Since its launch in the spring of 2009, NASA's Kepler Space Telescope has been hunting exoplanets. The holy grail being a planet that is essentially like ours in terms of size, composition, and habitability: an Earth-twin. While we still haven't found a planet that exactly fits that bill, Kepler has now confirmed the discovery of an Earth-sized exoplanet in its star’s habitable zone. The announcement was made at a press conference and the findings have been published in Science.

Kepler-186f is about 10% larger than Earth and orbits an M dwarf star around 500 light-years away in the constellation Cygnus. The star is about half of the size and mass of our sun, and it takes Kepler-186f about 130 Earth days to complete a revolution. On the outer edge of the star’s habitable zone, the planet receives about a third of the radiation from its parent star as we do from ours.

Life as we know it requires the presence of liquid water, so a planet with the potential for life would be not too close to the star (which would be too hot and the water would be vapor) yet not too far away (where it would be too cold and the water would be ice). Habitability requires a “Goldilocks Zone” where conditions are just right.

"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," said Elisa Quintana, lead author of the paper. "Finding a habitable zone planet comparable to Earth in size is a major step forward."

Co-author Thomas Barclay added: "Being in the habitable zone does not mean we know this planet is habitable. The temperature on the planet is strongly dependent on what kind of atmosphere the planet has. Kepler-186f can be thought of as an Earth-cousin rather than an Earth-twin. It has many properties that resemble Earth."

Determining the composition of planets out in the habitable zone isn’t as easy as those who are incredibly close to the star, because there isn’t as much radiation from the parent star available to determine what is or isn’t getting absorbed. While previous findings have indicated that Kepler-186f is a rocky planet, further analysis must be done before any definitive conclusions can be made.

Have questions for the scientists? Ask them using #Kepler186f on Twitter, Facebook, or G+.

Correction 4/17/2014 1:30 pm PDT: An earlier version of this article stated the planet was 40% larger than Earth. That was a typo; Kepler-186f is 10% larger than Earth. Apologies for any confusion that error may have caused