Characterizing 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 characterization by muting spectral features. In this study, we systematically investigate JWST instrument configurations for characterizing 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 and 0.1 mbar for K2-18 b and TOI-732 c, respectively, assuming a nominal 10× 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 characterizing temperate mini-Neptunes orbiting nearby M dwarfs.

Chemical conditions on Hycean worlds

“…Several recent studies have demonstrated the feasibility of detecting key molecular signatures, including biomarkers, in candidate Hycean worlds using transit spectroscopy with JWST. 17,34,124 Our goal here is to assess (a) the spectral features of relevant chemical species observable with JWST transit spectroscopy, and (b) chemical diagnostics for identifying a Hycean planet.…”

“…We model a transmission spectrum of K2-18 b using an adaptation of the AURA exoplanet atmospheric modeling and retrieval code. 125 We refer the reader to previous works 17,124 for details on the model setup using the AURA code and for the same planet bulk parameters as considered here. The model considers a plane parallel atmosphere at the day-night terminator of the planet in hydrostatic equilibrium and computes the transmission spectrum for a given temperature structure and composition.…”

“…17,28–31 Simulation studies show that such planets are indeed excellent candidates for detections of key volatile species and potential biomarkers in their atmospheres. 17,32–34,124…”

Hubble Space Telescope Transmission Spectroscopy for the Temperate Sub-Neptune TOI-270 d: A Possible Hydrogen-rich Atmosphere Containing Water Vapor

“…We consider an isothermal pressure-temperature (P-T) profile with the temperature as a free parameter. Assuming a non-isothermal profile does not significantly influence the results and is not warranted given the data quality (also see, e.g., Constantinou & Madhusudhan 2022). Our baseline model therefore includes 12 free parameters: volume mixing ratios of the six molecules noted above (X i for i = H 2 O, CH 4 , NH 3 , CO, CO 2 , HCN), the isothermal temperature (T 0 ), the reference pressure (P ref ) corresponding to the planet radius of 2.19 R ⊕ derived from the broadband light curve (Table 1), and four cloud/haze parameters-the haze amplitude and slope (a and γ), cloud-top pressure (P c ), and cloud/haze covering fraction (f).…”

Hubble WFC3 Spectroscopy of the Rocky Planet L 98–59 b: No Evidence for a Cloud-free Primordial Atmosphere

“…Future missions such as the James Webb Space Telescope (JWST) and Ariel (Tinetti et al 2018) will provide higher sensitivity and/or wider spectral coverage than HST/WFC3. This will have a huge impact on the detection of atmospheric features on small exoplanets, even with the presence of clouds at high altitudes (Hinkley et al 2022;Constantinou & Madhusudhan 2022;Whittaker et al 2022). If there is a significant amount of HCN in the atmosphere of L 98-59 b, we should be able to detect a very strong absorption feature near 2.5 µm, which is covered by the wavelength ranges of instruments from both of JWST and Ariel.…”