Smallest Earth-Like Exoplanets Found Orbiting in Habitable Zones

The two smallest exoplanets yet found in the habitable zone of a star system, and thus with the potential to be similar to Earth, were identified using the Kepler Space Telescope.

The two "super-Earth" planets are within the star Kepler 62's habitable zone, defined as the distances from a star that would potentially allow liquid water to exist on a planet's surface, thus giving life (as we know it) a chance. Eric Agol, a University of Washington associate professor of astronomy, has identified Kepler 62f, a small, probably rocky planet orbiting a Sun-like star in the Lyra constellation. The planet is about 1.4 times the size of Earth, receives about half as much solar flux, or heat and radiation, as Earth and circles its star in 267.3 (Earth) days.

Kepler 62's other super-Earth, nearby 62e, is 1.61 times Earth's size, circles the star in 122.4 days and gets about 20 percent more stellar flux than the Earth. A super-Earth is a planet greater in mass than our own planet, and smaller then the next class of Neptune-sized planets.

"The planets this small that we have found until now have been very close to their stars and much too hot to be possibly habitable. This is the first one Kepler has found in the habitable zone that satisfies this small size," Agol said. "Kepler 62f is the smallest size and the most promising distance from its star, which by these measures makes it the most similar exoplanet to Earth that has been found by Kepler."

But the Kepler 62 system is also quite far away from us, around 1,200 light-years. Lisa Kaltenegger, an astronomer at the Max-Planck Institute of astronomy in Germany who worked on the study, said that the far distance makes it impossible to analyze the atmosphere of the planets in greater detail even with the next generation of telescopes. But for Kepler 62f to contain liquid water, it would need to have a great amount of climate gases to keep warm enough, she said. That sounds a bit like Mars, which is in the habitable zone of the solar system but nowadays too cold.

Agol is the second author of a paper documenting the discovery published April 18 by Science Express, the online edition of the journal Science.

While the sizes of Kepler 62e and 62f are known, Agol said, their mass and densities are not -- but every planet found in their size range so far has been rocky, like Earth.

The Kepler telescope was launched in 2009 with the aim of finding Earth-like planets beyond the solar system. It detects planets by "transits" that cause their host stars to appear fainter when the planets pass in front as they orbit.

Kepler 62f was a late-arrival in terms of its discovery. Its planetary siblings were found by a team of researchers led by William Borucki of the NASA Ames Research Center, principal investigator for the Kepler Space Telescope. Kepler 62 b, c and d are 1.31, 0.54 and 1.95 times the size of the Earth, respectively, but orbit the star too close to be in the habitable zone.

"This type of discovery is the reason we launched the Kepler spacecraft -- to find small, Earth-sized, potentially Earth-temperature planets," Agol said. "At the same time, though, it isn't exactly the same as Earth. It is slightly larger and cooler than Earth. It tells me how special the Earth is and how it may take some time -- hopefully not too long -- to find its exact twin."