First Earth-sized planet found in star's habitable zone; liquid water may exist

Astronomers for the first time have discovered an Earth-sized world in the so-called habitable zone, an area of its orbit not too close nor too far from its star where liquid water, essential for life, could exist.

The discovery made by the Kepler space telescope was announced today by NASA at a 2 p.m. press conference.

“This is the first definitive Earth-sized planet found in the habitable zone around another star,” said Elisa Quintana of the SETI Institute at NASA Ames Research Center. She is the lead author of an article in the journal Science that describes the discovery.

Astronomers, using Kepler and various other techniques, have found about 1,800 of extrasolar planets during the past two decades. Yet only 20 orbit in the habitable zone, and these worlds are believed to be much larger than Earth and therefore more likely to be gaseous and resemble Uranus or Neptune than our smaller, rocky world.

“Theoretical models of how planets form suggest that those with diameters less than 1.5 times that of Earth are unlikely to be swathed in atmospheres of hydrogen and helium, the fate that’s befallen the gas giants of our own solar system," said Thomas Barclay, a staff scientist for the Kepler mission. "Consequently, Kepler-186f is likely a rocky world, and in that sense similar to Venus, Earth and Mars.”

The newly discovered planet orbits the red dwarf star Kepler-186, about 490 light-years away in the northern constellation Cygnus. The planet is called Kepler-186f, and is the fifth and outermost world discovered in this solar system.

The planet, just slightly larger than our world (1.1 Earth radii), orbits its star at a distance of 32.5 million miles. Astronomers are calling it not Earth's twin, but a cousin.

A news release from the SETI Institute said planets orbiting red dwarfs, generally smaller stars than the sun, traditionally were considered to be poor candidates for life. Such planets in the habitable zone would become tidally locked. In this scenario, one side of the planet constantly would face its parent star and become too hot; the dark side would be frozen.

Pete Zapadka:, 412-263-1857 or on Twitter @pzapadka. First Published April 17, 2014 11:50 AM

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