Trappist-1h transmission spectrum: knowing the star

TRAPPIST-1 is an ultracool dwarf star hosting seven rocky planets, all accessible for atmospheric characterization by transit spectroscopy with the upcoming James Webb Space Telescope. However, those measurements can be impacted by heterogeneities in the host star photosphere, as any spectral difference between the transited chord and the rest of the stellar disk can result in signals of stellar origin able to mimic or hide those of planetary ones . Although it makes the study of the planets atmospheres more challenging, it represents a unique opportunity to learn about the photospheric structure of the star. We use new HST/WFC3 infrared observations to put constraints on the outermost planet atmosphere, TRAPPIST-1h, while modeling the stellar photosphere. We show that TRAPPIST-1h is not likely to host an H/He dominated atmosphere and find no evidence of stellar contamination within its transmission spectrum. Finally we explore new avenues to probe time-varying heterogeneities at the surface of TRAPPIST-1, pushing towards a detailed atmospheric characterization of rocky planets around cool stars.