A second visit to Eps Ind Ab with JWST: new photometry confirms ammonia and suggests thick clouds in the exoplanet atmosphere of the closest super-Jupiter

TL;DR

This study uses JWST observations to analyze the atmosphere of the cold exoplanet Eps Ind Ab, revealing ammonia presence and thick water-ice clouds that influence its emission spectrum, challenging existing models.

Contribution

First JWST/MIRI observations of Eps Ind Ab's atmosphere, showing ammonia features and thick clouds, and providing updated orbital and physical parameters.

Findings

Eps Ind Ab is brighter at 11.3um than at 10.6um, indicating ammonia presence.

Ammonia feature is shallower than expected, suggesting thick water-ice clouds.

Cold giant exoplanets are fainter than models predict between 3-5um, supporting the water-ice cloud hypothesis.

Abstract

With JWST, we are directly imaging cold (~200-300K), solar-age giant exoplanets for the first time. At these temperatures many molecular features appear and water-ice clouds may condense and affect the emission spectrum; early photometric measurements of cold giant planets are already showing some tension with the predictions of cloud-free, solar-metallicity atmosphere models. Here we present new JWST/MIRI coronagraphic observations of the cold giant exoplanet Eps Ind Ab at 11.3um. Together with archival data, we use these new observations to study the atmosphere of this cold exoplanet, and we also re-fit its orbit, finding an updated mass of $7.6\pm0.7$ Mj and an eccentricity of $0.24^{+0.11}_{-0.08}$. The planet is significantly brighter (by $0.88\pm0.08$ mag) at 11.3um than at 10.6um, indicating the presence of ammonia. However, this ammonia feature is shallower than expected. This could indicate a low-metallicity or nitrogen-depleted atmosphere, but our preferred explanation is the presence of thick water-ice clouds that suppress the ammonia feature and the near-IR emission of Eps Ind Ab. Photometry of the small but growing sample of cold, giant exoplanets demonstrates that they are consistently fainter than expected between 3 to 5um, consistent with the water-ice cloud hypothesis. 10.6um and 11.3um photometry of this cold exoplanet sample would be valuable to determine whether the suppressed ammonia feature is universal, and to frame a new open question about the underlying physical cause.