Peering above the clouds of the warm Neptune GJ 436b with CRIRES+

TL;DR

This study uses high-resolution ground-based spectroscopy to probe the upper atmosphere of the warm Neptune GJ 436b, aiming to detect atmospheric features despite its featureless transmission spectrum, and demonstrates the potential of combining multiple transits for improved constraints.

Contribution

First ground-based high-resolution spectroscopy of GJ 436b's atmosphere, providing constraints on high-altitude clouds and metallicity, and highlighting the benefits of multi-transit data integration.

Findings

No transmission signals detected, but constraints on atmospheric composition were established.

If GJ 436b had a cloud deck above 10 mbar and metallicity below 300× solar, signals should have been detected.

Combining multiple transits could enhance atmospheric characterization capabilities.

Abstract

Exoplanets with masses between Earth and Neptune are amongst the most commonly observed, yet their properties are poorly constrained. Their transmission spectra are often featureless, which indicate either high-altitude clouds or a high atmospheric metallicity. The archetypical warm Neptune GJ 436b is such a planet showing a flat transmission spectrum in observations with the Hubble Space Telescope (HST). Ground-based high-resolution spectroscopy (HRS) effectively probes exoplanet atmospheres at higher altitudes and can therefore be more sensitive to absorption coming from above potential cloud decks. In this paper we aim to investigate this for the exoplanet GJ 436b. We present new CRIRES+ HRS transit data of GJ 436b. Three transits were observed, but since two were during bad weather conditions, only one transit was analyzed. The radiative transfer code petitRADTRANS was used to create atmospheric models for cross-correlation and signal-injection purposes, including absorption from H$_2$O, CH$_4$, and CO. No transmission signals were detected, but atmospheric constraints can be derived. Injection of artificial transmission signals indicate that if GJ 436b would have a cloud deck at pressures P>10 mbar and a <300$\times$ solar metallicity, these CRIRES+ observations should have resulted in a detection. We estimate that the constraints presented here from one ground-based HRS transit are slightly better than those obtained with four HST transits. Combining HRS data from multiple transits is an interesting avenue for future studies of exoplanets with high-altitude clouds.