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

¹ Astrobiology Research Unit, Université de Liège, Allée du 6 Août 19C, 4000 Liège, Belgium
e-mail: lgarcia@uliege.be
² Department of Earth and Planetary Sciences, Johns Hopkins University, 3400 N Charles St, Baltimore, MD 21218 USA
³ Bay Area Environmental Research Institute/NASA Ames Research Center, Moffett Field, CA 94035 USA
⁴ Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139 USA
⁵ Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139 USA
⁶ School of Physics, University of Bristol, HH Wills Physics Laboratory, Tyndall Avenue, Bristol BS8 1TL UK
⁷ Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139 USA
⁸ Department of Astronomy and Carl Sagan Institute, Cornell University, 122 Sciences Drive, Ithaca, NY 14853 USA

Received: 8 November 2021
Accepted: 24 February 2022

Abstract

Aims. 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.

Methods. 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.

Results. 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 (≥1000 × 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.