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 (0.31 $M_{\mathrm{Jup}}$; 1.46 $R_{\mathrm{Jup}}$) in a 4.9-day, near-aligned ($\lambda =8.1\pm 3.6^{\circ }$) orbit around CD-24 102 ($V$=10.7; F9). WASP-28b is an inflated, Jupiter-mass planet (0.91 $M_{\mathrm{Jup}}$; 1.21 $R_{\mathrm{Jup}}$) in a 3.4-day, near-aligned ($\lambda =8\pm 18^{\circ }$) orbit around a $V$=12, F8 star. As intermediate-mass planets in short orbits around aged, cool stars ($7_{-1}^{+2}$ Gyr for WASP-20 and $5_{-2}^{+3}$ Gyr for WASP-28; both with $T_{\mathrm{eff}}$ < 6250 K), 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.

Submitted to A&A. 9 pages, 6 figures, 6 tables. Fixed some typos

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