Image: Artists’ concepts of small exoplanets compared to our own planets Mars and Earth. As Kepler continues to hunt, how can we move beyond its findings to learn more about terrestrial planets around much closer stars?
Credit: NASA/JPL-Caltech

Exoplanet Missions Beyond Kepler


Because it’s going to be an interesting week for exoplanet studies (for reasons I’ll talk about soon, though not today), I’ll lead off with some thoughts on eta-Earth, defined as the fraction of Sun-like stars with a planet like Earth orbiting them. We have a lot to learn about the frequency of terrestrial worlds, and as Philip Horzempa points out in a recent article for The Space Review, the image that’s gradually emerging is of fewer ‘Earths’ than Carl Sagan once estimated when he said in the 1980s that half of all stars could have a planet like our own.

With Kepler’s continuing datastream and improving ground-based instrumentation, we’re learning more about planet distribution, but Horzempa notes that even now, estimates of Earth analogs have ranged from about 2 percent to as high as 35 percent. He cites John Rehling, who went to work on biases inherent in the Kepler data set, namely that 1) the Kepler data are more complete in regions close to the host star and 2) the transit method detects larger planets more readily than small ones (see Looking Into Kepler’s Latest for more on this analysis, which appeared last March). Rehling’s findings led him to support a low 2% figure for eta-Earth but more complete Kepler data now has him backing off the number. Here’s what Horzempa says on the matter:

[Rehling’s] analysis has shown that the abundance curves for certain planet types, such as super-Earths, do not follow a smooth distribution curve. Rather, there is a peak, followed by a falloff, with increasing distance from the parent star. The distribution curve for Earth-sized worlds shows a positive slope, so far. Therefore, at present the best that can be said about eta-Earth is that it probably has a value of 2–12%. Only an extended Kepler mission will be able to determine the actual fraction of stars that have an Earth orbiting it. A low value for eta-Earth will mean that the search for Earth analogs in nearby solar systems will need to be pursued with vigor, and with multiple approaches.

Read more: Exoplanet Missions Beyond Kepler — Centauri Dreams.

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