Constraining the Long-Term Stability and Habitability of Circumbinary Planets

Circumbinary planets (CBPs) can provide valuable exoplanet laboratories regarding the effects of flux variation on habitability. Because very few Earth-analog CBPs have been detected, we must numerically constrain which parameters are necessary for stable and potentially habitable circumbinary configurations. We present preliminary results from thousands of N-body REBOUND simulations, exploring uniform ranges for binary eccentricity, binary separation, binary mass ratio, and planet semi-major axis for an Earth-mass CBP orbiting two subsolar-mass stars; we integrate for 1 Gyr to ensure long-term stability. We characterize which subsets of parameters lead to stable orbits and which of these orbits may result in a potentially habitable planet, exploring the planets’ equilibrium temperatures as a proxy for habitability. We find that planets orbiting close to a highly eccentric binary quickly become unstable and that nearly all (87%) planets with semi-major axes at least three times greater than the distance between stars are stable, regardless of binary mass or eccentricity. We find that roughly one-third of stable planets experience habitable insolation for some fraction of their orbit, although only half have habitable equilibrium temperatures for at least 80% of their orbit. The planets that spend most of their orbit in the habitable zone tend to orbit close to their stars and around binaries of any eccentricity with low separation distances. We find that potentially-habitable CBPs rarely orbit binary stars that are similar in mass and that they tend to orbit closer to their host stars as the stellar mass ratio approaches 1.0.