It is a bit bigger and somewhat colder, but a planet circling a star 500 light-years away is otherwise the closest match of our home world discovered so far, astronomers announced on Thursday. The planet, known as Kepler 186f, named after NASA's Kepler planet-finding mission, which found it, has a diameter of 8,700 miles, 10 percent wider than Earth, and its orbit lies within the "Goldilocks zone" of its star, Kepler 186 - not too hot, not too cold, where temperatures could allow for liquid water to flow at the surface, making it potentially hospitable for life.

The researchers speculate that it is made of the same stuff as Earth - iron, rock, ice, liquid water, although the relative amounts could be very different. The gravity on Kepler 186f, too, is likely to be roughly the same as Earth's. "You could far more easily imagine someone being able to go there and walk around on the surface," Stephen Kane, an astronomer at San Francisco State and another member of the research team, said in an interview. Kepler 186f is not a perfect replica, however. It is closer to its star - a red dwarf that is smaller, cooler and fainter than our sun - than the Earth is; its year, the time to complete one orbit, is 130 days, not 365. It is also at the outer edge of the habitable zone, receiving less warmth, so perhaps more of its surface would freeze.