Occurrence Rates of Circumbinary Planets

Figure 1: Artist’s impression of circumbinary planets. Image credit: NASA/JPL-Caltech/T. Pyle.

Circumbinary planets are a subset of planets that orbit two stars instead of one. In recent years, precise photometric data from NASA’s Kepler space telescope has lead to the detection of several circumbinary planets. Kepler is a planet detection telescope that measures tiny dips in a star’s brightness when a planet happens to transit in front of the star. Using publicly available Kepler data, Armstrong et al. (2014) present the first ever estimations for the rate of occurrence of circumbinary planets. The study examines binary stars with orbital periods < 60 days, and planets > 4 Earth-radius with orbital periods < 300 days.

The rate of occurrence of circumbinary planets largely depend on the inclination distribution of their orbits with respect to the orbits of their host binaries. Results from the study show that if circumbinary planets have orbits that are preferentially coplanar with their host binaries, their rate of occurrence will be ~10 percent. Instead, if the orbits of circumbinary planets have an isotropic distribution, their rate of occurrence increases dramatically to at least ~50 percent. This is expected because in the isotropic distribution case, many more circumbinary planets must exist in order to produce the few detected transits.

Figure 2: Probability density functions for the rate of occurrence of circumbinary planets following a Gaussian inclination distribution with one-sigma inclination spread of 5 degrees. The distributions are shown for (from left to right) planets with >10, 8-10, 6-10 and 4-10 Earth-radius. The >10 Earth-radius density function has been scaled down by a factor of three for clarity, and takes a different form to the others due to the zero detections of planets within this group. Source: Armstrong et al. (2014).

Another result from the study is that Jupiter-like circumbinary planets, > 10 Earth-radius, in coplanar orbits, are much less common than Saturn-like or smaller equivalents. This is in line with the suggestion by Pierens & Nelson (2008) that higher mass planets in coplanar circumbinary orbits have increased chances of being ejected. So far, this study is merely a first rough estimate on the rate of occurrence of circumbinary planets. A larger sample of such planets is required to better understand their occurrence rates.

References:

- Armstrong et al. (2014), “On the Abundance of Circumbinary Planets”, arXiv:1404.5617 [astro-ph.EP]

- Pierens & Nelson (2008), “On the evolution of multiple low mass planets embedded in a circumbinary disc”, Astronomy and Astrophysics, 483, 633