Extreme Weather Variability on Hot Rocky Exoplanet 55 Cancri e Explained by Magma Temperature-Cloud Feedback

TL;DR

This paper proposes a magma temperature-cloud feedback mechanism to explain the observed brightness variability of the hot rocky exoplanet 55 Cancri e, linking surface magma heating, cloud formation, and oscillations in thermal emission.

Contribution

It introduces a novel magma-cloud feedback model that accounts for brightness variability and phase shifts observed in 55 Cancri e, supported by simple modeling and recent JWST data.

Findings

Brightness variability can be reproduced by the model.

Brightness at different wavelengths can oscillate out of phase.

Cloud cover variations affect phase curve amplitude and phase offset.

Abstract

Observations of the hot rocky exoplanet 55 Cancri e report significant but unexplained variability in brightness across visible and infrared bands, e.g., on sub-weekly timescales, its mid-infrared brightness temperature fluctuates by approximately 1400 K (with hundreds of Kelvin uncertainty). We propose a magma temperature-cloud feedback as a potential explanation that relies on the planet's atmosphere and surface. In this feedback, under cloud-free conditions, stellar radiation heats surface magma, releasing silicate vapor that condenses into clouds. Once formed, these clouds attenuate stellar insolation, thereby cooling the surface, reducing vapor supply, and decreasing cloudiness. A time lag between surface temperature increase and cloud formation, likely due to lagged atmospheric transport of cloud-forming vapor, enables self-sustained oscillations in surface temperature and cloudiness. These oscillations manifest as variations in both the planet's thermal emission and reflected starlight, causing variability in secondary eclipse depths across wavelengths without significantly affecting the transit depth. Using a simple model, we find that diverse planetary parameters can reproduce the observed infrared brightness variability. We also demonstrate that brightness at different wavelengths can oscillate out of phase, consistent with recent observations by the James Webb Space Telescope. Additionally, we propose that time-varying and spatially non-uniform cloud cover can result in changing amplitude and phase offset of the planet's phase curve, potentially explaining observations. Finally, we discuss observational strategies to test this proposed mechanism on 55 Cancri e. If confirmed, these observable ocean-atmosphere dynamics on exoplanets would provide valuable insights into the composition, evolution, and long-term fate of rocky planet volatiles.