One of the scientists who works on the Kepler planet-hunting mission, Dimitar Sasselov, inadvertently set off a bit of a controversy when he appeared to announce that its first big data release implied that our galaxy is rich in Earth-like planets, with approximately 100 million habitable ones. That might be great news, except for some awkward facts: he dropped the news during an informal TED talk, and nobody at NASA or elsewhere was prepared to back up his assertions. In fact, the Kepler team has faced a bit of a backlash for its decision to limit the release of data on Earth-like candidates. Had Sasselov spilled the beans?

If you look at the video carefully, however, it's clear that Sasselov was making the same sorts of arguments that appear in one of the Kepler papers. Because of our methods of detection, our collection of exoplanets was biased towards massive gas giants; the distribution of planetary candidates found by Kepler implies that smaller, rockier planets are far more common when you perform an unbiased search. And that, as Sasselov noted, allows you to make some statistical inferences.

For example, Kepler has only been doing observations long enough to spot exoplanets orbiting close to the host star, but you can infer the distribution of sizes seen in those applies further out from the star and into the habitable zone. You can also assume the portion of the galaxy being imaged is typical of the galaxy as a whole. Put those together, and you apparently get the 100 million figure mentioned in the talk.

Given that the talk wound up receiving significant coverage in the press, NASA also convinced Sasselov to post a clarification on the Kepler blog. In it, he helpfully points out that he was using a very liberal definition of "habitable" in his talk. There's a big difference between Earth-sized and Earth-like. Even then, by a lot of criteria, Venus is pretty Earth-like, but has a surface temperature that can melt lead.

In the end, there seems to be little here that couldn't have been inferred from papers that are already posted on the arXiv. The news itself was mostly in the eye-popping 100 million habitable planets figure, but that clearly relies on making some very significant assumptions. Unfortunately, it's probably safe to assume that more people read the initial press coverage of Sasselov's talk than will read his clarification.