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Humanity is likely to discover its first truly Earth-like planet in 2013, according to the director of the Planetary Habitability Laboratory of the University of Puerto Rico at Arecibo.

"I'm very positive that the first Earth twin will be discovered next year," Abel Mendez told Space.com.

Mendez is leading the Habitable Exoplanets Catalog project, which seeks to identify potentially habitable planets outside our solar system. Astronomers have already found a number of potentially habitable planets, based on chemical and physical characteristics that are theoretically conducive to life. Earlier this month, Mendez said seven potentially habitable planets have been found - Gliese 581d, HD 85512b, Kepler 22b, Gliese 667Cc, Gliese 581g, Gliese 163c, and HD 40307g.

Though these plants could theoretically support life, they are only marginally Earth-like. Most Earth-sized planets found to date are too close or too far from a star to support life. On the other hand, most planets found within the so-called "Goldilocks zone" where the temperature is neither too hot or too cold are much larger than Earth.

Kepler 22b, for instance, orbits within the "Goldilocks zone" but is 2.4 times the size of the Earth. It is also unknown whether Kepler 22b is rocky like Earth or gassy like Jupiter.

But Mendez and other astronomers believe it is only a matter of time before a truly Earth-like planet is spotted. With NASA's Kepler Space Telescope spotting hundreds of new planets every year, that discovery may come sooner than later.

"The first planet with a measured size, orbit and incident stellar flux that is suitable for life is likely to be announced in 2013," Geoff Marcy, a member of the Kepler team, told Space.com.

Watch a visual representation of the seven potentially habitable planets, courtesy of Space.com, below: