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