Kepler is the first spacecraft with the ability to find Earth-size planets orbiting stars like our sun in a zone where liquid water could exist.

Astronomers predict that the project should not only be able to discover Earth-size planets but also study their atmospheres for signs of life.

The next big milestone, astronomers say, will be the detection of Earth-size planets, although that will require going to space.

It is an Earth-size planet, but not in the habitable zone within which liquid water can exist.

If Earth-size planets are common then Kepler should detect hundreds of them.

Kepler is the first spacecraft capable of finding Earth-size planets around other stars.

Any Earth-size planets present, they said, must already have formed.

Scientists said the method had the potential to detect Earth-size planets someday.

Their observations should also improve to the point that they can see Earth-size planets.

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