TRAPPIST-1 h NIR spectrum

Finally, using a Bayesian atmospheric retrieval code we put new constraints on the atmosphere composition of TRAPPIST-1h. According to the retrieval analysis, there is no evidence of molecular absorption in the Near-InfraRed (NIR) spectrum. This suggests the presence of a high cloud deck or a layer of photochemical hazes in a primary atmosphere or a secondary atmosphere dominated by heavy species like nitrogen.

This result could even be the consequence of the lack of an atmosphere as the spectrum is better fitted using a flat-line. Variations of transit depth around 1.3um are likely due to remaining scattering noise and results are not improved while changing the spectral resolution. TRAPPIST-1 h has probably lost its atmosphere or possesses a layer of clouds and hazes blocking the NIR signal.

We can not distinguish yet between a primary cloudy or a secondary clear envelope using HST/WFC3 data but we can reject, in most cases with more than 3{sigma} confidence, the hypothesis of a clear atmosphere dominated by hydrogen and helium. By testing forced secondary atmospheric scenario, we find that a CO-rich atmosphere (i.e with a volume mixing ratio of 0.2) is one of the best fit to the spectrum with a Bayes Factor of 1.01 corresponding to a 2.1{sigma} detection.