NASA's Kepler mission has already found more than 1,200 potential alien planets, but it will likely be a few years before it hauls in the exoplanet "holy grail" – an alien Earth.

Scientists announced yesterday (Feb. 2) that the Kepler space telescope spotted 1,235 exoplanet candidates in its first four months of operation, including 54 that orbit their host stars in the so-called "habitable zone," that just-right range of distances that allow liquid water to exist.

While these finds are intriguing, none of the new planet candidates is likely to be a close Earth analogue, researchers said – even if a planet lies in the habitable zone, it may not be the same size and composition as Earth. Since our planet is the only world known to host life, finding and confirming an Earth twin could be a huge leap forward in the search for alien life.

"No one is more eager to get to that than the Kepler team," Douglas Hudgins, a Kepler program scientist at NASA headquarters in Washington, D.C., told reporters yesterday. "However, that's going to take time."

Making the finds

The Kepler space telescope, which launched in March 2009, finds alien planets by searching for tiny, telltale dips in a star's brightness caused when a planet transits — or crosses in front of — it from Earth's perspective.

The finds graduate from "candidates" to full-fledged planets after follow-up study — usually with large, ground-based telescopes — confirms that they're not false alarms. This process can take about a year. The Kepler team has already done a lot of vetting work on the 1,235 candidates, so most of them will probably pan out, researchers said.

"My feeling is, it'll be better than 80 percent," Bill Borucki, Kepler's science principal investigator, told

To flag a potential alien planet, Kepler needs to see a few transits, not just one. For that reason, it spots close-in planets more quickly and easily than planets found farther away, because the close-in ones move faster and transit more frequently.

And that's the pattern in the 54 potential planets — including the five that are around Earth's size — that Kepler found in their stars' habitable zones. The host stars are cooler than our own sun, and their habitable zones are thus closer in — which is where the planets are found.