Hubble Space Telescope transmission spectroscopy for the temperate sub-Neptune TOI-270d: a possible hydrogen-rich atmosphere containing water vapour

Abstract

TOI-270d is a temperate sub-Neptune discovered by the Transiting Exoplanet Survey Satellite (TESS) around a bright (J=9.1mag) M3V host star. With an approximate radius of 2RE and equilibrium temperature of 350K, TOI-270d is one of the most promising small exoplanets for atmospheric characterisation using transit spectroscopy. Here we present a primary transit observation of TOI-270d made with the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) spectrograph across the 1.126-1.644 micron wavelength range, and a 95% credible upper limit of $8.2\times 10^{-14}$ erg s${}^{-1}$ cm${}^{-2}$ A${}^{-1}$ arcsec${}^{-2}$ for the stellar Ly-alpha emission obtained using the Space Telescope Imaging Spectrograph (STIS). The transmission spectrum derived from the TESS and WFC3 data provides evidence for molecular absorption by a hydrogen-rich atmosphere at 4-sigma significance relative to a featureless spectrum. The strongest evidence for any individual absorber is obtained for H2O, which is favoured at 3-sigma 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 2.8-sigma. Further observations are therefore required to robustly determine the atmospheric composition of TOI-270d and assess the impact of stellar heterogeneity. If confirmed, our findings would make TOI-270d one of the smallest and coolest exoplanets to date with detected atmospheric spectral features.

Accepted for publication in AAS journals on November 22, 2022 (received July 5, 2022; revised October 30, 2022)

References in corpus (28)

• The NumPy array: a structure for efficient numerical computation
• The Transiting Exoplanet Survey Satellite
• The TESS Objects of Interest Catalog from the TESS Prime Mission
• Water Vapor in the Spectrum of the Extrasolar Planet HD 189733b: 1. the Transit
• A planet within the debris disk around the pre-main-sequence star AU Microscopii
• Mass-Metallicity Trends in Transiting Exoplanets from Atmospheric Abundances of H${}_2$O, Na, and K
• A nearby M star with three transiting super-Earths discovered by K2
• Habitability and Biosignatures of Hycean Worlds
• The California-Kepler Survey. X. The Radius Gap as a Function of Stellar Mass, Metallicity, and Age
• GENESIS: New Self-Consistent Models of Exoplanetary Spectra
• The interior and atmosphere of the habitable-zone exoplanet K2-18b
• New constraints on the planetary system around the young active star AU Mic. Two transiting warm Neptunes near mean-motion resonance
• A super-Earth and a sub-Neptune orbiting the bright, quiet M3 dwarf TOI-1266
• Masses and compositions of three small planets orbiting the nearby M dwarf L231-32 (TOI-270) and the M dwarf radius valley
• An emission spectrum for WASP-121b measured across the 0.8-1.1 micron wavelength range using the Hubble Space Telescope
• Aurora: A Generalised Retrieval Framework for Exoplanetary Transmission Spectra
• Diurnal variations in the stratosphere of the ultrahot giant exoplanet WASP-121b
• Unveiling shrouded oceans on temperate sub-Neptunes via transit signatures of solubility equilibria vs. gas thermochemistry
• How to identify exoplanet surfaces using atmospheric trace species in hydrogen-dominated atmospheres
• Confirmation of water emission in the dayside spectrum of the ultrahot Jupiter WASP-121b
• Stellar surface inhomogeneities as a potential source of the atmospheric signal detected in the K2-18 b transmission spectrum
• A planetary system with two transiting mini-Neptunes near the radius valley transition around the bright M dwarf TOI-776
• Predicting the Exoplanet Yield of the TESS Prime and Extended Missions Through Years 1-7
• Transmission spectroscopy for the warm sub-Neptune HD3167c: evidence for molecular absorption and a possible high metallicity atmosphere
• TOI-1231 b: A Temperate, Neptune-Sized Planet Transiting the Nearby M3 Dwarf NLTT 24399
• Characterising Atmospheres of Cloudy Temperate Mini-Neptunes with JWST
• Transit Timing Variations in the three-planet system: TOI-270
• GJ 3090 b: one of the most favourable mini-Neptune for atmospheric characterisation

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