NASA's Kepler Space Telescope has discovered two Earth-sized worlds orbiting a star in the Cygnus constellation, NASA announced Tuesday. They're the smallest exoplanets found to date and an encouraging sign that Kepler can spot planets as small as Earth.

"This demonstrates for the first time that Earth-size planets exist around other stars and that we can detect them," said Dr. Francois Fressin, of the Harvard-Smithsonian Center for Astrophysics and who led the team that made the observations. The team presented their findings in an online news conference held Tuesday.

The new planets called Kepler 20e and Kepler 20f, orbit a star called Kepler-20, a star located 980 light years away. However given their location around their star, the planets are far outside the "Goldilocks zone" where water can exist in liquid form. They would have been exposed to large doses of radiation and lighter elements like hydrogen and helium would have been blown away by stellar winds. It's unlikely the planets could harbor life but the scientists think that Kepler 20f could have had an atmosphere with water vapor early in its life.

Although the finds are significant, the next step according to Fressin would be finding an Earth-sized planet in the habitable zone.

"I don't foresee us finding a true, clear Earth twin very soon - there are so many different levels," Fressin said. "Maybe we don't only need a planet in a habitable zone and at the right size, but the star similarity. If we don't know what is in the atmosphere, or we don't have any constraints on the atmosphere, how can we say it has the right signature to be an Earth twin?"

The small planets are actually part of the solar system orbiting Kepler-20. The three largest planets, Kepler 20b, 20c and 20d have diameters of 15,000, 24,600 and 22,000 miles respectively. Kepler 20b has 8.7 times the mass of Earth. The new planets 20e and 20f are more like the Earth and Venus of the Kepler system. The smaller Kepler 20e is about 6,900 miles across while Kepler 20f measures 8,200 miles in diameters and is about the size of Earth. Both planets are likely rocky planets that migrated inward.

The Kepler-20 system has turned up many interesting finds. Kepler-22b has the right temperature for water to exist but is too big, while Kepler-20f and 20e are the right size but too hot. The Kepler-20 system is also arranged in a mix of rocky and gas planets that all orbit close to their parent star.

The study appears online in the online edition of the journal Nature.