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

Abstract With JWST, we are directly imaging cold (∼200–300 K) 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 11.3 μ m. Together with archival data, we use these new observations to study the atmosphere of this cold exoplanet, and we also refit its orbit, finding an updated mass of 7.6 ± 0.7 M Jup and an eccentricity of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>0.2</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.08</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.11</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> . The planet is significantly brighter (by 0.88 ± 0.08 mag) at 11.3 μ m than at 10.6 μ m, 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 and 5 μ m, consistent with the water-ice cloud hypothesis. 10.6 μ m and 11.3 μ m 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.