar claims in recent years.
Since 1995, astronomers have detected nearly 1,700 worlds orbiting nearby stars, using a variety of detection methods. About a half dozen claims of bigger Earth-size (or still larger "super-Earth") planets orbiting in habitable zones around red dwarfs have been made in recent years, Boss says. "Still, it once again proves what Kepler can do."
The next closest thing to Kepler-186f has a width 1.4 times
that of Earth, Quintana says. According to Seager, a planet whose
diameter is less than 1.75 Earths is likely to be rocky.
The Kepler report looks particularly reliable because of
the spacecraft's track record. It detects planets that dim the light
from their stars as they pass in front of them. Such transits, Quintana
says, are observable only in the roughly one percent of planetary
systems whose orbits can be seen edge on from Earth.
When transits occur regularly, their frequency allows
scientists to calculate the distance at which a planet is orbiting a
star. The amount of starlight dimming—typically on the order of 0.1
percent—is a measure of the planet's size.
Such searches are most sensitive to closer-in stars,
because fewer days of observations are required to see repeated
transits. That explains why the newly discovered planet's four closer-in
siblings had been spotted earlier by the space telescope. "They relied
on only two years of data," Quintana says. With so many planets in the
system, it's likely to be stable over billions of years.
Tickets on Hold:
Whether a life-friendly atmosphere exists on Kepler-186f
depends on a bevy of factors besides having the right orbit. "We see
planets in our own solar system—Venus but also Mars—that are Earthlike
but where things didn't work out," Kane says.
On Venus, a runaway greenhouse climate has cooked the
surface to temperatures that would melt lead. On Mars, the lack of a
strong magnetic field has allowed the solar wind to strip away much of
the planet's atmosphere. A magnetic field would be particularly
important for a planet orbiting a red dwarf, because such stars tend to
release strong flares that would sterilize the planet.
"Just because a planet is in the habitable zone doesn't mean it is habitable," Quintana says. "This is sort of a first step."
However, Kane argues that the greater mass of Kepler-186f
makes it more likely than Mars to have an interior heated by
radioactivity and stirred by the motion of fluids. Such motions are
required to power a dynamo that generates a protective magnetic field as
well as volcanoes, whose eruptions would help replenish a life-friendly
atmosphere. The planet's mass would also give it enough gravity to hold
on to that atmosphere.
"The other big question is whether it has water, delivered
by comets or some other means," Kane says. "Any place with liquid water
is a natural place to look for life."
Unfortunately, Kepler-186f is likely too dim and far away
to be seen directly with any telescope now in operation, or even with
NASA's James Webb Space Telescope, scheduled for launch in 2018.
"In reality we cannot know if the planet is actually
habitable. We need to get a sense of the atmosphere and its greenhouse
effect," Seager says. "Not possible for this particular planet, as it is
too distant from Earth for follow-up observations."
Kepler's Chase:
The latest Kepler discovery came from a trove of star
observations that the spacecraft made before a reaction wheel in its
steering system failed last year, hobbling the mission. A reduced "K2" mission was announced in March.
Hiding amid the existing Kepler observations, Kane says,
are more unconfirmed "candidate" planets orbiting stars as big as the
sun, at distances similar to Earth's 93-million-mile
(150-million-kilometer) distance from the sun.
"There are still a lot more Kepler 'habitable zone' worlds
out there to find," Kane says. "Almost certainly this is not the last
one."
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