JWST-TST High Contrast: Medium-resolution spectroscopy reveals a carbon-rich circumplanetary disk around the young accreting exoplanet Delorme 1 AB b

¹ Department of Physics & Astronomy, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA
² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
³ Department of Astronomy, Smith College, Northampton, MA 01063, USA
⁴ Earth and Planets Laboratory, Carnegie Institution for Science, 5241 Broad Branch Road, NW, Washington, DC 20015, USA
⁵ Astrophysics & Space Institute, Schmidt Sciences, New York, NY 10011, USA
⁶ Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 91405 Orsay, France
⁷ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
⁸ Gemini Observatory/NSF NOIRLab, 670 N. A’ohoku Pl., Hilo, HI 96720, USA
⁹ European Space Agency (ESA), European Space Astronomy Centre (ESAC), Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
¹⁰ European Space Agency (ESA), ESA Office, Space Telescope Science Institute, 3700 San Martin Dr, Baltimore, MD 21218, USA
¹¹ Division of Physics and Astronomy, Alfred University, 1 Saxon Drive, Alfred, NY 14802, USA
¹² Department of Astronomy and Carl Sagan Institute, Cornell University, 122 Sciences Drive, Ithaca, NY 14853, USA
¹³ Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
¹⁴ Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
¹⁵ Department of Aeronautics and Astronautics, MIT, 77 Massachusetts Avenue, Cambridge, MA 02139, USA

^★ Corresponding author: mmalin@stsci.edu

Received: 8 August 2025
Accepted: 7 October 2025

Abstract

Context. Young accreting planetary-mass objects are thought to draw material from a circumplanetary disk composed of gas and dust. While the gas within the disk is expected to disperse within the first million years, strong accretion has nonetheless been detected in older systems, including the 30-45 Myr-old planetary-mass companion Delorme 1 AB b.

Aims. We conducted spectroscopic observations with the James Webb Space Telescope’s Mid-Infrared Instrument (JWST/MIRI) to investigate the presence of circumplanetary material around this young accreting planet and to characterize the planet’s atmospheric properties and composition.

Methods. We performed forward modeling using atmospheric models to characterize the planet’s atmosphere, combining our MIRI observations with archival ground-based near-infrared data. We used slab models to analyze the circumplanetary gas and investigated H₂ emission.

Results. We derived the atmospheric parameters of Delorme 1 AB b, finding an effective temperature of T_eff = 1725 ± 134 K. To achieve a satisfactory fit to the observed spectrum, a secondary component is required, consistent with dust emission from a circumplanetary disk (CPD), characterized by a blackbody temperature of T_bb = 295 ± 27 K and an effective radius of R_bb = 18.8 ± 2.7 R_Jup. Beyond 10 μm, the spectral energy distribution (SED) becomes dominated by this circumplanetary disk rather than the planet itself. We detected strong emission from HCN and C₂H₂, along with tentative evidence of the isotopologue ¹³CCH₂, while no O-bearing species such as CO, CO₂, or H₂O are observed in the CPD spectrum. This suggests that the gas in the CPD has an elevated C/O. We also identified spatially extended H₂ emission around the planet, tracing warm gas, with indications that it may be at a higher temperature than the non-extended component.

Conclusions. The mid-infrared spectrum of the planetary-mass companion Delorme 1 AB b reveals the first detection of bright C-bearing species in a CPD together with an outflow traced by H₂ extended emission, which could be interpreted as a disk wind. The hot dust continuum emission suggests an inner cavity in the CPD. The presence of warm gas in the CPD provides constraints on the disk’s chemical composition and physical conditions, opening up new avenues for disk studies. The study of these long-lived “Peter Pan” disks will enhance our understanding of how accretion persists in evolved low-mass systems and shed light on their formation, longevity, and evolutionary pathways in planetary systems.