A Low-Density Transiting Super-Neptune

Bayliss et al. (2015) present the discovery of a low-density transiting super-Neptune identified as HATS-8b. This planet is in a close-in 3.58-day orbit around a Sun-like star. Photometric and spectroscopic observations indicate that HATS-8b has 0.873 times the radius and 0.138 times the mass of Jupiter, resulting in a remarkably low bulk density of only 0.259 g/cm³. HATS-8b is termed a super-Neptune as its mass is roughly halfway between that of Neptune and Saturn. Due to its large radius and low density, the gravity on HATS-8b is less than half the surface gravity on Earth and its atmospheric scale height (i.e. increase in altitude over which the atmospheric pressure decreases by a factor of 2.718) is expected to be almost 1,000 km.

HATS-8b is similar in mass as Kepler-101b, which is slightly more massive at 0.16 times the mass of Jupiter. However, the size of Kepler-101b is only 0.515 times the radius of Jupiter, giving it a much higher bulk density of 1.45 g/cm³, a factor of 5.6 times the bulk density of HATS-8b. The large range of densities amongst super-Neptunes suggests a huge compositional diversity and very different formation scenarios. HATS-8b is close enough to its host star that its dayside is heated to roughly 1,300 K. The orbit of HATS-8b around its host star is probably the limit for super-Neptunes. If it were any closer to its host star, evaporation would have reduced it to a super-Earth class planet.

Light curve showing the transit of HATS-8b in front of its host star. The transit depth indicates that HATS-8b has 0.873 times the radius of Jupiter. Bayliss et al. (2015).

Radial velocity curve showing the perturbation HATS-8b exerts on its host star. The amplitude of the radial velocity curve indicates that HATS-8b has 0.138 times the mass of Jupiter. Bayliss et al. (2015).

Reference:
Bayliss et al. (2015), “HATS-8b: A Low-Density Transiting Super-Neptune”, arXiv:1506.01334 [astro-ph.EP]