A Near-Infrared Chemical Inventory of the Atmosphere of 55 Cancri e

TL;DR

This study used high-resolution near-infrared spectroscopy during transits of 55 Cancri e to search for atmospheric molecules, placing strong constraints on certain gases and suggesting a high mean molecular weight or clouds if an atmosphere exists.

Contribution

First high-resolution near-infrared spectral analysis of 55 Cancri e's atmosphere using Doppler cross-correlation technique, setting new limits on molecular abundances.

Findings

No atmosphere detected around 55 Cancri e.

Strong constraints on HCN, NH3, and C2H2 abundances.

Atmosphere, if present, likely has high mean molecular weight or clouds.

Abstract

We present high-resolution near-infrared spectra taken during eight transits of 55 Cancri e, a nearby low-density super-Earth with a short orbital period (< 18 hours). While this exoplanet's bulk density indicates a possible atmosphere, one has not been detected definitively. Our analysis relies on the Doppler cross-correlation technique, which takes advantage of the high spectral resolution and broad wavelength coverage of our data, to search for the thousands of absorption features from hydrogen-, carbon-, and nitrogen-rich molecular species in the planetary atmosphere. Although we are unable to detect an atmosphere around 55 Cancri e, we do place strong constraints on the levels of HCN, NH${}_3$, and C${}_2$H${}_2$ that may be present. In particular, at a mean molecular weight of 5 amu we can rule out the presence of HCN in the atmosphere down to a volume mixing ratio (VMR) of 0.02%, NH${}_3$ down to a VMR of 0.08%, and C${}_2$H${}_2$ down to a VMR of 1.0%. If the mean molecular weight is relaxed to 2 amu, we can rule out the presence of HCN, NH${}_3$, and C${}_2$H${}_2$ down to VMRs of 0.001%, 0.0025%, and 0.08% respectively. Our results reduce the parameter space of possible atmospheres consistent with the analysis of HST/WFC3 observations by Tsiaras et al. (2016), and indicate that if 55 Cancri e harbors an atmosphere, it must have a high mean molecular weight and/or clouds.