Discovery of a Highly Inflated Hot Jupiter

Fulton et al. (2015) report the discovery of a highly inflated hot Jupiter identified as KELT-8b. Orbiting at a distance of 7 million km from a mildly evolved Sun-like star, KELT-8b has an orbital period of only 3.24 days and its dayside is intensely irradiated. KELT-8b was first detected as it periodically transits its host star. Follow-up observations show that KELT-8b has 0.87 times the mass and 1.86 times the radius of Jupiter. The large radius of KELT-8b makes it the second largest transiting exoplanet known; only WASP-17b has a larger radius. The size of KELT-8b places it well above the theoretical mass-radius curve for pure hydrogen planets, indicating that it is extremely inflated (Figure 3).

Figure 1: Artist’s impression of a transiting exoplanet. Image credit: ESO/L. Calçada.

Figure 2: Transit light curve of KELT-8b. Fulton et al. (2015)

Figure 3: Mass radius diagram of all confirmed transiting exoplanets (red circles). KELT-8b is the annotated black square. Solar System planets are represented by green triangles. Fulton et al. (2015)

Being so close to its host star, the equilibrium temperature on KELT-8b is estimated to be 1675 K. Also, given the planet’s extremely inflated size, the surface gravity on KELT-8b is only 3/5th of Earth’s. For comparison, the surface gravity of Jupiter is 2.5 times the surface gravity on Earth. It is important to note that KELT-8b is a gaseous planet like Jupiter, with no physical surface. For such planets, the “surface” is typically defined as the level where the atmospheric pressure is one bar.

Given the high temperature and low surface gravity, the scale height of a hydrogen-dominated atmosphere on KELT-8b is estimated to be 1113 km. The scale height is defined as the increase in altitude over which the atmospheric pressure decreases by a factor of 2.718. For comparison, the scale height of Earth’s atmosphere is 8.5 km and of Jupiter’s atmosphere is 27 km. The remarkably large scale height of KELT-8b implies that its atmosphere is extremely puffy. KELT-8b joins a small but intriguing class of highly inflated hot Jupiters. The host star of KELT-8b is currently expanding into a red giant. The star’s surface is encroaching on the planet and in the next few hundred million years, the planet is expected to be destroyed by its host star.

Reference:
Fulton et al. (2015), “KELT-8b: A highly inflated transiting hot Jupiter and a new technique for extracting high-precision radial velocities from noisy spectra”, arXiv:1505.06738 [astro-ph.EP]