Characterising the atmosphere of 55 Cancri e

1D forward model grid for current and future JWST observations

¹ SRON Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands
² Leiden Observatory, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands
³ Center for Astrophysics, Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
⁴ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
⁵ Department of Astronomy, Stockholm University, AlbaNova University Center, 10691 Stockholm, Sweden
⁶ Department of Earth Sciences, University of California, Riverside, CA 92521, USA
⁷ Department of Physics and Astronomy, University of New Mexico, 210 Yale Boulevard, Albuquerque, NM 87131, USA
⁸ Department of Astronomy and Astrophysics, University of Chicago, Chicago, IL, USA

^★ Corresponding author: mantas.zilinskas@jpl.nasa.gov

Received: 7 February 2025
Accepted: 24 March 2025

Abstract

Recent JWST observations with NIRCam and MIRI of the ultra-short-period super-Earth 55 Cancri e indicate a possible volatile atmosphere surrounding the planet. Previous analysis of the NIRCam spectra suggested potential absorption features from CO₂ or CO and significant sub-weekly variability. The MIRI low-resolution spectrum does not contain substantial features but was found to be consistent with effective heat redistribution models. For this study, we computed a grid of over 25 000 self-consistent 1D forward models incorporating H–N–O–C–S–P–Si–Ti equilibrium chemistry and assessed plausible atmospheric compositions based on the current JWST data. Despite exhaustive analysis, the composition and properties of the atmosphere remain elusive. While our results statistically favour a global, hydrogen-free, nitrogen-dominated atmosphere enriched in PO and CO₂, various alternative compositions, including H₂O–, CO–, PH₃–, or Si-bearing, remain viable explanations. Unconstrained heat redistribution efficiency and absolute NIRCam flux are among the largest sources of uncertainty in our analysis. We also find that the heat redistribution factor and surface pressure are highly degenerate with atmospheric composition, and that these parameters cannot be independently constrained using current JWST observations. Furthermore, we show that the observed variability may arise from dynamic interactions between the atmosphere and an underlying magma ocean, driving rapid shifts in atmospheric chemistry and thermal emission. Our results highlight the importance of using self-consistent forward models when analysing novel JWST spectra with limited signal-to-noise ratios – such as those of 55 Cancri e – as it allows for a more comprehensive evaluation of potential atmospheric scenarios while also being less sensitive to subtle spectral differences than retrievals. Future JWST observations, particularly at longer wavelengths with MIRI imaging mode to obtain broadband photometry, could help mitigate compositional degeneracies and provide further insight into variability. Constraining the heat redistribution value through phase curve measurements would also significantly reduce degeneracies. For a more complete characterisation of this iconic super-Earth, high-precision spectra are essential.