The discovery of Kepler-10b, the first rocky extrasolar planet, was confirmed yesterday by NASA’s Kepler mission. Only 1.4 times the size of Earth, it is the smallest exoplanet ever discovered.

Because its size is comparable to that of Earth, this is a first step to discovering how common such planets are, and if some of them could sustain life. This is not the case of Kepler-10b: the planet orbits too close to its star, about 1/20th as far from it as Mercury, the innermost planet in our solar system, orbits the sun. The planet orbits once every 0.84 days (about 20 hours), and its mass is about 4.6 times that of Earth; it is 560 light-years away and about eight billion years old.

Kepler’s 1-meter-diameter telescope, launched in 2009, searches for exoplanets by staring at the same 150,000 stars in the Cygnus constellation. It is looking for periodic dips in brightness caused by the passage of a planet in front of its star, in order to find Earth-sized planets in habitable orbits.

Kepler-10 was the first star identified that could potentially harbor a small transiting planet, early after the satellite’s launching. Precise measurements of the planet’s radius and mass were then made by the W.M. Keck Observatory 10-meter telescope in Hawaii.

Finding similar planets in habitable orbits will take a couple of more years of observations for sunlike stars, but the announcement of another habitable world never seemed so close.