Wednesday, June 8, 2016

Having Two Suns Makes Little Difference

Circumbinary planets are planets that orbit two host stars. As a result, they experience a time-varying irradiation pattern. Circumbinary planets appear to be relatively common. It has been predicted that planets larger than 6 times the radius of Earth occur for at least 10 percent of circumbinary systems. A number of circumbinary planets have already been detected and most of them are around the size of Neptune.

Figure 1: Artist’s impression of a circumbinary planet.

May & Rauscher (2016) present a study of the atmospheric effects of the time-varying irradiation pattern on known and hypothetical gaseous Neptune-like circumbinary planets. In the study, the parameter η defines the difference in temperature for the circumbinary case as compared to the single-star case. The results from the study show that for circumbinary planets on stable orbits around their host stars, η does not exceed 1 percent even for the most extreme cases.

For example, Kepler-47b is a Neptune-sized planet in a 49.5 day orbit around a pair of stars. For Kepler-47b, η is only 0.2 percent. This means that the maximum temperature deviation from the single-star model is only 6 K. Basically, such temperature differences are not large enough to induce discernable changes in the atmospheric circulation between circumbinary planets and planets orbiting single stars. This means that Neptune-like circumbinary planets can be treated as planets orbiting single stars when it comes to atmospheric modelling.

Figure 2: η values for 7 of the 10 known circumbinary systems. All have η values less than 0.1 percent. May & Rauscher (2016)

May & Rauscher (2016), "Examining Tatooine: Atmospheric Models of Neptune-Like Circumbinary Planets", arXiv:1605.08785 [astro-ph.EP]