Figure 1: Artist’s impression of a gas giant planet with a large terrestrial-size moon.
KOI-2939b is a gas giant planet similar to Jupiter. It has 1.52 ± 0.65 times the mass and 1.06 ± 0.01 times the radius of Jupiter (i.e. 483 ± 206 times the mass and 11.9 ± 0.1 times the radius of Earth). The next largest transiting circumbinary planet is Kepler-16b, with 0.75 times the radius of Jupiter. Observations of KOI-2939b by Kepler include two transits across the secondary star and one heavily blended transit across the primary star whereby the secondary star and the planet simultaneously pass in front of the disk of the primary star. These transit observations allow the size of KOI-2939b to be determined.
As for the mass of KOI-2939b, it was determined by measuring perturbations in the timing of the mutual eclipses of the central pair of stars that the planet orbits around. These perturbations are caused by the planet’s gravitational influence. The orbital period of KOI-2939b is ~1100 days. Despite having an orbital period ~3 times longer than Earth’s KOI-2939b is in the conservative habitable zone throughout its entire orbit. On average, KOI-2939b receives 0.71 ± 0.06 times the insolation that Earth gets from the Sun. Although KOI-2939b is not habitable as it is a gas giant planet, it can harbour large terrestrial-size moons that are capable of supporting life.
Figure 2: Three transit light curves indicating the presence of KOI-2939b. The left panel shows long-cadence data, the middle and right panels show short-cadence data. The first and third transits are across the secondary star. The second transit is heavily blended whereby the secondary star and the planet simultaneously pass in front of the disk of the primary star. Kostov et al. (2015).
Kostov et al. (2015), “KOI-2939b: the largest and longest-period Kepler transiting circumbinary planet”, arXiv:1512.00189 [astro-ph.EP]