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.”
“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.