Two years ago, the first transiting circumbinary planet, Kepler-16b, was discovered. Since then, several more circumbinary planets have been found. As the term suggests, a circumbinary planet is a planet that orbits two stars instead of one. Recently, Kostov et al. (2014) report the discovery of Kepler-413b - a Neptune-sized transiting circumbinary planet. Kepler-413b orbits a pair of stars, both of which are less massive and less luminous than the Sun. The two stars, one a spectral class K-type star and the other an M-type star, circle around each other every 10.1 days. Both stars form a K + M eclipsing binary system. Further out, Kepler-413b circles the pair every ~66 days on a somewhat eccentric orbit.
Figure 1: Artist’s impression of a possible view from the surface of a circumbinary planet. Credit: NASA/JPL-Caltech.
Kepler-413b was discovered using data from NASA’s Kepler space telescope. The Kepler light curve data for Kepler-413b show a set of 3 transits separated by ~66 days, followed by ~800 days with no transits, followed by another set of 5 transits with each transit again ~66 days apart. There is a small misalignment of ~2.5° between the orbital planet of the K + M binary and the orbital plane of Kepler-413b around the binary. As a consequence, the orbit of Kepler-413b precesses, causing long periods with no transits. In fact, the next transit is not expected to occur until 2020. Due to orbital precession from the influence of the central binary and the planet’s own eccentric orbit, Kepler-413b is likely to experience complex seasonal cycles with interesting climate patterns.
The combined incident stellar flux from the K + M binary at the orbital location of Kepler-413b varies from a minimum of ~1.64 to a maximum of ~3.86 times the average flux Earth receives from the Sun. This places Kepler-413b within the inner edge of the habitable zone around its host stars, suggesting temperatures that are probably too warm for life. Nevertheless, if a planet is a dry desert planet, it can remain habitable at even closer distances in a region known as the dry desert habitable zone. Interestingly, Kepler-413b is in this zone for most of its orbit (Figure 4) and a dry terrestrial-sized moon around Kepler-413b would be on the verge of habitability.
Figure 2: Photodynamical fits (red) to the 8 observed (and a possible 9th, labelled as “A” near time 188.35) transits. Kostov et al. (2014)
Figure 3: Orbital configuration of Kepler-413b over the course of 1/8 precession period (1/8 of ~11 years). Kostov et al. (2014)
Figure 4: Orbital location of Kepler-413b (black line) as a function of the orbital phase of the planet and equilibrium temperature, assuming a planetary Bond albedo of 0.34. The inner (red line) and outer (blue line) edges of the habitable zone are indicated, and the dashed line indicates the inner edge of the dry desert habitable zone. The planet is in the dry desert habitable zone for most of its orbit. Kostov et al. (2014)
Kostov et al. (2014), “Kepler-413b: a slightly misaligned, Neptune-size transiting circumbinary planet”, arXiv:1401.7275 [astro-ph.EP]