Astronomers have discovered what they say is the most Earth-like planet yet detected - a distant, rocky world that's similar in size to our own and exists in the Goldilocks zone where it's not too hot and not too cold for life.
The find, announced Thursday, excited planet hunters who have been scouring the Milky Way galaxy for years for potentially habitable places outside our solar system.
"This is the best case for a habitable planet yet found. The results are absolutely rock solid," University of California, Berkeley astronomer Geoff Marcy, who had no role in the discovery, said in an email.
The planet was detected by NASA's orbiting Kepler telescope, which studies the heavens for subtle changes in brightness that indicate an orbiting planet is crossing in front of a star. From those changes, scientists can calculate a planet's size and make certain inferences about its makeup.
The newfound object, dubbed Kepler-186f, circles a red dwarf star 500 light-years from Earth in the constellation Cygnus. A light-year is almost 6 trillion miles.
The planet is about 10 percent larger than Earth and may very well have liquid water - a key ingredient for life - on its surface, scientists said. That is because it resides at the outer edge of the habitable temperature zone around its star - the sweet spot where lakes, rivers or oceans can exist without freezing solid or boiling away.
The find "is special because we already know that a planet of this size and in the habitable zone is capable of supporting life as we know it," lead researcher Elisa Quintana of NASA's Ames Research Center said at a news conference.
The discovery was detailed in Friday's issue of the journal Science. It was based on observations that were made before the Kepler telescope was crippled by a mechanical failure last year.
The planet probably basks in an orange-red glow from its star and is most likely cooler than Earth, with an average temperature slightly above freezing, "similar to dawn or dusk on a spring day," Marcy said.