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Habitable? The newly discovered planet is a tad bigger than Earth and about as far from its parent star as Mercury is from the sun.

Habitable? The newly discovered planet is a tad bigger than Earth and about as far from its parent star as Mercury is from the sun.


Almost-Earth Tantalizes Astronomers With Promise of Worlds to Come

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.”