SAN FRANCISCO — NASA’s Kepler mission already has 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 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 findings 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, told reporters Feb. 2. “However, that’s going to take time.”

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 are not false alarms. This process can take about a year. The Kepler team already has 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 principal investigator, said.

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 is 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 sun, and their habitable zones are thus closer in, which is where the planets are found.

Spotting an Alien Earth

Finding an alien Earth — a rocky, Earth-size planet that orbits in the habitable zone of a Sun-like star — will take longer than four months of observing.

“It’ll require at least three years of Kepler data, as well as painstaking observations from some of the world’s largest ground-based telescopes, before those types of planets are going to begin to emerge from the data,” Hudgins said.

This is not to diminish the 54 planet candidates in their stars’ habitable zones, or to imply that life cannot evolve on them. But if scientists are after a true Earth analogue — and many are — they will probably have to wait a little longer.

As productive as it is proving to be, Kepler still represents just one step — and an early one at that — in humanity’s search for life beyond Earth, researchers said.

“We are in some sense the first generation,” Borucki |said. “We’re finding them [planets].”

But new instruments will be needed to look for signs of life on the many worlds that Kepler discovers, researchers said. Scanning most alien planets’ atmospheres for biomarkers, for example, is beyond the capabilities of today’s instruments.

New missions, such as NASA’s proposed Terrestrial Planet Finder (TPF) — which would use an array of telescopes orbiting Earth to generate detailed images of alien worlds — could carry the search forward, Borucki said.

Making that happen would take money and require some patience. TPF, for example, is in limbo, with no firm funding and no launch date. So how Kepler’s finds will be followed up remains a bit of a mystery.

“This is only one step,” Borucki said. “It’s an important step, but there are other steps that must follow.”

Kepler Telescope by the Numbers

$600 million: The cost of the Kepler planet-hunting mission at its launch in March 2009.

156,000: The number of stars in the constellations Lyra and Cygnus that the Kepler observatory is staring at 24 hours a day, seven days a week, in the search for extrasolar planets.

1,235: The number of potential alien planets that Kepler has discovered as of Feb. 2. NASA has repeatedly cautioned that all of Kepler’s findings must be confirmed by follow-up observations using other space and ground telescopes.

662: The number of planet candidates found by Kepler that would be about the size of Neptune.

288: The number of exoplanet candidates discovered by Kepler that are in the super-Earth class. Super-Earths are planets with masses between two and 10 times the mass of Earth.

170: The number of stars that seem to have more than one planetary candidate orbiting them, which would make them parts of alien solar systems.

165: The number of potential planets found to be the size of Jupiter, which is the largest planet in our solar system.

68: The number of Earth-size planet candidates in the Kepler data release.

54: The number of the planet candidates that appear to be in the “habitable zone” around their parent stars. The habitable zone is a belt just far enough from the star for any orbiting planet to be able to have liquid water on its surface.

19: The number of potential alien planets larger than Jupiter.

15: The number of planets discovered by Kepler that have been confirmed to exist by follow-up observations by astronomers using other instruments.

1: The number of Earth-size and Earthlike habitable planets confirmed to exist with intelligent life. We call this planet Earth.

Compiled by Tariq Malik