The GTC exoplanet transit spectroscopy survey. VI. A spectrally-resolved Rayleigh scattering slope in GJ 3470b

TL;DR

This study presents optical transmission spectroscopy of GJ 3470b, revealing a Rayleigh scattering slope indicative of a hydrogen-rich atmosphere, while assessing stellar activity effects to confirm atmospheric composition.

Contribution

First optical transmission spectrum of GJ 3470b in the 435-755 nm range showing Rayleigh scattering, confirming a hydrogen-rich atmosphere after accounting for stellar activity impacts.

Findings

Rayleigh scattering slope detected in optical spectrum

Stellar activity has minimal impact on the observed spectrum

Evidence supports a hydrogen-rich gaseous envelope around GJ 3470b

Abstract

Aims. As a sub-Uranus-mass low-density planet, GJ 3470b has been found to show a flat featureless transmission spectrum in the infrared and a tentative Rayleigh scattering slope in the optical. We conducted an optical transmission spectroscopy project to assess the impacts of stellar activity and to determine whether or not GJ 3470b hosts a hydrogen-rich gas envelop. Methods. We observed three transits with the low-resolution OSIRIS spectrograph at the 10.4 m Gran Telescopio Canarias, and one transit with the high-resolution UVES spectrograph at the 8.2 m Very Large Telescope. Results. From the high-resolution data, we find that the difference of the Ca II H+K lines in- and out-of-transit is only 0.67 +/- 0.22%, and determine a magnetic filling factor of about 10-15%. From the low-resolution data, we present the first optical transmission spectrum in the 435-755 nm band, which shows a slope consistent with Rayleigh scattering. Conclusions. After exploring the potential impacts of stellar activity in our observations, we confirm that Rayleigh scattering in an extended hydrogen/helium atmosphere is currently the best explanation. Further high-precision observations that simultaneously cover optical and infrared bands are required to answer whether or not clouds and hazes exist at high-altitude.