Wednesday, April 22, 2015

Two Eccentric Giant Planets around a Sun-Like Star

L. Mancini et al. (2015) report on the discovery of KOI-372, a planetary system consisting of two giant planets on wide and eccentric orbits around a Sun-like star. The two giant planets are identified as KOI-372 b and KOI-372 c, with the suffixes “b” and “c” indicating their planetary nature. KOI-372 b is a transiting giant planet that was first detected by NASA’s Kepler space telescope. Follow-up radial velocity measurements were then made using a ground-based telescope to determine the mass of KOI-372 b. The radial velocity measurements and observations by Kepler indicate that KOI-372 b is a dense Jupiter-like planet with 3.25 ± 0.20 times the mass and 0.882 ± 0.088 times the diameter of Jupiter.

Figure 1: Artist’s impression of a giant planet with a hypothetical moon in orbit around it.

KOI-372 b orbits its host star every 125.6 days in a fairly elongated orbit. The orbit of KOI-372 b brings the planet as close as 61 million km (0.41 AU) from its host star and out as far as 87 million km (0.58 AU). KOI-372 b is just interior to the habitable zone around its host star and any large Earth-size moon around KOI-372 b will be too hot to be habitable. Nevertheless, Kepler observed 12 transit events of KOI-372 b and analysis of the mid-transit times shows a clear variation in transit timing. This variation is due to the presence of a second planet in a wider orbit that is perturbing the orbit of KOI-372 b.

The second planet is identified as KOI-372 c and it has a mass between 0.13 to 0.31 times the mass of Jupiter. KOI-372 c also orbits its host star in an elongated orbit with an orbital period of roughly 460 days, at an average distance of about 1.2 AU from its host star. The orbit of KOI-372 c is right within the habitable zone. Any large Earth-size moon in orbit around KOI-372 c could potentially be habitable. Unlike its more massive sibling KOI-372 b, KOI-372 c does not transit its host star. Gyrochronological analysis shows that the planetary system of KOI-372 is relatively young, with an estimated age of 1.0 ± 0.3 billion years. In comparison, the Solar System is 4.57 billion years old.

Figure 2: When the known transiting planets are plotted in a semi-major axis versus planetary-mass diagram, it can be seen that KOI-372 b and KOI-372 c occupy sparsely-populated regions of the plot. The black circles indicate the positions of KOI-372 b and KOI-372 c (the size of the circle that denotes KOI-372 c is arbitrary since the planet does not transit its host star, so its size remains unknown). L. Mancini et al. (2015).

L. Mancini et al. (2015), “KOI-372: a young extrasolar system with two giant planets on wide and eccentric orbits”, arXiv:1504.04625 [astro-ph.EP]