Kepler 20E and Kepler 20F

A size comparison of two new planets with Earth and Venus.

Diagram courtesy Caltech/NASA

Two new planets found orbiting a sunlike star are the first truly Earth-size worlds discovered by NASA's Kepler Mission, scientists announced today.

The find comes on the heels of Kepler's first potentially Earthlike planet orbiting squarely within its star's water-friendly "Goldilocks zone"— the region that's not too hot and not too cold for liquid water to exist on a planet's surface.

Designated Kepler-20e and Kepler-20f, the two new planets are comparable in size to Earth and Venus. At 0.87 times the size of Earth, Kepler-20e is slightly smaller than Venus, while Kepler-20f is 1.03 times Earth's radius.

But both new extrasolar planets—or exoplanets—orbit their star much too closely to be within the habitable zone. In fact, the entire Kepler-20 system is believed to contain at least five planets all orbiting their star within a distance smaller than that between Mercury and the sun.

This orbital distance makes the planets very hot. For instance, Kepler-20e is estimated to have an average surface temperature of 1,400ºF (760ºC), while Kepler-20f is a "cooler" 800ºF (427ºC).
By contrast, Earth's average surface temperature is 57.2ºF (14°C).

Still, this is the first confirmation of truly Earth-size planets by the Kepler team—a key goal of the overall mission.more here