JWST/MIRI observations of the young TWA 27 system: Hydrocarbon disk chemistry, silicate clouds, and evidence of a circumplanetary disk

¹ ETH Zürich, Institute for Particle Physics and Astrophysics, Wolfgang-Pauli-Str. 27, 8093 Zürich, Switzerland
² Centro de Astrobiología (CAB), CSIC-INTA, ESAC Campus, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
³ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
⁴ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁵ Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA
⁶ Department of Astrophysics, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland
⁷ Max-Planck-Institut für Astronomie (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
⁸ Department of Physics & Astronomy, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA
⁹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁰ Department of Astrophysics, American Museum of Natural History, New York, NY 10024, USA
¹¹ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
¹² Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
¹³ HFML - FELIX, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
¹⁴ SRON Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands
¹⁵ Department of Astrophysics, University of Vienna, Türkenschanzstr. 17, 1180 Vienna, Austria
¹⁶ STAR Institute, Université de Liège, Allée du Six Août 19c, 4000 Liège, Belgium
¹⁷ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
¹⁸ LERMA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Paris, France
¹⁹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
²⁰ UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
²¹ Earth and Planets Laboratory, Carnegie Institution for Science, 5241 Broad Branch Road, NW, Washington, DC 20015, USA
²² Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
²³ Department of Astronomy, Stockholm University, AlbaNova University Center, 106 91 Stockholm, Sweden
²⁴ School of Physics & Astronomy, Space Park Leicester, University of Leicester, 92 Corporation Road, Leicester, LE4 5SP, UK
²⁵ School of Physics and Astronomy, Sun Yat-sen University, Zhuhai 519082, PR China
²⁶ Université Paris-Saclay, UVSQ, CNRS, CEA, Maison de la Simulation, 91191 Gif-sur-Yvette, France
²⁷ Centro de Astrobiología (CAB, CSIC-INTA), Carretera de Ajalvir, 8850 Torrejón de Ardoz, Madrid, Spain
²⁸ School of Cosmic Physics, Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin, D02 XF86, Ireland
²⁹ Department of Astronomy, Oskar Klein Centre, Stockholm University, 106 91 Stockholm, Sweden

^★ Corresponding authors: polychronis.patapis@phys.ethz.ch; gabriele.cugno@uzh.ch

Received: 7 July 2025
Accepted: 24 September 2025

Abstract

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.