Kepler-16 (AB) b, an exoplanet with a Saturn-like mass, Venus-like orbit, and Mars-like temperatures.
Kepler-16 (AB) b, aka "Tatooine," is the first exoplanet detected around binary stars (Doyle et al., 2011). The planet has a mass and size comparable to Saturn but orbits the star system with a period and distance similar to Venus. Its parent binary system is fainter than the Sun therefore making the planet colder than Mars instead of a hot desert as suggested by its colloquial name. In any case, Kepler-16 (AB) b is probably a gas giant and not suitable for life. However, it lies in the outer edge of the habitable zone (HZ) and its moons, if any, could be habitable if large enough to hold a dense atmosphere like Titan does precisely on Saturn.
Here we calculated the HZ for all the 684 confirmed planets so far from the Extrasolar Planets Encyclopedia (Figure 1). We used the Habitable Zone Distance (HZD) habitability metrics as a measure of the planet location with respect to its parent star HZ. Planets in the middle of the HZ have a HZD of zero while minus or plus one in the edges, depending if they are closer or farther from the star, respectively. Planets in the HZ are only considered habitable if they have a terrestrial-like size. Smaller ones will not be able to hold an atmosphere (i.e. look at our Moon right next to us in the habitable zone) while gas giants will have crushing pressures. Kepler-16 (AB) b is in the cold side of the HZ.
A moons mass scaling (Canup and Ward, 2006) analysis suggest that any Kepler-16 (AB) b largest moons are probably around 0.01 Earth masses, a size of 0.3 Earth radii assuming a mass-radius relationship for ocean bodies (Sotin et al., 2007). That is comparable to the size of our own Moon or Io. Any such moons will be a barren world closer to the star systems but at their distance it is plausible to have an atmosphere. Unfortunately, they will probably be a Mars-cold environments. Only the presence of much larger moons will allow for a habitable environment. Another more interesting possibility is an oceanic tidal-heated hot-Europa moon.
Our calculation for the HZ of Kepler-16 (AB) b only considered the main star and ignored the colder contribution of the secondary binary. Therefore, we expect that Kepler-16 (AB) b is actually a little bit closer to the center of the HZ but still a cold world. The dynamic nature of its HZ will be later explored but we do not expect large variations because the binary system is close together and the contribution of the secondary star is smaller even considering its higher emission in the infrared.
Figure 1. Comparison of the habitable zone (HZ) using the Habitable Zone Distance (HZD) habitability metrics for 684 confirmed exoplanets (red) with Solar System planets (blue). The HZD provides a simple way to compare and assess the HZ for many exoplanets using Habitable Zone Units (HZU) with those within the green shade inside it (-1 to +1 HZU). The Solar System, Gliese 581, HD 85512 b, and Kepler-16 (AB) b planets are labeled (outer Solar System planets are out of the scale in the Cold Zone). Only 439 of the 684 exoplanets in the database had enough data to calculate the HZD. Most fall between the -3 to +3 HZU range. Of those, 305 fall in the Hot Zone (a detection bias), 52 in the Cold Zone, and 82 in the HZ. Kepler-16 (AB) b has a HZD = 0.6 HZU which puts it in a worst position than Mars but similar to Gl 581 d, a potential habitable exoplanet.