The Kepler spacecraft, launched in 2009 to scour distant stellar systems for Earth-like planets, has yet to attain that lofty goal, but it is now returning a flood of data about all manner of planets outside the solar system. On June 15, the Kepler team released information on possible planets identified in the first month or so of the spacecraft's three-plus-year mission—a massive set of more than 300 candidates that promises to significantly augment the known catalogue of extrasolar planets. The data were published online at the preprint repository and will be submitted to The Astrophysical Journal.

On top of that bountiful, publicly released haul, the Kepler team is holding onto a list of 400 of the most intriguing potential host stars for further observation. With these two sets of objects, Kepler is poised to rewrite the catalogue of known exoplanets, which currently contains about 450 objects, including five confirmed exoplanets that the Kepler team announced in January.

At least a dozen of the candidates are of comparable size to COROT 7 b, the smallest exoplanet known at just 1.6 times the diameter of Earth, and some are estimated to be slightly smaller. And those are just the smallest contestants from the set the Kepler team has not retained for follow-up observation. The withheld set of 400 possible stellar systems contains objects that appear even smaller, perhaps as small as Earth. "The cat is out of the bag," says Natalie Batalha, a professor of physics and astronomy at San Jose State University and a co-investigator on the Kepler team. "Kepler has seen Earth-size planet candidates."

But before COROT 7 b loses its crown as the smallest known exoplanet, Kepler's candidates must be verified with more data to rule out planetary decoys. "We fully expect half of the candidates to be false positives," Batalha says. One common false positive is an eclipsing binary star behind one of Kepler's target stars; the background light blocked by such an eclipse can mimic the periodic dimming that Kepler uses to identify planets passing in front of its target stars. The spacecraft, trained on a patch of more than 150,000 stars near the constellation Cygnus, trails Earth as both orbit the sun.

Without follow-up observations, fine details on the 312 publicly released candidates remain scarce—Kepler can determine a planet's diameter but not its mass, for instance. But even the rough outlines of the Kepler sample are intriguing.

For instance, more than half of Kepler's newly announced candidates are smaller than Neptune. That stands in stark contrast to the current catalogue of exoplanets, which because of observational biases is loaded with gas giant–size worlds; of the nearly 100 catalogued exoplanets whose diameters are known, all but two are Neptune-size or larger.

Kepler may also have spotted the first planetary system in which more than one planet can be seen passing in front of its star. One star in the public data set bears the signature of three distinct planets causing periodic dimming effects, and four more stars show the possible presence of two planets apiece. Such multiple-planet systems have been difficult to spot by Kepler's chosen search method—the monitoring of stars for planet-induced shading events known as transits—because the planets' orbits must be almost perfectly planar so both are aligned with the space telescope's line of sight.

But the ultimate prize of Kepler's hunt, a potentially habitable terrestrial world in an Earth-like orbit around a sunlike star, remains years away. The Kepler team's protocols require that three transits must be recorded, along with other observations, before a candidate can be confirmed as a true planet, and an Earth-like orbit will carry a planet into Kepler's view just once a year. (The planetary transits that Kepler looks for only occur once per planetary orbit, when the three objects—the planet, the host star and Kepler—fall into alignment. For a planet that takes around one Earth year to circle its star the transit would occur once annually.) With that requirement, plus time for data processing and confirmation, a true Earth–sun analogue will not be announced until after the end of the three- and-a-half-year mission, Batalha says.

In the meantime, the public data release should spur a rash of independent involvement as astronomers work to confirm the candidates Kepler has identified. "We're calling it an embarrassment of riches—so many candidates," Batalha says. "It's become abundantly clear that we're not going to be able to follow up all of these candidates ourselves."

The decision to allow Kepler scientists to keep some of their data private until February 2011 has drawn criticism from some fellow researchers. Jon Morse, director of the astrophysics division for NASA's Science Mission Directorate, addressed the issue in May during a town hall with astronomers at the semiannual meeting of the American Astronomical Society in Miami. Morse explained that launch delays had caused the Kepler team to miss much of its annual follow-up season, the months when the spacecraft's target stars are in view from telescopes on Earth. "Having the team be able to hold back 400 or so targets to do some follow-up from the ground to look for false positives was consistent" with the spirit of the original data release agreement, Morse said. "Folks in this room understand the concept of proprietary data rights."