The first planet that can be considered a true "alien Earth" will probably be discovered in the next two years, a NASA scientist says.

Astronomers have so far found more than 750 so-called "exoplanets" orbiting distant stars, and NASA's Kepler Space Telescope has identified 2,300 possible "candidates" that will require analysis and further study, but an Earth-like exoplanet has yet to be found.

Such a planet would be the size of Earth and orbit its star in the "Goldilocks" zone, a distance "just right" to make liquid water, and thus possibly life, possible.

"I believe Kepler will find a 'Goldilocks planet' within the next two years," said Shawn Domagal-Goldman, a researcher at NASA Headquarters in Washington. "We'll be able to point at a specific star in the night sky and say 'There it is -- a planet that could support life!'"

Such a faraway Earth-like planet would be small and faint, difficult to detect as its dim light would be overwhelmed by the bright glare of its star.

But an indirect approach, called transit spectroscopy, could reveal a lot about such a planet, researchers said.

The technique analyzes starlight bouncing off the atmosphere of an alien planet that would carry information astronomers can analyze to learn about its composition.

"The reflected light of an exoplanet tells its story," Doug Hudgins, Kepler program scientist at NASA Headquarters, told SPACE.com