A combined transmission spectrum of the Earth-sized exoplanets TRAPPIST-1 b and c

TL;DR

This study presents a combined transmission spectrum of TRAPPIST-1 b and c, ruling out hydrogen-dominated atmospheres and suggesting denser atmospheres are more likely, thus providing insights into their atmospheric composition.

Contribution

First combined transmission spectrum of TRAPPIST-1 b and c, constraining their atmospheric compositions and ruling out hydrogen-dominated atmospheres.

Findings

Hydrogen-dominated atmospheres are unlikely for both planets.

The spectrum is featureless, indicating possible dense atmospheres.

Cloud-free hydrogen atmospheres are ruled out at 10-sigma level.

Abstract

Three Earth-sized exoplanets were recently discovered close to the habitable zone of the nearby ultracool dwarf star TRAPPIST-1. The nature of these planets has yet to be determined, since their masses remain unmeasured and no observational constraint is available for the planetary population surrounding ultracool dwarfs, of which the TRAPPIST-1 planets are the first transiting example. Theoretical predictions span the entire atmospheric range from depleted to extended hydrogen-dominated atmospheres. Here, we report a space-based measurement of the combined transmission spectrum of the two inner planets made possible by a favorable alignment resulting in their simultaneous transits on 04 May 2016. The lack of features in the combined spectrum rules out cloud-free hydrogen-dominated atmospheres for each planet at 10-$\sigma$ levels; TRAPPIST-1 b and c are hence unlikely to harbor an extended gas envelope as they lie in a region of parameter space where high-altitude cloud/haze formation is not expected to be significant for hydrogen-dominated atmospheres. Many denser atmospheres remain consistent with the featureless transmission spectrum---from a cloud-free water vapour atmosphere to a Venus-like atmosphere.