HST Transmission Spectra of the Hot-Neptune HD 219666 b: Detection of Water and the Challenge of Constraining Both Water and Methane
arXiv:2503.03895 · doi:10.3847/1538-3881/adc684
Abstract
Although Neptunian-sized (2 - 5 R) planets appear to be extremely common in the Galaxy, many mysteries remain about their overall nature. To date, only 11 Neptunian-sized planets have had their atmospheres spectroscopically characterized, and these observations hint at interesting diversity within this class of planets. Much of our understanding of these worlds and others derive from transmission spectroscopy with the Hubble Space Telescope's Wide Field Camera 3 (HST/WFC3). One key outcome of HST/WFC3 observations has been the consistent detection of water but no methane in Neptunian atmospheres, though recent James Webb Space Telescope (JWST) observations are potentially starting to overturn this "missing methane" paradigm. In this work, we present the transmission spectrum of the hot Neptune HD 219666 b from 1.1 - 1.6 m from two transit observations using HST/WFC3 G141. Our fiducial atmospheric retrieval detects water at ~3- in HD 219666 b's atmosphere and prefers no contribution from methane, similar to these previous observations of other planets. Motivated by recent detections of methane in Neptunian atmospheres by JWST, we explore additional models and find that a methane-only scenario could adequately fit the data, though it is not preferred and likely unphysical. We discuss the impact of this methane detection challenge on our understanding of planetary atmospheres based on HST/WFC3 observations alone, and where JWST observations offer a solution.
This work has been published in the Astronomical Journal. 25 pages including references. 5 figures and 3 tables in Main Text, 5 figures in Appendix
References in corpus (44)
- Array Programming with NumPy
- The Astropy Project: Sustaining and Growing a Community-oriented Open-source Project and the Latest Major Release (v5.0) of the Core Package
- EXOFAST: A fast exoplanetary fitting suite in IDL
- Rayleigh scattering in the transit spectrum of HD 189733b
- Water Vapor and Clouds on the Habitable-Zone Sub-Neptune Exoplanet K2-18b
- Water Vapour Absorption in the Clear Atmosphere of an exo-Neptune
- Water vapour in the atmosphere of the habitable-zone eight Earth-mass planet K2-18 b
- A Sub-Neptune Exoplanet with a Low-Metallicity Methane-Depleted Atmosphere and Mie-Scattering Clouds
- Mass-Metallicity Trends in Transiting Exoplanets from Atmospheric Abundances of HO, Na, and K
- Hubble Space Telescope Near-IR Transmission Spectroscopy of the Super-Earth HD 97658b
- A reflective, metal-rich atmosphere for GJ 1214b from its JWST phase curve
- The Transit Light Source Effect II: The Impact of Stellar Heterogeneity on Transmission Spectra of Planets Orbiting Broadly Sun-like Stars
- Trends in Atmospheric Properties of Neptune-Size Exoplanets
- K-H_2 Quasi-molecular absorption detected in the T-dwarf epsilon Indi Ba
- Variability in the Atmosphere of the Hot Giant Planet HAT-P-7 b
- A High Internal Heat Flux and Large Core in a Warm Neptune Exoplanet
- On degeneracies in retrievals of exoplanetary transmission spectra
- Statistical Analysis of Hubble/WFC3 Transit Spectroscopy of Extrasolar Planets
- Empirical limb-darkening coefficients & transit parameters of known exoplanets from TESS
- An Ultra-Hot Neptune in the Neptune desert
- 1D atmospheric study of the temperate sub-Neptune K2-18b
- Coupled Evolution with Tides of the Radius and Orbit of Transiting Giant Planets: General Results
- Model Atmospheres From Very Low Mass Stars to Brown Dwarfs
- ExoClock Project III: 450 new exoplanet ephemerides from ground and space observations
- Near-infrared transmission spectrum of the warm-uranus GJ 3470b with the Wide Field Camera-3 on the Hubble Space Telescope
- Aurora: A Generalised Retrieval Framework for Exoplanetary Transmission Spectra
- Updated Parameters and a New Transmission Spectrum of HD 97658b
- Clouds and Clarity: Revisiting Atmospheric Feature Trends in Neptune-size Exoplanets
- A Hubble PanCET Study of HAT-P-11b: A Cloudy Neptune with a Low Atmospheric Metallicity
- Hubble Space Telescope transmission spectroscopy for the temperate sub-Neptune TOI-270d: a possible hydrogen-rich atmosphere containing water vapour
- Progress in Modeling Very Low Mass Stars, Brown Dwarfs, and Planetary Mass Objects
- On the Application of Bayesian Leave-One-Out Cross-Validation to Exoplanet Atmospheric Analysis
- Variability in the Atmosphere of the Hot Jupiter Kepler-76b
- Transmission spectroscopy for the warm sub-Neptune HD3167c: evidence for molecular absorption and a possible high metallicity atmosphere
- Methane as a dominant absorber in the habitable-zone sub-Neptune K2-18 b
- Phase Curves of Hot Neptune LTT 9779b Suggest a High-Metallicity Atmosphere
- Is the atmosphere of the ultra-hot Jupiter WASP-121b variable?
- First semi-empirical test of the white dwarf mass-radius relationship using a single white dwarf via astrometric microlensing
- Water absorption in the transmission spectrum of the water-world candidate GJ9827d
- Spitzer Reveals Evidence of Molecular Absorption in the Atmosphere of the Hot Neptune LTT 9979b
- Tentative Evidence for Water Vapor in the Atmosphere of the Neptune-Size Exoplanet HD 106315 c
- Reassessing the Evidence for Time Variability in the Atmosphere of the Exoplanet HAT-P-7 b
- Characterization of a set of small planets with TESS and CHEOPS and an analysis of photometric performance
- Taranga DR1: Analysis of TESS Short Cadence data for years 1 and 2