Wednesday, October 8, 2014

One Planet, Two Stars

Welsh et al. (2014) present the discovery of KIC 9832895b, a circumbinary planet in a 240.5 day orbit around an eclipsing binary. Basically, an eclipsing binary is a pair of stars that appear to eclipse one another as they orbit around each other. In the case of KIC 9832895b, it orbits around an eclipsing binary consisting of a pair of stars with 0.93 and 0.194 times the Sun’s mass. Both stars orbit around one another with a period of 27.3 days. The orbital period of KIC 9832895b is 8.8 times the orbital period of the eclipsing binary. This places the planet safely outside the dynamical instability zone.

Figure 1: Artist’s impression of a gaseous planet which KIC 9832895b might resemble.

Figure 2: Face-on view of KIC 9632895b’s orbit, showing the habitable zone (HZ). The dark green region corresponds to the narrow (conservative) HZ and the light green corresponds to the nominal (extended) HZ. The dashed red circle represents the dynamical instability zone. The orbit of KIC 9632895b is shown in white. Welsh et al. (2014).

KIC 9832895b was detected from its three transits across the primary star (i.e. the more mass star) of the eclipsing binary. The transit depth indicates that KIC 9832895b is 6.2 times the radius of Earth, indicating that KIC 9832895b is somewhat larger than Neptune. The mass of KIC 9832895b is estimated to be most likely less than 16 times the mass of Earth due to the absence of any noticeable perturbations it has on the eclipsing binary. This constrains the mean density of KIC 9832895b to be less than 0.38 g/cm³ and demonstrates that it is an unusually low density planet, probably of gaseous composition.

KIC 9832895b is the 10th circumbinary planet discovered using data collected from NASA’s Kepler space telescope. In addition, KIC 9832895b is also in the circumbinary habitable zone where temperatures are relatively clement. The time-averaged insolation that KIC 9832895b receives is estimated to be 94 percent the intensity of insolation Earth receives from the Sun. Although KIC 9832895b is itself unlikely to harbour life, it could host a large moon capable of supporting life. Of the 10 circumbinary planets known so far, KIC 9832895b is the third found to lie within the circumbinary habitable zone.

Interestingly, the inclination of KIC 9832895b oscillates with a 102.8 year period. As a result, transits only occur ~8 percent of the time. This explains why the three detected transits of KIC 9832895b were only found in the later portion of the Kepler dataset. The transits will not be observable after 2015 and will only return on 2066. Since the transits do not always happen, for every system like the one hosting KIC 9832895b, there are ~12 similar systems where planetary transits are not observed.

Welsh et al. (2014), “KIC 9632895 - The 10th Kepler Transiting Circumbinary Planet”, arXiv:1409.1605 [astro-ph.EP]