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.

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

References in corpus (63)

• SciPy 1.0--Fundamental Algorithms for Scientific Computing in Python
• Astropy: A Community Python Package for Astronomy
• emcee: The MCMC Hammer
• The NumPy array: a structure for efficient numerical computation
• The Transiting Exoplanet Survey Satellite
• Binary companions of evolved stars in APOGEE DR14: Search method and catalog of ~5,000 companions
• MultiNest: an efficient and robust Bayesian inference tool for cosmology and particle physics
• A new extensive library of PHOENIX stellar atmospheres and synthetic spectra
• X-ray spectral modelling of the AGN obscuring region in the CDFS: Bayesian model selection and catalogue
• The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets
• batman: BAsic Transit Model cAlculatioN in Python
• A super-Earth transiting a nearby low-mass star
• The Transiting Exoplanet Survey Satellite: Simulations of planet detections and astrophysical false positives
• Infrared Transmission Spectroscopy of the Exoplanets HD209458b and XO-1b Using the Wide Field Camera-3 on the Hubble Space Telescope
• An asteroseismic view of the radius valley: stripped cores, not born rocky
• ExoMol line lists IV: The rotation-vibration spectrum of methane up to 1500 K
• The TESS Objects of Interest Catalog from the TESS Prime Mission
• The Transit Light Source Effect: False Spectral Features and Incorrect Densities for M-dwarf Transiting Planets
• A variationally computed line list for hot NH3
• How to Distinguish between Cloudy Mini-Neptunes and Water/Volatile-Dominated Super-Earths
• ExoMol line lists III: An improved hot rotation-vibration line list for HCN and HNC
• Water Vapor in the Spectrum of the Extrasolar Planet HD 189733b: 1. the Transit
• A Framework for Characterizing the Atmospheres of Low-Mass Low-Density Transiting Planets
• Compositional diversity in the atmospheres of hot Neptunes, with application to GJ 436b
• A Revised Exoplanet Yield from the Transiting Exoplanet Survey Satellite (TESS)
• Detection of transits of the nearby hot Neptune GJ 436 b
• The rotation and Galactic kinematics of mid M dwarfs in the Solar Neighborhood
• Three Possible Origins for the Gas Layer on GJ 1214b
• A planet within the debris disk around the pre-main-sequence star AU Microscopii
• LDTk: Limb Darkening Toolkit
• Mass-Metallicity Trends in Transiting Exoplanets from Atmospheric Abundances of H${}_2$O, Na, and K
• Detection of H2O and evidence for TiO/VO in an ultra hot exoplanet atmosphere
• Stellar and Planetary Properties of K2 Campaign 1 Candidates and Validation of 17 Planets, Including a Planet Receiving Earth-like Insolation
• A nearby M star with three transiting super-Earths discovered by K2
• Habitability and Biosignatures of Hycean Worlds
• Retrieval of planetary and stellar properties in transmission spectroscopy with Aura
• High Metallicity and Non-equilibrium Chemistry in the Dayside Atmosphere of hot-Neptune GJ 436b
• GENESIS: New Self-Consistent Models of Exoplanetary Spectra
• The California-Kepler Survey. X. The Radius Gap as a Function of Stellar Mass, Metallicity, and Age
• The interior and atmosphere of the habitable-zone exoplanet K2-18b
• H-alpha Activity of Old M Dwarfs: Stellar Cycles and Mean Activity Levels For 93 Low-Mass Stars in the Solar Neighborhood
• A super-Earth and two sub-Neptunes transiting the nearby and quiet M dwarf TOI-270
• 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
• A pair of TESS planets spanning the radius valley around the nearby mid-M dwarf LTT 3780
• A Physical Model-based Correction for Charge Traps in the Hubble Space Telescope's Wide Field Camera 3 Near-IR Detector and Applications to Transiting Exoplanets and Brown Dwarfs
• 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
• The CARMENES search for exoplanets around M dwarfs. Two planets on the opposite sides of the radius gap transiting the nearby M dwarf LTT 3780
• 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
• Bulk composition of the transiting hot Neptune around GJ 436
• 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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