NASA: Kepler candidates include dozens of Earth-sized planets

When NASA scheduled a press briefing to talk about Kepler results, the safe assumption was that the focus would be on the system it had spotted with six planets orbiting a single star. That system was mentioned, but the Kepler scientists took the opportunity to provide a status report on the entire mission. And the orbiting observatory has quite a haul, with over 1,200 planet candidates (the vast majority announced today), and a few dozen each that are Earth-sized or orbiting within their star's habitable zone.

Since most of these candidates will have to be confirmed by ground-based observatories, Kepler has probably turned the competition for telescope time rather cutthroat. Still, the Kepler team members are suggesting that almost all of them will end up being real planets; they seem to expect that the final number of this list will be over 1,000. And that's after only four months of observation data, short enough so that only planets orbiting close to their host stars are likely to be picked up by the hardware.

Kepler's also looking at a tiny fraction of the sky (less than a percent). Extrapolating the 1,200 candidates based on all these limitations, you get a very large number of planets. "The fact that we’ve found so many planet candidates in such a tiny fraction of the sky suggests there are countless planets orbiting stars like our sun in our galaxy," said one of the Kepler scientists. "Kepler can find only a small fraction of the planets around the stars it looks at because the orbits aren’t aligned properly. If you account for those two factors, our results indicate there must be millions of planets orbiting the stars that surround our sun." Eighty-six of the planetary candidates are present in multi-planet system.

Kepler can be used to estimate the size of the planets, since we know how much of the star's light they block. These results are shown in the graphic above, and NASA has provided numbers: 68 are Earth sized, and another 288 are super-Earths. The biggest category is Neptune-like, with 662 representatives. Under 200 are the size of Jupiter or above. That's a rather significant result, given that earlier detection methods had been heavily biased towards heavy planets, raising the prospect that smaller bodies were relatively rare. Clearly, with a more complete catalog of planets, that's turning out not to be the case.

Finally, with four months of data, Kepler's been able to look at the habitable zone of some of the dimmer stars in its field of view. It's spotted 54 planet candidates there, five of which are close to Earth-sized. You can safely assume a lot of effort is already going into confirming these. In the mean time, Kepler is continuing to take data and, with each passing month, it's able to look for planets orbiting further from their host star. As a result, the next data release like this will probably include a lot of planetary candidates in the habitable zone.