Hubble Space Telescope Transmission Spectroscopy for the Temperate Sub-Neptune TOI-270 d: A Possible Hydrogen-rich Atmosphere Containing Water Vapor
T. M. Evans, Nikku Madhusudhan, Jason Dittmann, Maximilian N. Günther, Luis Welbanks, Vincent Van Eylen, Ian J. M. Crossfield, Tansu Daylan, Laura Kreidberg
Abstract
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 μ m 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 Ly α 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 4 σ significance relative to a featureless spectrum. The strongest evidence for any individual absorber is obtained for H 2 O, which is favored at 3 σ significance. When retrieving on the WFC3 data alone and allowing for the possibility of a heterogeneous stellar brightness profile, the detection significance of H 2 O is reduced to 2.8 σ . 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.