Characterising Atmospheres of Cloudy Temperate Mini-Neptunes with JWST

Abstract

The upcoming James Webb Space Telescope (JWST) promises a generational shift in the study of temperate mini-Neptune atmospheres using transit spectroscopy. High-altitude clouds however threaten to impede their atmospheric characterisation by muting spectral features. In this study, we systematically investigate JWST instrument configurations for characterising cloudy mini-Neptune atmospheres, assessing the importance of instrument choice and wavelength coverage, focusing on NIRISS and NIRSpec. We consider two temperate mini-Neptunes orbiting nearby M dwarfs, K2-18 b and TOI-732 c, with equilibrium temperatures below 400 K, as case studies and assess observations using different instrument configurations with one transit per instrument. We find that their JWST transmission spectra with modest observing time and adequate wavelength coverage can provide precise abundance constraints of key molecules H2O, CH4, and NH3 even in the presence of clouds at significantly high altitudes. The best constraints are obtained by combining all three high-resolution NIRSpec gratings (G140H+G235H+G395H) that together span the ~1-5 $\mu$m range. Single-transit observations with this three-instrument configuration allow precise abundance constraints for cloud-top pressures as low as 3 mbar and 0.1 mbar for K2-18 b and TOI-732 c, respectively, assuming a nominal 10x solar metallicity. The constraints vary with instrument combinations. We find that NIRSpec G235H+G395H is the optimal two-instrument configuration, while NIRISS or NIRSpec G235H is optimal for single-instrument observations. Absent high-altitude clouds, even single-instrument observations can provide good abundance constraints for these planets. Our findings underscore the promise of JWST transmission spectroscopy for characterising temperate mini-Neptunes orbiting nearby M dwarfs.

Accepted for publication in MNRAS, 19 pages, 12 figures

References in corpus (32)

• Most 1.6 Earth-Radius Planets are not Rocky
• Atmospheric Retrieval for Super-Earths: Uniquely Constraining the Atmospheric Composition with Transmission Spectroscopy
• Water Vapor in the Spectrum of the Extrasolar Planet HD 189733b: 1. the Transit
• Possible thermochemical disequilibrium in the atmosphere of the exoplanet GJ 436b
• Observing the Atmospheres of Known Temperate Earth-sized Planets with JWST
• Mass-Metallicity Trends in Transiting Exoplanets from Atmospheric Abundances of H${}_2$O, Na, and K
• Observing transiting planets with JWST -- Prime targets and their synthetic spectral observations
• A consistent retrieval analysis of 10 Hot Jupiters observed in transmission
• Habitability and Biosignatures of Hycean Worlds
• H2O abundances in the atmospheres of three hot Jupiters
• The L 98-59 System: Three Transiting, Terrestrial-Sized Planets Orbiting a Nearby M-dwarf
• The interior and atmosphere of the habitable-zone exoplanet K2-18b
• Characterization of the K2-18 multi-planetary system with HARPS: A habitable zone super-Earth and discovery of a second, warm super-Earth on a non-coplanar orbit
• On degeneracies in retrievals of exoplanetary transmission spectra
• Masses and compositions of three small planets orbiting the nearby M dwarf L231-32 (TOI-270) and the M dwarf radius valley
• 1D atmospheric study of the temperate sub-Neptune K2-18b
• How to identify exoplanet surfaces using atmospheric trace species in hydrogen-dominated atmospheres
• Unveiling shrouded oceans on temperate sub-Neptunes via transit signatures of solubility equilibria vs. gas thermochemistry
• Information Content Analysis for Selection of Optimal JWST Observing Modes for Transiting Exoplanet Atmospheres
• Detectable molecular features above hydrocarbon haze via transmission spectroscopy with JWST: Case studies of GJ 1214b-, GJ 436b-, HD 97658b-, and Kepler-51b-like planets
• Seeing above the Clouds with High Resolution Spectroscopy
• On the Temperature Profiles and Emission Spectra of Mini-Neptune Atmospheres
• A planetary system with two transiting mini-Neptunes near the radius valley transition around the bright M dwarf TOI-776
• Prospects for Characterizing the Haziest Sub-Neptune Exoplanets with High Resolution Spectroscopy
• Characterizing Atmospheres of Transiting Earth-like Exoplanets Orbiting M Dwarfs with James Webb Space Telescope
• An Information-Theoretic Approach to Optimize JWST Observations and Retrievals of Transiting Exoplanet Atmospheres
• Information content of JWST-NIRSPEC transmission spectra of warm Neptunes
• The Detectability and Constraints of Biosignature Gases in the Near & Mid-Infrared from Transit Transmission Spectroscopy
• JexoSim: A time domain simulator of exoplanet transit spectroscopy with JWST
• Detecting Biosignatures in the Atmospheres of Gas Dwarf Planets with the James Webb Space Telescope
• JexoSim 2.0: End-to-End JWST Simulator for Exoplanet Spectroscopy -- Implementation and Case Studies
• Photochemistry and Spectral Characterization of Temperate and Gas-Rich Exoplanets

Cited by in corpus (8)

• Hubble Space Telescope transmission spectroscopy for the temperate sub-Neptune TOI-270d: a possible hydrogen-rich atmosphere containing water vapour
• Exoplanet Spectroscopy with JWST NIRISS: Diagnostics and Case Studies
• Hubble WFC3 Spectroscopy of the Terrestrial Planets L~98-59~c \& d: No Evidence for a Clear Hydrogen Dominated Primary Atmosphere
• Hubble WFC3 Spectroscopy of the Rocky Planet L 98-59 b: No Evidence for a Cloud-Free Primordial Atmosphere
• Peering above the clouds of the warm Neptune GJ 436b with CRIRES+
• The effect of spectroscopic binning on atmospheric retrievals
• Exploring the habitability and interior composition of exoplanets lying within the extended habitable zone
• Ground-based Atmospheric Characterization of Super-Earth L 98-59 d at High Spectral Resolution