Kepler-186f is the first planet ever discovered that is both about the same size as the Earth, and orbits in the "habitable zone" around its star. This is the distance from a star at which a planet would be likely to sustain liquid water. This combination could make Kepler-186f the most Earth-like planet ever discovered outside the solar system. 

The exoplanet has a diameter about ten percent greater than that of Earth. Astronomers have, so far, not been able to determine the mass or composition of the planet. Theoretical models predict most worlds this size should be rocky. 

The orbit of the exoplanet brings it around Kepler-186 once every 130 days. The planet lies at the outer edge of the star's habitable zone, receiving just one-third as much solar energy as our home world. 

"Kepler-186f resides in the Kepler-186 system about 500 light-years from Earth in the constellation Cygnus. The system is also home to four inner planets, seen lined up in orbit around a host star that is half the size and mass of the sun," NASA officials wrote in a press release announcing the discovery. 

Astronomers now know of more than a thousand planets orbiting around other stars. Hundreds of these were discovered by the Kepler space observatory. 

"The quest for Earth-like planets is a major focus of current exoplanet research. Although planets that are Earth-sized and smaller have been detected, these planets reside in orbits that are too close to their host star to allow liquid water on their surfaces... if Kepler-186f has an Earth-like atmosphere and water at its surface, then some of this water is likely to be in liquid form," researchers wrote in the journal article announcing the discovery.  

Kepler-186f may be one of the most likely places for life to rise, but scientists are being careful not to be too excited about the prospect. Other planets have been found before that orbit their companion stars in the habitable zone, but each of these is 40 percent larger than the Earth, or more. 

"We know of just one planet where life exists - Earth. When we search for life outside our solar system we focus on finding planets with characteristics that mimic that of Earth," Elisa Quintana, research scientist at the SETI Institute and lead author of the study, said. 

Kepler 186 is a M-class dwarf star, which are the most common stars in the galaxy. Quintana believes the first signs of life on planets in other systems may come from worlds circling one of these distant suns. Still, these stars produce large quantities of radiation that could kill off any life that did develop on the planet. 

Discovery of the Earth-like planet in the habitable zone of its star was detailed in the journal Science.