Ever since the first exoplanet
was discovered in 1996, astronomers have been scanning the heavens for
another Earth: a rocky planet orbiting its star at just the right
distance for it to harbor liquid water and thus, potentially, life. Now,
sifting through data collected by NASA’s Kepler orbiting observatory,
they have discovered just such a planet, although it’s not quite Earth
2.0. Named Kepler-186f, the planet orbits a star that is less than half
the size of the sun and much cooler.
“Very exciting,” says James Kasting, an exoplanet researcher at
Pennsylvania State University, University Park, who was not involved in
the work. “This is probably the most potentially Earth-like planet yet.”
David Charbonneau, an exoplanet researcher at Harvard University, calls
it “one of the most significant discoveries from Kepler.”
The new world is the outermost of five planets orbiting Kepler-186, a
red dwarf star some 500 light-years from Earth. Such M stars typically
have a fraction of the mass of the sun, burn more slowly, and are too
faint to be seen with the naked eye. (Hotter sunlike stars are
classified as type G.) Kepler detected the planet from a minuscule
dimming of the star each time the planet transited, or crossed the face
of the star.
From the extent and timing of that dimming, the researchers
calculated that the planet is almost the same size as Earth—just 10%
bigger in diameter—and goes around its star once every 130 days.
Although its mass is unknown, astronomers say its size almost guarantees
that it is rocky like Earth. Its distance from its star—about as far
from the star as Mercury is from the sun—puts it in the outer reaches of
Kepler-186’s habitable zone.
Planets previously found in the habitable zones of other stars have
been substantially larger than Earth and unlikely to have a rocky
surface. But because it orbits a dwarf, “we consider this planet more of
an Earth cousin than an Earth twin,” says Elisa Quintana, a researcher
at NASA Ames Research Center in Mountain View, California, and lead
author of a paper announcing the discovery this week in
Science.
Small, faint stars like type Ms make up more than three-quarters of the
billions of stars in the Milky Way, however, so the finding could open a
wide new hunting ground for extraterrestrial life. “Our galaxy is
probably littered with cousins of Kepler-186f,” Quintana says.
Several factors could make such planets less promising abodes for
life than planets circling sunlike stars. For one, their close-in
habitable zones could leave them extra vulnerable to perils such as
stellar flares. On the plus side, M stars keep burning billions of years
longer than sunlike stars do. “That is good news for life, because
there is a longer period of time for it to take hold on the surface of
the planet,” says Stephen Kane, a co-author and researcher at San
Francisco State University in California.
Kepler-186f is a late bonus from Kepler, which monitored the
brightness of some 150,000 stars from March 2009 to May 2013 in search
of planets. Analyzing Kepler data, scientists have identified more than
3800 planetary candidates, of which 961 have been confirmed as planets.
As the software to search Kepler’s data improves, scientists keep
finding planets that they missed before.
Researchers had detected Kepler-186’s four inner planets by the
spring of 2013. Then, a routine analysis of all of Kepler’s light
curves—a procedure that typically takes weeks of supercomputer
time—flagged the possible existence of a small fifth planet. Quintana’s
team conducted a series of checks to ensure that what the software had
found was a genuine transit.
To learn more about the planet, Quintana and colleagues had to learn
more about the star. By taking spectra of Kepler-186 with ground-based
telescopes, they nailed down its mass and size—information that helped
them determine the planet’s radius. “I remember walking to Elisa’s
office one afternoon, and she looked up at me and said, ‘The planet’s
about the size of the Earth,’ ” says Steve Howell, project scientist for
Kepler at NASA Ames and a co-author of the paper. Further analysis
placed the planet in the outer reaches of the star’s habitable zone.
“We are not saying that there’s water on the surface,” Howell says.
“All we know is that the surface has the right temperature that water
could exist there in a liquid state.” To support water, however, the
planet would also need to have an atmosphere to protect it. It’s unclear
from the available data whether the planet has an adequate atmospheric
blanket.
Even those key ingredients wouldn’t guarantee that Kepler-186f is
habitable. If it orbited slightly closer in, Quintana says,
gravitational interactions would leave the planet tidally locked:
rotating so that one side always faced the star. Such two-faced
planets—with the night side eternally frigid and the day side blistering
hot—are considered long shots for life. But Quintana and her colleagues
say Kepler-186f is far enough out that it might avoid that fate.
Unfortunately, the planet is too far away from Earth for follow-up
studies. However, researchers hope it heralds many similar worlds soon
to come. “The biggest impact of this discovery is to know that there are
planets that are the same size as Earth in the habitable zones of small
stars,” Charbonneau says. He says the next step will be “to find a
close example” so that upcoming instruments like the Giant Magellan
Telescope and NASA’s James Webb Space Telescope can “study the
atmosphere of such planets and perhaps even deduce the presence of life
on them.”
