To date, we have discovered thousands of exoplanets, most of which orbit a single star. Interestingly, some of these exist in the "habitable zone", the region around a star that would allow liquid water to exist on the surface of a rocky planet. The location of the habitable zone depends on the temperature of the star. Low-mass stars are generally cool stars and so the habitable zone is close to the star (for high-mass stars, the habitable zone needs to be farther out). Some planets orbit two stars, which is not surprising because about half of the stars in the sky are in binary pairs. A planet that orbits a binary star is called a "circumbinary planet". However, for binary stars, there is a limit called the “critical instability radius” that dictates how close the planet can be from the stars. Any planet inside this “radius” cannot maintain stability in its orbit and will get thrown out of the system. If this critical instability radius is larger than the habitable zone, it is not possible for a planet to be in the habitable zone. Therefore, some types of binary stars simply cannot have any habitable zone planets. We explore the conditions that create such "uninhabitable" zones: stellar masses, mass ratios, temperatures, radii, orbital periods, and eccentricities.
