Nearly all giant planets discovered to date have orbital periods either less than ~10 days (i.e. the hot-Jupiters) or more than ~100 days (i.e. like Jupiter). Only a few giant planets are known to have orbital periods between 10 to 100 days. A. Santerne et al. (2014) report on the discovery of KOI-1257b, a warm giant planet in a highly eccentric 86.6 day orbit around one of two stars in a binary system. KOI-1257b’s orbit is such that it periodically transits its host star. Its discovery serves as an important addition to a list comprising of just a handful transiting giant planets with orbital periods greater than one month - HD80606 b (111 day period); CoRoT-9 b (95 day period); Kepler-30 c (60 day period) and Kepler-87 b (115 day period).
KOI-1257b was first detected by NASA's Kepler space telescope as the planet periodically passes in front of its host star and causes the apparent brightness of its host star to dip by a small amount. The amount of dimming allows the size of KOI-1257b to be estimated at 0.94 ± 0.12 times Jupiter's radius. Subsequently, radial velocity observations were made using the SOPHIE and HARPS-N spectrographs to measure the amount of gravitational tugging KOI-1257b exerts on its host star. This allows the planet’s mass to be estimated at 1.45 ± 0.35 times Jupiter's mass. With the size and mass determined, the bulk density of KOI-1257b is 2.1 ± 1.2 g/cm^3.
Figure 1: Artist’s impression of a giant planet.
Figure 2: Phase-folded transit light curve of KOI-1257b as observed by NASA's Kepler space telescope. The best-fit model is superimposed on the data and residuals from the best-fit model in parts per thousand (ppt) are shown below. A. Santerne et al. (2014).
KOI-1257b goes around its host star on an orbit with a high eccentricity of 0.772 ± 0.045, implying a highly elongated orbit. An eccentricity of zero denotes a circle, while an eccentricity of 1 denotes a parabola. At closest approach, KOI-1257b is only ~10 million km from its host star. On the farthest point along its orbit, KOI-1257b is ~100 million km from its host star. In comparison, the Earth orbits the Sun at an average distance of 149.6 million km. When KOI-1257b is nearest its host star, it receives ~100 times more stellar flux than when it is at its farthest. The equilibrium temperature of KOI-1257b is estimated to be 511 ± 50 K. Nevertheless, temperatures on KOI-1257b are expected to swing wildly due to the highly eccentric orbit.
A. Santerne et al. (2014), “SOPHIE velocimetry of Kepler transit candidates XII. KOI-1257 b: a highly-eccentric 3-month period transiting exoplanet”, arXiv:1406.6172 [astro-ph.EP]