Thursday, July 14, 2016

KELT-11b is a Highly Inflated Sub-Saturn

Pepper et al. (2016) present the discovery of KELT-11b, a highly-inflated, low-mass gas giant planet in a 4.736529 ± 0.00006 day orbit around a sub-giant star. The host star of KELT-11b has ~1.44 times the mass and ~2.72 times the radius of the Sun, and its effective temperature is 5370 ± 51 K. As for KELT-11b itself, it has 0.195 ± 0.018 times the mass and ~1.37 times the radius of Jupiter, giving it an exceptionally low density of less than ~10 percent the density of water. The planet's remarkably low density makes it one of the most inflated planets known. Furthermore, the estimated equilibrium temperature on KELT-11b is ~1712 K, and its surface gravity is only ~1/4 as strong as the surface gravity on Earth.

The low gravity gives KELT-11b has an exceptionally large scale height of almost 2800 km. The scale height is basically the vertical distance in a planet's atmosphere over which the atmospheric pressure changes by a factor of approximately 2.718. As a result of its large scale height, the planet's atmospheric transmission signal is expected to be relatively large, making KELT-11b a good target for follow-up and atmospheric characterization. Currently, the host star of KELT-11b is the brightest star in the southern hemisphere with a known transiting planet.

Estimated atmospheric scale height of known transiting hot Jupiters versus the V-band brightness of the host star. KELT-11b is indicated by a filled green circle. Pepper et al. (2016)

Pepper et al. (2016), "KELT-11b: A Highly Inflated Sub-Saturn Exoplanet Transiting the V=8 Subgiant HD 93396", arXiv:1607.01755 [astro-ph.EP]