Nasa's Kepler telescope mission has discovered two more Earth-like planets orbiting a Sun-like star outside our solar system, it has announced. Planets Kepler-20e and Kepler-20f are too close to their star so no liquid water could exist on their surface, but they are the smallest exoplanets ever found around a star similar to our Sun. Kepler-20e orbits its parent star every 6.1 days and Kepler-20f every 19.6 days, which make them very hot and thus inhospitable worlds, according to Nasa. Kepler-20f has a similar temperature as Mercury - some 800 degrees Fahrenheit, while the surface temperature of Kepler-20e is higher than 1,400 degrees Fahrenheit – which would melt glass.

Nasa scientists discovered another Earth-size planet at the beginning of December. Called Kepler-22b, it is 600 light years away from the Earth.

However, the recent discovery of the two new planets is an important milestone in the search for planets like Earth. Both of them are believed to be rocky; one is smaller than Venus, and the other slightly larger than Earth. The two planets are in a five-planet system called Kepler-20, which is around ,000 light-years away in the constellation Lyra.

"This discovery demonstrates for the first time that Earth-size planets exist around other stars, and that we are able to detect them,” said Francois Fressin of the Harvard-Smithsonian Center for Astrophysics in Cambridge.

The Kepler space telescope can detect planets and planet candidates, which are then confirmed by using other telescopes and computer technologies.

"In the cosmic game of hide and seek, finding planets with just the right size and just the right temperature seems only a matter of time," said Natalie Batalha, Kepler deputy science team lead and professor of astronomy and physics at San Jose State University. "We are on the edge of our seats knowing that Kepler's most anticipated discoveries are still to come," she concluded.

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(photo source: Nasa)