The GTC exoplanet transit spectroscopy survey

XI. Possible detection of Rayleigh scattering in the atmosphere of the Saturn-mass planet WASP-69b^★

¹ Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
e-mail: fmurgas@iac.es
² Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
³ Key Laboratory of Planetary Sciences, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210023, PR China

Received: 13 April 2020
Accepted: 3 July 2020

Abstract

Context. One of the main atmospheric features in exoplanet atmospheres, detectable both from ground- and space-based facilities, is Rayleigh scattering. In hydrogen-dominated planetary atmospheres, Rayleigh scattering causes the measured planetary radius to increase toward blue wavelengths in the optical range.

Aims. We aim to detect and improve our understanding of several features in the optical range observable in planetary atmospheres. We focus on studying transiting exoplanets that present a wide range of orbital periods, masses, radii, and irradiation from their host star.

Methods. We obtained a spectrophotometric time series of one transit of the Saturn-mass planet WASP-69b using the OSIRIS instrument at the Gran Telescopio Canarias. From the data we constructed 19 spectroscopic transit light curves representing 20 nm wide wavelength bins spanning from 515 to 905 nm. We derived the transit depth for each curve individually by fitting an analytical model together with a Gaussian process to account for systematic noise in the light curves.

Results. We find that the transit depth increases toward bluer wavelengths, indicative of a larger effective planet radius. Our results are consistent with space-based measurements obtained in the near infrared using the Hubble Space Telescope, which show a compatible slope of the transmission spectrum. We discuss the origin of the detected slope and argue between two possible scenarios: a Rayleigh scattering detection originating in the planet’s atmosphere or a stellar activity induced signal from the host star.