NASA’s Kepler Mission discovered two more Earth-size planets, Kepler-20e and Kepler-20f, beyond our Solar System. The discovered system is jam-packed with five planets revolving round a star, all circling within a distance almost equal to Mercury’s orbit.

The other three larger gas planets are Kepler-20b, Kepler-20c and Kepler-20d. The arrangement of the planets from the closest to its star to the farthest is: b, e, c, f and d, with the ordering of the letters reflecting the time at which the planets were initially discovered.

Astronomers think that Kepler-20e is entirely rocky and without an atmosphere. In fact, they believe that the planet always shows the same side to its host star, like the moon to the Earth, and thus could have large temperature differences between its permanent nights and days (technically, tidally locked). However, the planet may be geologically active, due to its own formation process and the strong gravitational interactions with its host star.

“The primary goal of the Kepler mission is to find Earth-sized planets in the habitable zone,” said Francois Fressin of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., lead author of a new study published in the journal Nature. “This discovery demonstrates for the first time that Earth-size planets exist around other stars, and that we are able to detect them.”

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