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Scientific Method / Science & Exploration

Latest Kepler data includes over 700 exoplanet candidates

Despite some apparent confusion on NASA's part, the data release for the …

The Kepler spacecraft

Today should be a historic moment for the field of astronomy. With a single data release, the team behind NASA's Kepler instrument has nearly doubled the number of extrasolar planets we're aware of, including many that fall between the sizes of Earth and Neptune, a class of planets that was poorly represented in our existing collection. Instead, NASA's handling of the announcement appears to have been very confused, and attention will likely focus on the candidate planets that were left out: 400 objects that are even smaller than the ones being announced.

The decision to hold back on the most Earth-like objects had been made months ago. The intent is to allow the project scientists the chance to confirm these are not false positives and to have priority in publishing on the exoplanets. These candidates are mentioned only once in a draft paper that describes the data that is being released. Referring to Earth's radius as Re, the draft states, "those [stars] with the small-size candidates (ie, those with radii less than 1.5 Re), are among the 400 withheld targets and are thus not among those considered here."

That controversy aside, NASA's handling of the data release has been positively bizarre. Word of the hundreds of new planets in the data release appears to have been first disclosed by The New York Times. A NASA video describing the data appeared briefly on YouTube, but was then pulled, while two draft papers that describe some of the results appeared in the arXiv last night.

But, despite the apparent publicity, NASA has remained silent. There have been no releases about the data, and NASA's Kepler mission page still lists its planet count as an anemic five. It's not clear what exactly is going on with the space agency.

Fortunately, there are the draft papers in the arXiv to rely on. One of them describes five systems that appear to be hosts to multiple planets (one with at least three, the rest with at least two). All of these appear to be very heavy planets—the smallest, at a minimum, is five times the mass of Earth—but three of them are fairly compact, with radii that are less than three times that of Earth's. Most of them have orbital periods of less than 20 days, so the authors expect that we'll be able to observe changes in orbital dynamics of some of these systems as these heavy bodies exert gravitational pulls on each other.

But it's the full catalog of objects that Kepler appears to be tracking that's really jaw dropping. Prior to these results, we were aware of 460 exoplanets, most of them big. The typical planet identified by transit observations (the method used by Kepler) was about 13 Re. The data that has been released includes about 100 planets that are somewhere between two and four Earth radii—and that's excluding up to 400 smaller objects. Kepler's total exoplanet candidate list (including those 400) is over 700.

The bias towards large planets in our existing catalog was at least partially a product of the fact that bigger objects are easier to detect. But the Kepler data seems to put to rest any question of whether it was also the product of a real bias towards the formation of heavy planets. In the area that Kepler's imaging, the authors conclude:

Small candidate planets with periods less than 30 days are much more common than large candidate planets with periods less than 30 days and that the ground-based discoveries are sampling the large-size tail of the size distribution. Note that for a substantial range of planet sizes, a 1/R2 curve fits the Kepler data well. Assuming the false positive rate and other biases discussed above are independent of planet size for planets larger than two Earth radii, this implies that the frequency of planets decreases with the area of the planet.

In other words, for at least those planets close to their host stars, there seem to be a lot more Earth analogs than super-Jupiters.

Kepler will continue to train its 95 megapixel eye on over 150,000 stars within its study area for several years more. As time goes on, it's more likely to spot and validate transits of planets further from the host stars, including some that may be able to play host to water in its liquid form. Hopefully, NASA will have its house a bit more in order as the future data comes in—and when the 400 systems currently being withheld are released this coming February.

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