Five potential Earth-sized planets that may support liquid water — and so possibly life — have been found by NASA's Kepler mission.

All five orbit the habitable zone — the region around a star where the temperature is just right for the existence of liquid water — of stars that are smaller and cooler than the sun, NASA announced at a news conference Wednesday. Liquid water could make it possible for life as we know it to exist on those planets.

However, follow-up observations are needed to verify that the signals observed by the Kepler space telescope are actual planets, the U.S. space agency cautioned in a news release.

'The fact that we've found so many planet candidates in such a tiny fraction of the sky suggests there are countless planets orbiting sun-like stars in our galaxy.'— William Borucki, NASA

NASA released new data Wednesday from the telescope on more than 1,000 possible new planets outside our solar system — more than doubling the count of what astronomers call exoplanets.

They haven't been confirmed as planets yet, but some astronomers estimate that 90 per cent of what Kepler has found will eventually be verified.

54 planets in habitable zones

Overall, Kepler found 54 potential planets around the habitable zones of several stars, although most are at least double the size of Earth.

The telescope has been searching 156,000 stars in its field of view — about 1/400th of the sky — for signs of planets since September 2009.

New planets by the numbers

Of the 1,235 planets discovered by Kepler to date:

  • 68 are approximately Earth-sized.
  • 288 are about double the size of Earth.
  • 662 are Neptune-sized (20 times the mass of Earth).
  • 165 are Jupiter-sized (300 times the mass of Earth).
  • 19 are larger than Jupiter.

"The fact that we've found so many planet candidates in such a tiny fraction of the sky suggests there are countless planets orbiting sun-like stars in our galaxy," said William Borucki of NASA's Ames Research Center in Moffett Field, Calif., the mission's science principal investigator.

In a statement, Borucki noted that many could also have moons with liquid water. That even holds for planets so large that their gravity would crush life as we know it.

Another major finding announced Wednesday was six confirmed planets orbiting a sun-like star called Kepler-11 — the most planets ever found orbiting a single star outside our solar system. All are larger than Earth and the largest around about the size of Uranus and Neptune (about 20 times the mass of Earth.)

Kepler orbits the sun between Earth and Mars. It searches for planets by measuring tiny decreases in the brightness of stars caused by planets crossing in front of them.

With files from The Associated Press