“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.

"December 2011 may be remembered as the first time humanity was able to discover an Earth-size planet in orbit around another star," lead author Francois Fressin, of the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts, said today during a NASA teleconference.

The discovery "demonstrates for the first time that Earth-size planets exist around other stars, and that we are able to detect them."

New Planets a Tight-Knit Group

Previously discovered exoplanets have all been considerably larger than Earth.

Even the just announced Goldilocks world, Kepler-22b, was estimated to be 2.4 times Earth's radius and is thought to have a fairly low density, meaning it could have a thick atmosphere and a surface quite unlike Earth's.

With the new Kepler discovery, we now have confirmation of planets that "are exactly the right size [to be Earthlike] ... but are too hot" for life as we know it, team member David Charbonneau, also with the CfA, told National Geographic News…”

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