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Limits on Circumbinary Planet Habitable Zones

Presentation #142.04 in the session SETI and Planetary Habitability — iPoster Session.

Published onJun 29, 2022
Limits on Circumbinary Planet Habitable Zones

To date, we have discovered roughly 5000 exoplanets, 14 of which orbit around two stars. Both single-star and binary-star systems can have planets in their habitable zones: the planets simply need to be at the correct distance from the star(s) to have a temperate climate. However, there is a distinct difference between a single-star and binary-star scenario which severely limits the possibility of circumbinary planets being located in the habitable zone: the critical instability radius. Interior to this radius, the time-varying gravitational perturbations from the two stars prohibit a planet from having a stable orbit. While both the habitable zone and the critical instability radius depend on the stars’ masses, separation, and eccentricity, their dependencies on these parameters are different, and thus grow at different rates as these parameters increase. Therefore, it is possible for the critical radius to exceed the outer (cold) edge of the habitable zone. If this happens, then it is impossible for the binary to host any planets in the habitable zone. We use the Holman and Wiegert (1998) formulation for the instability radius, the Eggl et al. (2020) calculation of the habitable zone of a binary star (based on the Kopparapu et al. (2014) determination of the habitable zone boundaries), and stellar isochrones to get the stellar radii, temperatures, and luminosities for a given stellar mass. With these, we show that the instability radius does indeed prohibit the existence of planets in the habitable zone around many short-period binary star configurations. This “uninhabitability” is particularly important for low mass stars, and this potentially puts a significant constraint on the number of habitable-zone circumbinary planets in the galaxy.

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