Today, NASA's Kepler team has announced that it has developed a new technique to verify the existence of many of the planetary candidates in its back catalog. The technique, which relies on the presence of multiple planets in the system, has led to the single largest announcement of new planets in history: 715 of them, orbiting a total of 305 stars. Most of these are small, between the sizes of Earth and Neptune, and are tightly packed in the inner regions of the systems in which they reside, but four appear to be in the habitable zone.

If you visit Kepler's home page, you'll see a count of confirmed planets in the upper right (it's currently at 961). Hover over it, and you'll see there are over 3,800 unconfirmed planetary candidates. Those candidates come from the method that Kepler uses to discover planets: watching for a mini-eclipse that causes a slight dimming of their host star's light. A similar pattern can be caused by a dim star orbiting in the system (a configuration called an eclipsing binary system), which raises the prospect of false positives.

In the past, this has generally involved multiple follow-up observations with a large telescope, which has held back the announcement of confirmed planets to a relative trickle. However, there have been a number of discoveries that have been based on Kepler data alone. These discoveries have come from multi-planet systems, where the planets gravitationally interacted, speeding up or slowing each other down. This activity creates regular variations in the timing and duration of the eclipses as the exoplanets transit between their host star and Earth.

The new technique takes a different approach to these orbital interactions. If the bodies interacting are as massive as stars, then the systems would be dynamically unstable; one or more of the stars would be ejected from the system. That leaves one other likely candidate for causing a false positive in these multiple-transit systems: planets orbiting some other star in the foreground. Based on the rate of these sorts of false positives and their distribution in the Kepler data, the chance of the multi-transit signals coming from a false positive works out to be about one percent—which places the probability that they're planets at a statistically significant 99 percent.

As a result, the Kepler team is now willing to call them planets and not just planetary candidates. Although there are still some hitches, as the SETI Institute's Jason Rowe referred to them as candidates during today's announcement. "I've been training myself for four years to say candidates," he said by way of excuse.

Using the first two years of Kepler data, over 300 stars come through this statistical analysis, and they are host to 715 new exoplanets—the largest collection ever announced at once. The vast majority are in the inner regions of their exosolar systems, and the exoplanets themselves are generally small, hovering awkwardly between super-Earths and mini-Neptunes. The tight packing and large size mean that these exosolar systems have a lot more mass in their inner reaches, which suggests that either the planets have migrated inward or the star started out with a much larger planet-forming disk (or some combination of the two).

The changing picture of our planet catalog was also emphasized during the press conference, driven home by the graph below.

Meanwhile, preparations for the next phase of Kepler's scientific career are continuing. Although the probe lost one of its 20 imaging sensors to a hardware failure, controllers are confident that they can gather sufficient data from the remaining 19. And initial tests of the new pointing procedure suggest that it will be able to outperform ground-based observatories despite the loss of two of Kepler's reaction wheels.

Materials from today's press call, including two papers describing the new planet-validating technique, have been placed online by NASA.