At least one of 1,000 "planet candidates" discovered by NASA's Kepler mission is a verified planet located in the habitable zone of its star, the mission's scientists have confirmed.

"This is a major milestone on the road to finding Earth's twin," said Douglas Hudgins, a scientist with the Kepler program, in a statement Monday. The confirmation will be published in the Astrophysical Journal.

Kepler-22b, located about 600 light years away, has a radius 2.4 times bigger than the Earth, making it the smallest planet ever found in the middle of the habitable zone around a star – the area where liquid water, and therefore life as we know it, can theoretically exist. The planet orbits a star similar to our sun, but slightly smaller and cooler. It completes the orbit every 290 days. Scientists don't yet know whether it is a rocky, gaseous or liquid-covered planet.

The Kepler space telescope, which orbits the sun between Earth and Mars, has been searching 156,000 stars in its field of view — about 1/400th of the sky — for signs of planets since September 2009.

In February, the mission announced it had found more than 1,000 possible new planet candidates, including 54 around the habitable zones of stars. Five were thought to be Earth-sized. However, scientists said at the time that more work was needed to confirm that the signals detected by Kepler were actual planets.

Kepler 22-b is the first of the 54 habitable-zone planet candidates to be confirmed.

Kepler searches for planets by measuring tiny decreases in the brightness of stars caused by planets crossing in front of them. A planet is considered "confirmed" after it has witnessed the same crossing or "transit" three times.