Tuesday, September 30, 2014

Hot Giant Planet that is Blacker than Coal

Gandolfi et al. (2014) report on the discovery of a half-Jupiter mass planet transiting an old Sun-like star every 2.7 days. This discovery combines data collected by NASA’s Kepler space telescope from 13 May 2009 to 11 May 2013 with spectroscopic follow-up observations performed with the FIES spectrograph at the Nordic Optical Telescope in La Palma, Spain. Photometric data from Kepler indicates how much starlight is blocked when the planet transits in front of its host star, allowing the size of the planet to be estimated. The FIES spectrograph measures the amount of gravitational tugging the planet has on its host star and provides the estimated mass of the planet.

Figure 1: Artists’ illustration of a hot-Jupiter orbiting a Sun-like star. Image credit: Haven Giguere & Nikku Madhusudhan.

Figure 2: Phase-folded transit light curve of KOI-183b showing the best fitting model and residuals. Gandolfi et al. (2014).

Figure 3: Radial velocity data from the FIES spectrograph with the median, 68th and 99th percentile limits. Gandolfi et al. (2014).

The planet, identified as KOI-183b, is estimated to have 0.595 ± 0.081 times the mass of Jupiter and 1.192 ± 0.052 times the radius of Jupiter. Given the mass and size of the planet, its bulk density is 0.459 ± 0.083 g/cm³. KOI-183b orbits its host star at a distance of only ~1/28th the Earth-Sun distance. As a result, KOI-183b is intensely heated and is classified as a hot-Jupiter. The radius of KOI-183b is consistent with theoretical models for heavily irradiated coreless gas-giant planets. Being so near to its host star, temperatures on KOI-183b can reach ~2000 K, hot enough to melt titanium metal.

Data from Kepler also indicates that KOI-183b periodically passes behind its host star in what is known as a secondary eclipse. The secondary eclipse signal has a depth of 14.2 ± 6.6 ppm. From the depth of its secondary eclipse signal, KOI-183b is estimated to have a very low Bond albedo of only 0.037 ± 0.019, making it one of the “darkest” gas-giant planets known so far. Basically, KOI-183b reflects only ~4 percent of the incoming radiation from its host star back into space. For comparison, that is darker than coal. Other hot-Jupiters with similarly low Bond albedos include TrES-2b and Kepler-77b.

Gandolfi et al. (2014), “KOI-183b: a half-Jupiter mass planet transiting a very old solar-like star”, arXiv:1409.8245 [astro-ph.EP]