Worlds Next Door. III. Indirect Evidence for Enhanced Atmospheric Metallicity and/or the Presence of Water Clouds in the Nearest Jupiter-analog $\hat{I}\mu$ Eri b

Abstract

We present the most sensitive direct imaging search for the nearest ($d=3.2$ pc) Jupiter-analog exoplanet, $\hat{I}\mu$ Eri b, with JWST/NIRCam coronagraphy between 4-5 $\hat{I}¼$m (F444W). We achieve a 5$\ddot{I}ƒ$ contrast sensitivity $\approx 3.0\times 10^{-7}$ ($\hat{I}”\approx 16.3$ mag) in the F444W filter at the expected planet separation of $\approx$1". This is the deepest 4-5 $\hat{I}¼$m contrast performance achieved for any JWST/NIRCam observation to date at these separations (and $>10\times$ better than ground-based limits). Yet, the planet remains elusive to imaging. We update the star's age to $1.1\pm 0.1$ Gyr, older than previous age estimates, using the latest gyrochronology relations. This significantly impacts $\hat{I}\mu$ Eri b's inferred effective temperature ($T_{\mathrm{eff}}$), which is now expected to lie between 150-200 K based on evolutionary models for a 1 $M_{\mathrm{Jup}}$ planet. Using cloud-free Sonora Flame Skimmer models and custom PICASO patchy cloud models in the above $T_{\mathrm{eff}}$ range, we find that the F444W non-detection of $\hat{I}\mu$ Eri b can be explained by a metal-enriched atmosphere and/or an atmosphere containing water ice clouds. Both possibilities suggest that $\hat{I}\mu$ Eri b's atmosphere is strikingly similar to that of Jupiter in our Solar System. Alternatively, if we do not enforce the dynamical mass ($0.98\pm 0.09M_{\mathrm{Jup}}$), a solar metallicity, cloud-free, $\lesssim 0.81M_{\mathrm{Jup}}$ planet would be consistent with the NIRCam upper limit based on the Sonora Flame Skimmer evolutionary models. Finally, we place limits on the size of a potential ring system using the NIRCam/F210M data and discuss the opportunity to directly image $\hat{I}\mu$ Eri b with additional JWST observations, the Roman Coronagraph Instrument, the ExtraSolar Coronagraph on the Lazuli Observatory, and EELT/METIS.

Accepted to AJ. 27 pages, 14 figures, 5 tables