Washington, April 18 - Astronomers said Thursday they have found a planet roughly Earth's size that may have liquid water on its surface and thus, theoretically, be habitable.

The discovery proves the existence of worlds that might be similar to our own and could also shape future investigations of exoplanets that could have terrestrial surface environments, the researchers reported in the US journal Science, according to Xinhua.

"This is the first definitive Earth-sized planet found in the habitable zone around another star," lead author Elisa Quintana of the SETI Institute and NASA Ames Research Centre said in a statement.

The new world, designated Kepler-186f, is the outmost of five planets orbiting Kepler-186, a red dwarf star some 500 light years from Earth.

Of the nearly 1,800 confirmed exoplanets found in the past two decades, about 20 orbit their host star in the habitable zone, a range of orbital distances at which surface water on a planet with an atmosphere would neither freeze nor boil.

However, all of these previously discovered worlds are larger than Earth, and consequently their true nature -- rocky or gaseous- - is unknown.

Kepler-186f was discovered using NASA's Kepler space telescope, which was launched in March 2009 to search for Earth-sized planets orbiting other stars.

On the basis of the observed dimming of starlight from Kepler- 186, the researchers estimated that this newly discovered planet is close to Earth's size -- just 10 percent larger in diameter -- and goes around its star once every 130 days.

Finding Kepler-186f is a first, but "it's not a record we wish to keep," Quintana said. "We want to find more of these."(IANS)