A JWST/MIRI view of κ Andromedae b: Refining its mass, age, and physical parameters

¹ Aix Marseille Université, CNRS, CNES, LAM, Marseille, France
² Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
³ Department of Physics & Astronomy, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA
⁴ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁵ LIRA, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université, CNRS, 92190 Meudon, France
⁶ Université Paris-Saclay, UVSQ, CNRS, CEA, Maison de la Simulation, 91191 Gif-sur-Yvette, France
⁷ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁸ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France

^★ Corresponding author: nicolas.godoy@lam.fr

Received: 19 March 2025
Accepted: 4 August 2025

Abstract

Context. κ And b is a substellar companion with a mass near the planet–brown dwarf boundary orbiting a B9IV star at ~50–100 au. Estimates of its age and mass vary, which has fueled a decade-long debate. Additionally, the atmospheric parameters (T_eff 1650–2050 K and log(g) 3.5–5.5 dex) remain poorly constrained. The differences in atmospheric models and inhomogeneous datasets contribute to the varied interpretations.

Aims. We aim to refine the characterization of κ And b by using mid-infrared data to capture its full bolometric emission. Combined with near-infrared (NIR) measurements, we aim to constrain T_eff, log(g), and the radius to narrow down the uncertainties in age and mass.

Methods. We obtained JWST/MIRI coronagraphic data in the F1065C, F1140C, and F1550C filters and recalibrated existing NIR photometry using an updated ATLAS stellar model. We used MIRI color–magnitude diagrams to probe the likelihood of species (e.g., CH₄, NH₃, and silicates). We compared the H and F1140C colors and magnitudes of the companion to isochrones to constrain the age and mass. We then modeled its spectral energy distribution with atmospheric models to refine the estimates of T_eff, radius, and log(g) and to constrain age and mass using evolutionary models.

Results. Cloudy atmosphere models fit the spectral energy distribution of κ And b best. This is consistent with its L0/L2 spectral type and its position near silicate-atmosphere field objects in the MIRI color–magnitude diagram. We derived an age of 47 ± 7 Myr and a mass of 17.3 ± 1.8 M_Jup by weight-mean combining the models. Atmospheric modeling yielded T_eff = 1791 ± 68 K and a radius of 1.42 ± 0.06 R_Jup. This improves the precision by ~30% over previous estimates. log(g) was constrained to 4.35 ± 0.07 dex, which is an improvement in the precision by ~70% relative to the most precise literature value of 4.75 ± 0.25 dex.

Conclusions. Our new mass estimate places κ And b slightly above the planet–brown dwarf boundary determined by the deuterium-burning limit. Our age estimate is ~75% more precise than previous values and aligns the object with the Columba association (42 Myr). The derived T_eff suggests silicate clouds, but this needs to be confirmed spectroscopically. MIRI data were crucial to refine the radius and temperature, which led to stronger constraints on the age and mass (both dependent on the model) and improved the overall characterization of κ And b.