A Warm Gas Giant Planet Circling a Fast-Spinning Star

Bourrier et al. (2015) present the discovery of a transiting warm gas giant planet identified as KOI-12b. It was first detected by NASA’s Kepler space telescope. Follow-up observations were performed using the SOPHIE (Spectrographe pour l’Observation des Phénomènes des Intérieurs stellaires et des Exoplanètes) échelle spectrograph on the 1.93 m reflector telescope at the Haute-Provence Observatory in south-eastern France. KOI-12b has 1.43 ± 0.13 times the diameter of Jupiter and a 3σ upper mass limit of about 10 times the mass of Jupiter. The diameter of KOI-12b was directly measured from the amount of starlight the planet blocks as it transits in front of its host star.

Figure 1: Artist’s rendition of a gas giant planet.

KOI-12b orbits its host star at a distance of about 0.14 AU, with an orbital period of 17.86 days. The host star of KOI-12b is a fast-rotating star (spins with a speed of at least 60 km/s) with 1.45 ± 0.09 times the mass and 1.63 ± 0.15 times the diameter of the Sun. Furthermore, it is a relatively hot star with an effective surface temperature of 6820 ± 120 K. For comparison, the Sun has an effective temperature of 5778 K. The larger size and higher surface temperature means that the host star of KOI-12b is at least ~5 times more luminous than the Sun.

The distance of KOI-12b from its host star suggests that it is a moderately irradiated warm gas giant planet with an estimated temperature of just over 1000 K. However, the diameter of KOI-12b appears inflated given the moderate stellar irradiation it receives. KOI-12b is the largest exoplanet known that has an orbital distance greater than 0.1 AU and the host star of KOI-12b is one of the hottest known to host an exoplanet. Both KOI-12b and its host star are located at a distance of about 1400 light-years away.

Figure 2: Transit light curve of KOI-12b. The best fit to the data is displayed as a red line, with residuals in the lower panel. Bourrier et al. (2015).

Reference:
Bourrier et al. (2015), “SOPHIE velocimetry of Kepler transit candidates XVI. Tomographic measurement of the low obliquity of KOI-12b, a warm Jupiter transiting a fast rotator”, arXiv:1504.04130 [astro-ph.EP]