NASA's Kepler space telescope has found the first confirmed Earth-size planets orbiting another star, astronomers announced Tuesday, a major milestone in an ongoing project aimed at finding out how commonplace-or rare-Earth-like worlds may be across the cosmos.

In a solar system 1,000 light years away with at least five planets, the newly confirmed Earth-size worlds orbit too close to their star to support life. But proving the Kepler observatory can, in fact, spot worlds as small as Earth across the vast reaches of interstellar space gives astronomers confidence many more such planets are awaiting discovery among the 2,326 planet candidates found by the telescope to date.

"The first of these two planets has a diameter just 3 percent larger than the Earth, which makes it the closest object to the Earth in terms of size in the known universe," Francois Fressin of the Harvard-Smithsonian Center for Astrophysics, told reporters during a teleconference. "The second planet is 13 percent smaller than the Earth, with a diameter of around 7,000 miles. It is also smaller than Venus, and this is, in fact, the smallest planetary body ever discovered in orbit around an Earth-like star.

"Most importantly, it is the first time we've crossed the Earth-size threshold. In other words, December 2011 could be remembered as the first time humanity has been able to detect a planet of Earth-size or smaller around another star."

On Dec. 5, the Kepler team announced the discovery of a world twice the size of Earth orbiting in its star's habitable zone, where liquid water can exist, the first time a relatively Earth-size world had been found at the right distance to possibly support life.