Kepler Finds First Earth-sized Planet in Habitable Zone

For the first time, scientists have discovered an Earth-sized alien planet in the habitable zone of its host star, an “Earth cousin” that just might have liquid water and the right conditions for life.

The newfound planet, called Kepler-186f, was first spotted by NASA’s Kepler space telescope and circles a dim red dwarf star about 490 light-years from Earth. While the host star is dimmer than Earth’s sun and the planet is slightly bigger than Earth, the positioning of the alien world coupled with its size suggests that Kepler-186f could have water on its surface, scientists say. 

“One of the things we’ve been looking for is maybe an Earth twin, which is an Earth-size planet in the habitable zone of a sunlike star,” Tom Barclay, Kepler scientist and co-author of the new exoplanet research, said in an interview. “This [Kepler-186f] is an Earth-size planet in the habitable zone of a cooler star. So, while it’s not an Earth twin, it is perhaps an Earth cousin. It has similar characteristics, but a different parent.” 

Scientists think Kepler-186f — the outermost of five planets found to be orbiting the star Kepler-186 — orbits at a distance of 52.4 million kilometers, theoretically within the habitable zone for a red dwarf.

Earth orbits the sun from an average distance of about 150 million kilometers, but the sun is larger and brighter than the Kepler-186 star, meaning that the sun’s habitable zone begins farther out from the star by comparison to Kepler-186.

“This is the first definitive Earth-sized planet found in the habitable zone around another star,” Elisa Quintana of the SETI Institute and NASA’s Ames Research Center, the lead author of a new study detailing the findings, said in a statement.

Other planets of various sizes have been found in the habitable zones of their stars. However, Kepler-186f is the first alien planet this close to Earth in size found orbiting in that potentially life-supporting area of an extrasolar system, according to exoplanet scientists.

“This is the best case for a habitable planet yet found,” Geoff Marcy, an astronomer at the University of California, Berkeley, said via email. “The results are absolutely rock-solid. The planet itself may not be, but I’d bet my house on it. In any case, it’s a gem.”

The new research was published online April 17 in the journal Science.