NASA's planet-hunting Kepler spacecraft has confirmed the existence of a planet that is 600 light years away that could possibly have water on its surface.

NASA said the planet, Kepler-22b, is the first the mission has found in the "habitable zone." It is said to be the smallest planet found orbiting in the middle of the habitable zone of a star similar to the Earth's sun.

Kepler-22b has twice the radius of the Earth and scientists are still uncertain if the planet is mostly rocky, gaseous or has a liquid composition.

Scientists believe Kepler-22b's discovery brings them a step closer to finding a planet that is much like Earth.

"This is a major milestone on the road to finding Earth's twin," Douglas Hudgins, Kepler program scientist at NASA Headquarters in Washington, stated in a press release. "Kepler's results continue to demonstrate the importance of NASA's science missions, which aim to answer some of the biggest questions about our place in the universe."

Kepler-22b orbits around a sun-like star in 290 days. This is comparable to the Earth's 365 days even though the new planet is much bigger.

NASA said the planet's host star belongs to the same class as the Earth's sun, which is known as the G-type. However, it is slightly smaller and cooler.

If there is water on this new planet, there could be life.

The search for planets

The Kepler observatory was launched in March 2009 with a mission to hunt down Earth-size alien planets that are in the habitable zone of their parent stars.

NASA said Kepler has discovered more than 1,000 new planet candidates -- almost double the previously known count - with 10 of them being near-Earth-size, orbiting in the habitable zone of their host star.

Kepler finds alien planets by using the "transit method," which involves looking for dips in a star's brightness. These dips are caused by a planet transits, or when it crosses in front of the star, blocking some of the star's light.

"If it does that three times in succession then the NASA scientists think that is good enough to conclude that what's causing the dip in light is a planet," Ian Ridpath of the Royal Astronomical Society told The Associated Press.

NASA will determine if a planet candidate is truly a planet after follow up observations.

The space agency has said that of the 54 habitable zone planet candidates reported in February this year, Kepler-22b is the first to be confirmed.

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