Figure 1: Artist's impression of an Earth-sized planet.
TRAPPIST-1b and TRAPPIST-1c are two Earth-sized planets in orbit around a low-mass red dwarf star. The equilibrium temperatures of TRAPPIST-1b and TRAPPIST-1c are estimated to be ~366 K and ~315 K, respectively. On 4 May 2016, both TRAPPIST-1b and TRAPPIST-1c simultaneously transited their host star. This rare transit event was observed with the Hubble Space Telescope (HST). The simultaneous transits allowed the combined transmission spectrum of the two planets to be measured. The lack of features in the observed combined transmission spectrum indicates that TRAPPIST-1b and TRAPPIST-1c both do not have cloud-free hydrogen-dominated atmospheres.
If the planets have hydrogen-dominates atmospheres, they must contain clouds or hazes at pressures less than ~10 mbar to account for the featureless transmission spectrum. However, theoretical predictions for hydrogen-dominated atmospheres at the irradiation levels of TRAPPIST-1b and TRAPPIST-1c indicate that cloud formation only occurs at pressures greater than 100 mbar. At such pressures, the clouds are too deep in the atmosphere to noticeably affect the transmission spectrum. As a result, TRAPPIST-1b and TRAPPIST-1c are unlikely to have hydrogen-dominated atmospheres. Nevertheless, many denser types of atmospheres (e.g. cloud-free water-vapour atmosphere or Venus-like atmosphere with high-altitude hazes) remain consistent with the observed featureless transmission spectrum. Future observations will be needed to distinguish between such atmospheres.
Figure 2: Light curve showing the simultaneous transits of TRAPPIST-1b and TRAPPIST-1c. Julien de Wit et al. (2016)
Figure 3: Transmission spectra of TRAPPIST-1b and TRAPPIST-1c compared to models. The observed transmission spectrum from HST measurements is indicated by black circles with 1σ error bars. Julien de Wit et al. (2016)
Julien de Wit et al. (2016), "A combined transmission spectrum of the Earth-sized exoplanets TRAPPIST-1 b and c", arXiv:1606.01103 [astro-ph.EP]