WASP-20b and WASP-28b : a hot Saturn and a hot Jupiter in near-aligned orbits around solar-type stars

Abstract

We report the discovery of the planets WASP-20b and WASP-28b along with measurements of their sky-projected orbital obliquities. WASP-20b is an inflated, Saturn-mass planet (0.31M; 1.46R) in a 4.9-day, near-aligned (λ = 12.7 ± 4.2) orbit around CD-24 102 (V = 10.7; F9). Due to the low density of the planet and the apparent brightness of the host star, WASP-20 is a good target for atmospheric characterisation via transmission spectroscopy. WASP-28b is an inflated, Jupiter-mass planet (0.91M; 1.21R) in a 3.4-day, near-aligned (λ = 8 ± 18) orbit around a V = 12, F8 star. As intermediate-mass planets in short orbits around aged, cool stars (7 Gyr and 6000 ± 100 K for WASP-20; 5 Gyr and 6100 ± 150 K for WASP-28), their orbital alignment is consistent with the hypothesis that close-in giant planets are scattered into eccentric orbits with random alignments, which are then circularised and aligned with their stars' spins via tidal dissipation.