A Re-Inflated Planet Orbiting a Red Giant Star

Inflated gas giant planets have been known for quite awhile. These planets orbit very close to their host stars and consequently receive high incident fluxes. However, it is unclear whether the inflated size of such a planet is primarily caused by the deposition of energy from the host star into the interior of the planet or by an inhibition in the planet’s cooling process. Grunblatt et al. (2016) present the discovery of EPIC 211351816.01. This planet appears to be an inflated gas giant planet in orbit around a red giant star. EPIC 211351816.01 has 1.27 ± 0.09 times the radius and 1.10 ± 0.11 times the mass of Jupiter. Its orbital period around its host star is 8.408 days. The host star of EPIC 211351816.01 has 4.20 ± 0.14 times the radius and 1.16 ± 0.12 times the mass of the Sun. Its age is estimated to be 7.8 ± 2.0 billion years old and its effective temperature is 4790 ± 90 K.

EPIC 211351816.01 seems to have been re-inflated when its host star evolved into a much more luminous red giant star. This is because when its host star has yet to evolve into a red giant star, the planet is at a large enough distance that the amount of irradiation it received was probably not sufficient to have kept the planet inflated. At that time, the planet would have received only ~200 times the flux Earth gets from the Sun, and that was probably not enough to keep the planet inflated. In a way, the discovery of EPIC 211351816.01 provides evidence that the inflation of a planet is driven by the deposition of energy from the planet’s host star rather than by a slowdown in the planet’s cooling process.

Reference:
Grunblatt et al. (2016), “EPIC 211351816.01: A (Re-?)Inflated Planet Orbiting a Red Giant Star”, arXiv:1606.05818 [astro-ph.EP]