In the race to find another Earth, scientists have just hit a major milestone.
In a NASA teleconference yesterday, a team of astronomers announced that they’ve discovered a planet in a far-away solar system that’s the most Earth-like yet—almost certainly rocky, and at the right temperature for oceans of liquid water to coat its surface. Its name is Kepler–186f and its discovery, reported in the journal Science, is the first confirmed example of a planet that is made of solid rock, like Earth, and exists in the habitable zone of another star.
Just how closely its surface conditions resemble Earth—and whether it can sustain life—remains a mystery. “Perhaps instead of an Earth twin, we’ve discovered an Earth cousin,” quipped Thomas Barclay, one of the team members at NASA’s Ames Research Center in a prepared statement.
Nevertheless, this is a watershed moment. “This is one of the big milestones we’ve been looking for in our attempts to find out if there are places just like home, if there’s life out there,” Barclay said.
Let’s start with what it has going for it. If you were to make the 500-light-year trek to Kepler–186f and stand on its surface, many things would feel familiar. There would be solid ground beneath you, and you’d probably weigh roughly the same as you do now. Look around you, and the distance to the horizon would look very Earthly, as the planet is nearly the same size as Earth (only 10% larger).
But gaze up at its sun and the view would be quite different. Its star is only about half as big as ours and thousands of degrees cooler, the type of star astronomers classify as an M dwarf—the smallest and most common type of star in the galaxy. These stars emit a cool red glow, but we can’t tell you what the color of the planet’s sky would be; we don’t know for sure if it even has an atmosphere.
Kepler–186f and four other planets in the same solar system were first detected by NASA’s Kepler space telescope, which from 2009 to 2013 searched for exoplanets by monitoring the brightness of over 100,000 stars in our galaxy. A slight and temporary dim indicated that an orbiting planet had crossed in front of the star, blocking some of its light. The team, led by Elise Quintana of NASA Ames and the SETI Institute, confirmed the discovery by following up with the powerful Keck and Gemini telescopes on Mauna Kea in Hawaii using another method—looking for the telltale “wobble” of the star as its orbiting planets give it a slight gravitational tug to and fro. As the star moves back and forth, the wavelength of its light shifts like the pitch of a siren as it rushes past you—a phenomenon called the Doppler effect.
Since its mission began in 2009, Kepler has found 962 confirmed planets, and over 2600 suspected candidates are waiting for further analysis or follow-up observations. The easiest planets to detect are gas giants, the size of Jupiter or larger, inhabiting scorching hot orbits very close to their stars. But as Kepler accumulated more data and scientists became increasingly clever at analyzing it, they’ve been able to pick out smaller and more Earth-like planets in the habitable zone.
Kepler–186f is the smallest confirmed one yet, beating out the previous titleholder, Kepler–62e, a planet roughly 1.6 times the size as Earth. Astronomers also have a better handle on Kepler–186f’s mass than another oft-floated habitable candidate, Gliese 667 Cc, only 22 light years away in the constellation Scorpius, which astronomers aren’t sure is rocky or gaseous.
But just because Kepler–186f is Earth-sized and in the habitable zone doesn’t mean it’s actually habitable. Scientists aren’t sure about the prospects for life around any M dwarf systems. These stars are diminutive, but have very strong magnetic fields that twist and writhe, causing violent flares that could sterilize a planet with dangerous ultraviolet radiation or scrub away any atmosphere it might have had.
Seth Shostak, an astronomer at the SETI Institute, wrote on the prospects for detecting life on Kepler–186f for Space.com:
OK, so as a lottery ticket that might deliver on the big payoff — a life-bearing world — how good is Kepler-186f? Can we swing some other telescopes in its direction and actually verify that it has an atmosphere, maybe loaded up with water vapor?
Quintana is doubtful. “This star is too dim for follow-up surveys, even with large next-generation telescopes. But it does show you can form Earth-size planets in the habitable zone.”
And meanwhile, the SETI Institute is continuing its radio reconnaissance of Kepler-186f, examining a wide swath of microwave frequencies looking for alien transmissions. So far, the inhabitants — if there are any — are mum.
Kepler hasn’t been collecting data since the spring of 2013, when a mechanical failure robbed it of its ability to precisely maintain its orientation in space. But the discovery of Kepler–186f shows that there’s still much to be uncovered in its backlog of data. The goal of Kepler wasn’t just to find a habitable planet like Kepler–186f, but to find enough of them so scientists can figure out how common they are—and preferably around Sun-like stars, not M dwarfs like Kepler–186. Kepler’s premature end will make that determination a little less precise than they’d hoped, but finding a planet like Kepler–186f bodes well for the hunt for more like it—and possibly alien life, too.