Hubble Space Telescope Transmission Spectroscopy for the Temperate Sub-Neptune TOI-270 d: A Possible Hydrogen-rich Atmosphere Containing Water Vapor

Abstract

TOI-270 d is a temperate sub-Neptune discovered by the Transiting Exoplanet Survey Satellite (TESS) around a bright (J = 9.1 mag) M3V host star. With an approximate radius of 2 R ⊕ and equilibrium temperature of 350 K, TOI-270 d is one of the most promising small exoplanets for atmospheric characterization using transit spectroscopy. Here we present a primary transit observation of TOI-270 d made with the Hubble Space Telescope Wide Field Camera 3 (WFC3) spectrograph across the 1.126–1.644 μm wavelength range, and a 95% credible upper limit of 8.2 × 10−14 erg s−1 cm−2 Å−1 arcsec−2 for the stellar Lyα emission obtained using the Space Telescope Imaging Spectrograph. The transmission spectrum derived from the TESS and WFC3 data provides evidence for molecular absorption by a hydrogen-rich atmosphere at 4σ significance relative to a featureless spectrum. The strongest evidence for any individual absorber is obtained for H2O, which is favored at 3σ significance. When retrieving on the WFC3 data alone and allowing for the possibility of a heterogeneous stellar brightness profile, the detection significance of H2O is reduced to 2.8σ. Further observations are therefore required to robustly determine the atmospheric composition of TOI-270 d and assess the impact of stellar heterogeneity. If confirmed, our findings would make TOI-270 d one of the smallest and coolest exoplanets to date with detected atmospheric spectral features.