2 Earth-sized planets discovered

Two Earth sized planetsbilled as the smallest ever found orbiting a sun-like star outside our solar systemhave been discovered in a five-planet system about 1,000 light years away.

Kepler 20e has a radius about 0.87 times that of Earth and Kepler 20f has a radius about 1.03 times Earth's radius, making them slightly smaller than Venus and slightly larger than Earth respectively, NASA announced Tuesday.

However, both planets orbit too close to their star to be habitablethat is, they are too hot for liquid water to be able to exist on their surface.

Kepler 20e, which circles its star every 6.1 Earth days, has a surface temperature of 760 C, making it hotter than any planet on our solar system. Kepler 20f, which has an orbit of 19.6 Earth days, has a temperature comparable to that on Mercuryaround 430 C.

The planets are among 33 confirmed among more than 2,000 planet candidates so far detected by NASA's Kepler mission. The Kepler space telescope has been scanning 56,000 stars in its field of view about 1/400th of the sky for signs of planets since September 2009. It measures tiny decreases in the brightness of stars that may be caused by planets crossing in front of them. A planet is considered "confirmed" after it has witnessed the same crossing or "transit" three times.

Most exoplanets detected so far by Kepler and other devices, such as the European Southern Observatory's HARPS instrument, have been super-Earths or gas giants. Super-Earths are a class of planet more massive than the Earth, but less than 10 times more massive. They do not exist in our own solar system, but seem to be common around other stars.

The other three planets in the Kepler-20 system are all larger than Earth but smaller than Neptune, which is about 17 times more massive than the Earth. All five are very close to their stara distance that would be within Mercury's orbit in our own solar systemand the larger planets alternate with the smaller ones.

"The Kepler data are showing us some planetary systems havearrangements of planets very different from that seen in our solar system," said Jack Lissauer, planetary scientist and Kepler science team member at NASA's Ames Research Center in Moffett Field, Calif., in a statement.