The hot Neptune WASP-166 b with ESPRESSO III: A blue-shifted tentative water signal constrains the presence of clouds

TL;DR

This study uses high-resolution ESPRESSO spectroscopy to investigate water presence in the atmosphere of the Neptune-sized exoplanet WASP-166 b, revealing a tentative blue-shifted water signal and insights into cloud coverage, contributing to understanding planets in the Neptune desert.

Contribution

First detection attempt of water in a Neptune desert exoplanet's atmosphere using high-resolution visible spectroscopy with ESPRESSO, highlighting the method's potential for small exoplanets.

Findings

Tentative blue-shifted water signal detected

Low water abundance or high clouds inferred

High significance detection possible for cloud-free atmospheres

Abstract

With high-resolution spectroscopy we can study exoplanet atmospheres and learn about their chemical composition, temperature profiles, and presence of clouds and winds, mainly in hot, giant planets. State-of-the-art instrumentation is pushing these studies towards smaller exoplanets. Of special interest are the few planets in the 'Neptune desert', a lack of Neptune-size planets in close orbits around their hosts. Here, we assess the presence of water in one such planet, the bloated super-Neptune WASP-166 b, which orbits an F9-type star in a short orbit of 5.4 days. Despite its close-in orbit, WASP-166 b preserved its atmosphere, making it a benchmark target for exoplanet atmosphere studies in the desert. We analyse two transits observed in the visible with ESPRESSO. We clean the spectra from the Earth's telluric absorption via principal component analysis, which is crucial to the search for water in exoplanets. We use a cross-correlation-to-likelihood mapping to simultaneously estimate limits on the abundance of water and the altitude of a cloud layer, which points towards a low water abundance and/or high clouds. We tentatively detect a water signal blue-shifted ~5 km s-1 from the planetary rest frame. Injection and retrieval of model spectra show that a solar-composition, cloud-free atmosphere would be detected at high significance. This is only possible in the visible due to the capabilities of ESPRESSO and the collecting power of the VLT. This work provides further insight on the Neptune desert planet WASP-166 b, which will be observed with JWST.