WASP-86b and WASP-102b: super-dense versus bloated planets

Abstract

We report the discovery of two transiting planetary systems: a super dense, sub-Jupiter mass planet WASP-86b (Mpl = 0.82 ± 0.06 MJ; Rpl = 0.63 ± 0.01 RJ), and a bloated, Saturn-like planet WASP-102b (Mpl = 0.62 ± 0.04 MJ; Rpl = 1.27 ± 0.03 RJ). They orbit their host star every ~5.03, and ~2.71 days, respectively. The planet hosting WASP-86 is a F7 star (Teff = 6330±110 K, [Fe/H] = +0.23 ± 0.14 dex, and age ~0.8–1 Gyr); WASP-102 is a G0 star (Teff = 5940±140 K, [Fe/H] = -0.09± 0.19 dex, and age ~1 Gyr). These two systems highlight the diversity of planetary radii over similar masses for giant planets with masses between Saturn and Jupiter. WASP-102b shows a larger than model-predicted radius, indicating that the planet is receiving a strong incident flux which contributes to the inflation of its radius. On the other hand, with a density of ?pl = 3.24± 0.3 ?J, WASP-86b is the densest gas giant planet among planets with masses in the range 0.05 < Mpl < 2.0 MJ. With a stellar mass of 1.34 M? and [Fe/H]= +0.23 dex, WASP-86 could host additional massive and dense planets given that its protoplanetary disc is expected to also have been enriched with heavy elements. In order to match WASP-86b’s density, an extrapolation of theoretical models predicts a planet composition of more than 80% in heavy elements (whether confined in a core or mixed in the envelope). This fraction corresponds to a core mass of approximately 210M? for WASP-86b’s mass of Mpl~260 M?. Only planets with masses larger than about 2 MJ have larger densities than that of WASP-86b, making it exceptional in its mass range.