HST/WFC3 transmission spectroscopy of the cold rocky planet TRAPPIST-1h

TL;DR

This study used Hubble's WFC3 to observe TRAPPIST-1h, aiming to characterize its atmosphere, and found it likely has a high molecular weight atmosphere, no atmosphere, or an opaque layer, with current data limitations.

Contribution

First transmission spectroscopy of TRAPPIST-1h with Hubble, modeling stellar contamination and constraining its atmospheric properties despite data limitations.

Findings

TRAPPIST-1h unlikely to have an aerosol-free H/He atmosphere

Data suggests a flat, featureless spectrum due to high molecular weight atmosphere or no atmosphere

Stellar contamination modeling presents significant challenges

Abstract

TRAPPIST-1 is a nearby ultra-cool dwarf star transited by seven rocky planets. We observed three transits of its outermost planet, TRAPPIST-1h, using the G141 grism of the Wide Field Camera 3 instrument aboard the Hubble Space Telescope to place constraints on its potentially cold atmosphere. In order to deal with the effect of stellar contamination, we model TRAPPIST-1 active regions as portions of a cooler and a hotter photosphere, and generate multi-temperature models that we compare to the out-of-transit spectrum of the star. Using the inferred spot parameters, we produce corrected transmission spectra for planet h under five transit configurations and compare these data to planetary atmospheric transmission models using the forward model CHIMERA. Our analysis reveals that TRAPPIST-1h is unlikely to host an aerosol-free H/He-dominated atmosphere. While the current data precision limits the constraints we can put on the planetary atmosphere, we find that the likeliest scenario is that of a flat, featureless transmission spectrum in the WFC3/G141 bandpass due to a high mean molecular weight atmosphere (>1000x solar), no atmosphere, or an opaque aerosol layer, all in absence of stellar contamination. This work outlines the limitations of modeling active photospheric regions with theoretical stellar spectra, and those brought by our lack of knowledge of the photospheric structure of ultracool dwarf stars. Further characterization of the planetary atmosphere of TRAPPIST-1h would require higher precision measurements over wider wavelengths, which will be possible with the James Webb Space Telescope.