A paper has been accepted for publication in a science journal (PDF) where the author has analyzed data from NASA’s Kepler planet-finding observatory, trying to figure out how many Earth-sized planets there might be in the galaxy orbiting their stars in their habitable zones; that is, at the right distance so that the star warms the planet enough to have liquid water. In the paper, he estimates that on average 34% (+/-14%) of Sun-like stars have terrestrial planets in that Goldilocks zone.
I can explain how he got this number. But I can also explain why I think this needs to be taken with a grain of salt. Let me be clear: it’s possible he’s right, and I suspect he may very well be. His math looks good to me. But a couple of assumptions he had to make need to be pointed out, and I want that to be clear before the media start running around saying there are billions of Earths in the galaxy based on this.
Here’s the deal. Kepler is an orbiting observatory that’s staring at about 100,000 stars, looking for dips in their light when an orbiting planet passes in front of them from our perspective. The length of time the dip takes gives us the orbital period of the planet, and the size of the planet (if the star’s size is known, generally true) can be determined by how much light is blocked. I talk about how this works in a little more detail in an earlier post.
The astronomer, Wesley Traub of Caltech, based his analysis on only the first few months (136 days) of Kepler data, what was available at the time. This introduces a bias into the calculations, because that length of time is too short to conclusively find planets in their stars’ habitable zones! Even being generous, the length of such an orbit is at least 200 days, much longer than the Kepler sample. So he was forced to look at only short-period planets (with periods of 42 days or less), much closer to their stars, and extrapolate the data from there. I’ll note that Dr. Traub was up front about potential biases in the data and his analysis.