Detection of an atmosphere around the super-Earth 55 Cancri e

TL;DR

This study presents spectroscopic evidence suggesting that the super-Earth 55 Cancri e has a hydrogen-rich atmosphere possibly containing HCN, indicating a high C/O ratio and a carbon-rich environment, based on HST observations and Bayesian spectral retrieval.

Contribution

First detection of atmospheric features around 55 Cancri e using HST near-infrared spectroscopy, identifying HCN as a likely atmospheric molecule and suggesting a carbon-rich composition.

Findings

Detection of transit depth modulations at 6σ confidence level.

Identification of HCN as the most probable molecular candidate.

Implication of a high C/O ratio indicating a carbon-rich environment.

Abstract

We report the analysis of two new spectroscopic observations of the super-Earth 55 Cancri e, in the near infrared, obtained with the WFC3 camera onboard the HST. 55 Cancri e orbits so close to its parent star, that temperatures much higher than 2000 K are expected on its surface. Given the brightness of 55 Cancri, the observations were obtained in scanning mode, adopting a very long scanning length and a very high scanning speed. We use our specialized pipeline to take into account systematics introduced by these observational parameters when coupled with the geometrical distortions of the instrument. We measure the transit depth per wavelength channel with an average relative uncertainty of 22 ppm per visit and find modulations that depart from a straight line model with a 6$\sigma$ confidence level. These results suggest that 55 Cancri e is surrounded by an atmosphere, which is probably hydrogen-rich. Our fully Bayesian spectral retrieval code, T-REx, has identified HCN to be the most likely molecular candidate able to explain the features at 1.42 and 1.54 $\mu$m. While additional spectroscopic observations in a broader wavelength range in the infrared will be needed to confirm the HCN detection, we discuss here the implications of such result. Our chemical model, developed with combustion specialists, indicates that relatively high mixing ratios of HCN may be caused by a high C/O ratio. This result suggests this super-Earth is a carbon-rich environment even more exotic than previously thought.