2025A&A...704A...5P

Context. The Mid-Infrared Instrument (MIRI) on board the James Webb Space Telescope (JWST) enables the characterisation of young self-luminous gas giants in a previously inaccessible wavelength range, revealing insights into physical processes of the gas, dust, and clouds.
Aims. We aim to characterise the young planetary system TWA 27 (2M1207) in the mid-infrared, revealing the atmosphere and disk spectra of the M9 brown dwarf TWA 27A and its L6 planetary-mass companion TWA 27b.
Methods. We obtained data from the MIRI Medium Resolution Spectrometer (MRS) from 4.9 to 20 μm, and MIRI Imaging in the F1000W and F1500W filters. We applied high-contrast imaging data processing methods in order to extract the companion spectral energy distribution up to 15 μm at a separation of 0.″78 and contrast of 60. Using published spectra from JWST/NIRSpec, we analysed the 1-20 μm spectra with self-consistent atmosphere grids, and the molecular disk emission from TWA 27A with 0D slab models.
Results. We find that the atmosphere of TWA 27A is well fitted with the BT-SETTL model of effective temperature T_eff ∼ 2780 K, logg ∼ 4.3, and a blackbody component of ∼740 K for the circumstellar disk inner rim. The disk consists of at least 11 organic molecules, and neither water nor silicate dust emission are detected. The atmosphere of the planet TWA 27b matches with a T_eff ∼ 1400 K low-gravity model when adding extinction, with the ExoREM grid fitting the best. MIRI spectra and photometry for TWA 27b reveal a silicate cloud absorption feature between 8-10 μm, and evidence (>5σ) of infrared excess at 15 μm that is consistent with predictions from circumplanetary disk emission.
Conclusions. The MIRI observations present novel insights into the young planetary system TWA 27, showing a diversity of features that can be studied to understand the formation and evolution of circumplanetary disks and young dusty atmospheres.