The discoveries of WASP-91b, WASP-105b and WASP-107b: Two warm Jupiters and a planet in the transition region between ice giants and gas giants^⋆,⋆⋆

¹ Astrophysics Group, Keele University, Staffordshire ST5 5BG, UK
e-mail: d.r.anderson@keele.ac.uk
² SUPA, School of Physics and Astronomy, University of St. Andrews, North Haugh, Fife KY16 9SS, UK
³ Institut d’Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août, 17, Bât. B5C, Liège 1, Belgium
⁴ Cavendish Laboratory, J J Thomson Avenue, Cambridge CB3 0HE, UK
⁵ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
⁶ Observatoire de Genève, Université de Genève, 51 chemin des Maillettes, 1290 Sauverny, Switzerland
⁷ N. Copernicus Astronomical Centre, Polish Academy of Sciences, Bartycka 18, 00-716 Warsaw, Poland
⁸ Institute of Planetary Research, German Aerospace Center, Rutherfordstrasse 2, 12489 Berlin, Germany
⁹ Centre for Planetary Sciences, University of Toronto at Scarborough, 1265 Military Trail, Toronto, ON M1C 1A4, Canada
¹⁰ Department of Astronomy & Astrophysics, University of Toronto, Toronto, ON M5S 3H4, Canada
¹¹ Institute of Astronomy, Madingley Road, CB3 0HA, UK

Received: 14 January 2017
Accepted: 11 February 2017

Abstract

We report the discoveries of three transiting exoplanets. WASP-91b is a warm Jupiter (1.34 M_Jup, 1.03 R_Jup) in a 2.8-day orbit around a metal-rich K3 star. WASP-105b is a warm Jupiter (1.8 M_Jup, 0.96 R_Jup) in a 7.9-day orbit around a metal-rich K2 star. WASP-107b is a warm super-Neptune/sub-Saturn (0.12 M_Jup, 0.94 R_Jup) in a 5.7-day orbit around a solar-metallicity K6 star. Considering that giant planets seem to be more common around stars of higher metallicity and stars of higher mass, it is notable that the hosts are all metal-rich, late-type stars. With orbital separations that place both WASP-105b and WASP-107b in the weak-tide regime, measurements of the alignment between the planets’ orbital axes and their stars’ spin axes may help us to understand the inward migration of short-period, giant planets. The mass of WASP-107b (2.2 M_Nep, 0.40 M_Sat) places it in the transition region between the ice giants and gas giants of the Solar System. Its radius of 0.94 R_Jup suggests that it is a low-mass gas giant with a H/He-dominated composition. The planet thus sets a lower limit of 2.2 M_Nep on the planetary mass above which large gaseous envelopes can be accreted and retained by proto-planets on their way to becoming gas giants. We may discover whether WASP-107b more closely resembles an ice giant or a gas giant by measuring its atmospheric metallicity via transmission spectroscopy, for which WASP-107b is a very good target.