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Scientists say a world that's 490 light-years away qualifies as the first confirmed Earth-sized exoplanet that could sustain life as we know it — but in an environment like nothing we've ever seen.
The planet, known as Kepler-186f, is "more of an Earth cousin than an Earth twin," Elisa Quintana, an astronomer at the SETI Institute at NASA Ames Research Center, told the journal Science. Quintana is the lead author of a report on the planet published by Science this week.
"This discovery does confirm that Earth-sized planets do exist in the habitable zones of other stars," Quintana said during a Thursday news briefing at NASA Headquarters.
Kepler-186f goes around an M-type dwarf star that's smaller and cooler than our sun. But it orbits much closer to its parent star than Earth does, within what would be Mercury's orbit in our own solar system. Those two factors combine to produce an environment that could allow for liquid water on the surface, assuming that the planet had a heat-trapping atmosphere.
"The star, to our eyes, would look slightly orange-y," about a third again as big as our sun but only a third as bright, said co-author Thomas Barclay, a staff scientist for NASA's Kepler mission who is also affiliated with NASA and the Bay Area Environmental Research Institute. At midday, Kepler-186f's landscape might look similar to what we see on Earth an hour before sunset, he told NBC News.
Or it might not: If the planet lacked an atmosphere to retain and redistribute its sun's warmth, it would be a cold, dry, lifeless world.
Kepler-186f probably rates as the most potentially Earthlike planet discovered so far, said Jim Kasting, a geoscientist at Penn State University who did not play a role in the Science study. But he told NBC News that it's still "less likely to be habitable than planets around more sunlike stars." Even better prospects for alien habitability might well be identified in the months and years to come.
How the world was found
Kepler-186f is just the latest discovery to be pulled out of terabytes' worth of data collected by the Kepler mission. Before it went on the fritz last year, the Kepler space telescope stared at more than 150,000 stars in a patch of sky, looking for the telltale dimming of starlight as planets passed over the stars' disks. Nearly 1,000 exoplanets have been confirmed using Kepler data, and almost 3,000 more candidates are still awaiting confirmation.
It takes years of observation to confirm the pattern of dimming and brightening that's associated with alien planets, particularly if the planets are small and far from their parent stars. In February, astronomers reported that at least four worlds circled the dwarf star known as Kepler-186 or KOI-571. In this week's Science paper, Quintana and her colleagues confirm the existence of Kepler-186f as the fifth and outermost world.
They report that Kepler-186f is about 10 percent wider than Earth, tracing a 130-day orbit around its sun at a mean distance of 0.35 astronomical units. (An astronomical unit is the distance between Earth and our sun, which is 93 million miles or 150 million kilometers.) That would put Kepler-186f on the cooler, outer side of the star's habitable zone — the range of orbital distances where liquid water could exist on a planet's surface.
Astronomers have confirmed the existence of other planets in their stars' habitable zone, but those prospects are super-Earth-size. Smaller habitable-zone candidates also have been found, but they have yet to be confirmed as planets.
Barclay said Kepler-186f was particularly promising because it's less than 1.5 times the size of Earth. Planets in that size range are more likely to be rocky with a thinner atmosphere, like Earth, Mars and Venus. But worlds exceeding that size stand a better chance of retaining a thick atmosphere of hydrogen and helium, like the giant planet Neptune.
"While those planets also could be rocky, they don't remind us of home," Barclay said.
M-dwarf stars are thought to be the most numerous stars, accounting for as much as 70 percent of the stars in the Milky Way. As a result, "the first signs of other life in the galaxy may well come from planets orbiting an M dwarf," Quintana said.
The promise of worlds to come
Could we actually detect signs of life on Kepler-186f? That's a tough one. The astronomers behind the discovery acknowledge that the planet might be just too far away for follow-up studies. The SETI Institute has been searching for radio signals from the Kepler-186 system over a wide frequency range (1 to 10 GHz), but so far nothing has been detected.
Kasting, the author of "How to Find a Habitable Planet," said worlds around M-class dwarf stars faced several disadvantages in the habitability department. For one thing, such planets generally end up being tidally locked to their stars — meaning that one side of the planet is always facing its parent sun while the other is always turned away.
"This is not a show-stopper for habitability, but it's a problem nonetheless," Kasting said.
Also, M-class stars tend to throw off strong stellar winds and flares that could blast away a planet's atmosphere. In Kasting's view, the most serious problem is that such stars are thought to glow much brighter early in their lifetime, and only later settle down to become dimmer than the sun.
"Thus, a planet like Kepler 186f that is within the star's habitable zone today may have been strongly heated soon after it formed, in which case it may have lost its water early by way of a runaway greenhouse," Kasting said in an email.
These are the reasons why astronomers are so interested in searching for Earth-sized planets that lie in the habitable zones around stars that are more like our sun. "We just need a lot of money and political will to do that," Kasting said.
Further analysis of the data from Kepler could produce those prospects, but scientists are also banking on future missions such as NASA's Transiting Exoplanet Survey Satellite and James Webb Space Telescope, the European Space Agency's PLATO and CHEOPS probes, and the Giant Magellan Telescope in Chile. Such observing instruments could look for the potential signatures of life in alien atmospheres.
In addition to Quintana and Barclay, the authors of "An Earth-Sized Planet in the Habitable Zone of a Cool Star" include Sean Raymond, Jason Rowe, Emeline Bolmont, Douglas Caldwell, Steve Howell, Stephen Kane, Daniel Huber, Justin Crepp, Jack Lissauer, David Ciardi, Jeffrey Coughlin, Mark Everett, Christopher Henze, Elliott Horch, Howard Isaacson, Eric Ford, Fred Adams, Martin Still, Roger Hunter, Billy Quarles and Franck Selsis.
NBC News science editor Alan Boyle is among the speakers at this week's Norwescon science-fiction and fantasy convention, presented at the Doubletree by Hilton Hotel Seattle Airport.