Astronomers discovered the first exoplanet orbiting a sunlike star in 1995. Since then, they've spotted more than 800 worlds beyond our own solar system, and many more candidates await confirmation by follow-up observations. [The Strangest Alien Planets (Gallery)]

NASA's prolific Kepler Space Telescope, for example, has flagged more than 2,300 potential planets since its March 2009 launch. Only 100 or so have been confirmed to date, but mission scientists estimate that at least 80 percent will end up being the real deal.

The first exoplanet finds were scorching-hot Jupiter-like worlds that orbit close to their parent stars, because they were the easiest to detect. But over time, new instruments came online and planet hunters honed their techniques, enabling the discovery of smaller and more distantly orbiting planets — places more like Earth.

Last December, for instance, Kepler found a planet 2.4 times larger than Earth orbiting in its star's habitable zone — that just-right range of distances where liquid water, and perhaps life as we know it, can exist.

The Kepler team and other research groups have detected several other worlds like that one (which is known as Kepler-22b), bringing the current tally of potentially habitable exoplanets to nine by Mendez' reckoning.