In a major find, NASA's Kepler Space Telescope has discovered three new Earth-size habitable or "Goldilocks" zones around Sun-like stars. Of the three, two are likely made of rock like our Earth.

Of more than 1,000 verified planets found by Kepler, eight are less than twice Earth-size and in their stars' habitable zone. All eight orbit stars are cooler and smaller than our Sun, the US space agency said in a statement.

"Three of the newly-validated planets are located in their distant Suns' habitable zone, the range of distances from the host star where liquid water might exist on the surface of an orbiting planet," informed John Grunsfeld, associate administrator of NASA's Science Mission Directorate at the agency's headquarters in Washington DC.

Two of the newly validated planets, Kepler-438b and Kepler-442b, are less than 1.5 times the diameter of Earth.

Kepler-438b, 475 light-years away, is 12 percent bigger than Earth and orbits its star once every 35.2 days.

Kepler-442b, 1,100 light-years away, is 33 percent bigger than Earth and orbits its star once every 112 days.

Both Kepler-438b and Kepler-442b orbit stars smaller and cooler than our Sun, making the habitable zone closer to their parent star in the direction of the constellation Lyra.

"Each result from the planet-hunting Kepler mission's treasure trove of data takes us another step closer to answering the question of whether we are alone in the Universe," Grunsfeld noted.

The Kepler team has also added another 554 candidates to the roll of potential planets, six of which are near-Earth-size and orbit in the habitable zone of stars similar to our Sun.

"With each new discovery of these small, possibly rocky worlds, our confidence strengthens in the determination of the true frequency of planets like Earth," added co-author Doug Caldwell, SETI Institute Kepler scientist at NASA's Ames Research Center at Moffett Field, California.

Work is underway to translate these recent discoveries into estimates of how often rocky planets appear in the habitable zones of stars like our Sun, a key step toward NASA's goal of understanding our place in the universe.

The Kepler Space Telescope continuously monitored more than 150,000 stars beyond our solar system, and, to date, has offered scientists an assortment of more than 4,000 candidate planets for further study -- the 1,000th of which was recently verified.

The research paper reporting this finding has been accepted for publication in The Astrophysical Journal.