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

Abstract: Young accreting planetary-mass objects are thought to draw material from a circumplanetary disk (CPD) 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. We conducted spectroscopic observations with JWST/MIRI to investigate the presence of circumplanetary material around this young, accreting planet, and to characterize the planet's atmospheric properties and composition. We perform forward modeling using atmospheric models to characterize the planet's atmosphere. Beyond 10 microns, the SED becomes dominated by this CPD rather than the planet itself. We detect strong emission from HCN and C$_2$H$_2$, while no O-bearing species are observed in the CPD spectrum, suggesting the gas in the CPD has an elevated C/O. We also identify spatially extended H$_2$ emission around the planet, tracing warm gas, with indications that it may be at a higher temperature than the non-extended component. 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 H$_2$ extended emission, that 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, shedding light on their formation, longevity, and evolutionary pathways in planetary systems.

• The paper presents JWST/MIRI medium-resolution spectroscopy that distinguishes the planetary atmosphere from its carbon-rich, oxygen-poor circumplanetary disk.
• It reports the absence of silicate features and evidence of grain growth and a dust cavity, indicating advanced disk evolution.
• The study detects extended H₂ emission indicative of disk winds, quantifying mass-loss rates that challenge traditional disk dispersal models.

Medium-Resolution JWST/MIRI Spectroscopy of Delorme 1 AB b: A Carbon-Rich Circumplanetary Disk and Evidence for Outflows

Introduction

The study presents JWST/MIRI Medium Resolution Spectrometer (MRS) observations of the young, accreting planetary-mass companion Delorme 1 AB b, a 13 $M_\mathrm{Jup}$ object at 84 AU from a close M-dwarf binary. The system is notable for its advanced age (30–45 Myr) relative to typical disk-bearing planetary-mass objects, challenging canonical disk dispersal timescales. The primary goals are to characterize the planet's atmosphere and to provide the first detailed mid-infrared (MIR) spectroscopic analysis of its circumplanetary disk (CPD), with a focus on molecular gas content, dust properties, and evidence for disk winds.

Observations and Data Reduction

JWST/MIRI MRS IFU data were obtained, covering 4.9–27.9 μm at $R_\lambda \sim 3700$. The data reduction pipeline included background subtraction, PSF fitting, and robust uncertainty estimation via aperture statistics. The resulting data cubes enable spatially and spectrally resolved analysis of both the planet and the host binary.

Figure 1: Median of the cubes for band A across all channels after removing the background contribution. The planet Delorme 1 AB b is in the middle of the field of view, while the point source in the upper corner represents the binary Delorme 1 AB. The vertical stripes observed in band 4A are caused by detector artifacts.

Spectral Morphology and Qualitative Features

The extracted spectrum of Delorme 1 AB b is characterized by a flat MIR SED, with a pronounced IR excess at $\lambda > 8~\mu$m, inconsistent with a pure planetary photosphere. The contrast between the planet and the host binary decreases at longer wavelengths, indicating significant circumplanetary emission. The spectrum reveals strong molecular features, including the $Q$-branches of C$_2$H$_2$ and HCN, and multiple H$_2$ emission lines. Notably, no silicate features are detected, implying grain growth and/or dust settling in the CPD.

Figure 2: Top: Extracted spectra of the binary star (gray) and its companion Delorme 1 AB b (color-coded) across the MIRI/MRS bands. The dashed region outlined in red indicates the wavelength range where the planet's atmosphere is detected, while the brown, dotted region marks the wavelengths dominated by the circumplanetary disk. Bottom: S/N as a function of wavelength for the companion.

Planetary Atmosphere Characterization

Atmospheric Model Fitting

The planetary spectrum, combined with archival NIR data, was fit using multiple atmospheric model grids (Exo-REM, ATMO, Sonora, BT-Settl) via the species framework. All models require the addition of a blackbody component to account for the MIR excess, interpreted as CPD dust emission. The best-fit atmospheric parameters are:

• $T_\mathrm{eff} = 1725 \pm 134$ K
• $R = 1.9 \pm 0.1~R_\mathrm{Jup}$
• Super-solar C/O and metallicity (model-dependent)
• Blackbody (CPD): $T_\mathrm{bb} = 295 \pm 27$ K, $R_\mathrm{bb} = 18.8 \pm 2.7~R_\mathrm{Jup}$

  Figure 3: Delorme 1 AB b spectrum and photometric data from the visible to MIR, along with the best-fit Exo-REM atmospheric models. The black curve includes an additional blackbody component to account for the observed MIR excess.

Molecular Detections

Cross-correlation analysis confirms robust detection of H$_2$O and CO in absorption in the planetary atmosphere up to $\sim$10 μm. No significant CH$_4$ or NH$_3$ is detected.

Figure 4: Cross-correlation with Exo-REM model in all bands individually. The planetary atmosphere is detected until band 2C (wavelengths $\sim$10 μm).

Figure 5: Cross-correlation with Exo-REM molecular absorption templates for the planetary atmosphere. Strong detections of H$_2$O and CO are observed, while no significant signal is found for CH$_4$ or NH$_3$.

Model Limitations and Degeneracies

All atmospheric models exhibit degeneracies between $T_\mathrm{eff}$, $\log g$, metallicity, and cloud properties. The best-fit radii are consistent with evolutionary models only when strong priors are imposed. The C/O and [Fe/H] values are highly model-dependent, and the fit quality is sensitive to the treatment of clouds and the inclusion of the CPD component.

Circumplanetary Disk: Gas and Dust Properties

Molecular Gas Emission

After continuum subtraction, the CPD spectrum reveals strong emission from C$_2$H$_2$ (both optically thick and thin components), HCN, and tentative $^{13}$CCH$_2$. No O-bearing species (CO, CO$_2$, H$_2$O) are detected in emission, indicating a high C/O ratio in the CPD gas.

Figure 6: Modeling of the CPD gas. Top: The continuum-subtracted MIRI spectrum (black) compared to the best-fit total model (red). Emission from C$_2$H$_2$ is shown in yellow for the thin component and dark orange for the thick component, HCN is in orange, and $^{13}$CCH$_2$ in dark red. Bottom: Residuals.

Extended H$_2$ Emission and Disk Winds

Multiple H$_2$ pure rotational lines are detected, with the S(3) and S(5) lines exhibiting spatially extended emission up to $\sim$40 AU, exceeding the expected CPD outer radius. The excitation temperature of the extended component is $T_\mathrm{exc} \sim 1160$ K, while the unresolved component is cooler ($\sim$720 K). The mass of warm H$_2$ is $\sim 1.2 \times 10^{-6}~M_\mathrm{Jup}$, and the inferred mass-loss rate is $2 \times 10^{-10}~M_\mathrm{Jup}~\mathrm{yr}^{-1}$, yielding a wind-to-accretion ratio of $0.004$–$0.1$.

Figure 7: Residuals after PSF subtraction for each H$_2$ line. Circles indicate concentric apertures at 1$\times$FWHM in green and 2.5$\times$FWHM in purple. This physical scale in AU depends on the pixel scale, which varies across spectral bands.

Figure 8: H$_2$ amplitude maps: each pixel value corresponds to the amplitude of the H$_2$ line Gaussian fit, in erg/cm$^2$/s. The dashed circles indicate fixed-radius apertures of 40 AU for comparison. A Gaussian smoothing with $\sigma = 1$ pixel has been applied to each image. The color scale is the same for each image.

Figure 9: Rotation diagram to measure the excitation temperature of H$_2$.

Disk Structure: Cavity, Grain Growth, and Chemistry

The absence of a 10 μm silicate feature indicates grain growth ($a > 5~\mu$m) and/or dust settling. The CPD dust emission is well fit by a single blackbody at 295 K, with an effective radius of $\sim$19 $R_\mathrm{Jup}$, suggesting a dust cavity. The inferred cavity size ($\sim$33 $R_\mathrm{Jup}$) is much larger than the dust sublimation radius, implying a depleted inner disk. The CPD outer radius, set by the Hill sphere, is $\sim$8 AU, but the H$_2$ emission extends beyond this, consistent with a disk wind.

Figure 10: Schematic of the Delorme 1 AB b system: the planet and its CPD. Top: Temperature as a function of radius with both axes on a logarithmic scale. The upper and lower purple bounds correspond to scenarios with high and low accretion, respectively. Representative radius values are indicated. The beige region denotes scales below the spatial resolution of MIRI/MRS. Bottom: A schematic illustration of the CPD structure with the molecular gas species, intended as a simplified representation; it does not necessarily reflect the true spatial distribution of molecular species.

Comparative Disk Chemistry and Evolution

The CPD around Delorme 1 AB b is chemically distinct from T Tauri disks, with a lack of O-bearing molecules and a dominance of hydrocarbons, paralleling trends seen in disks around very low-mass stars and brown dwarfs. The molecular inventory is more limited than in higher-mass analogs, with no detection of C$_4$H$_2$, C$_6$H$_6$, or water emission. This supports a scenario where disks around substellar and planetary-mass objects evolve toward carbon-rich, oxygen-poor chemistries, possibly due to rapid dust evolution, accretion-driven chemistry, or initial conditions in low-mass environments.

Implications for Disk Dispersal and Moon Formation

The persistence of a gas-rich, carbon-rich CPD at $\sim$30–45 Myr challenges standard disk dispersal models and supports the existence of "Peter Pan" disks around low-mass objects. The high C/O ratio in the CPD may have significant implications for the composition of any forming satellites, potentially favoring carbon-rich moon systems. The detection of disk winds suggests ongoing mass loss, which may regulate disk lifetime and satellite formation efficiency.

Theoretical and Practical Implications

• Disk longevity: The presence of a massive, gas-rich CPD at this age requires revised models for disk dispersal, especially in low-mass, wide-separation systems.
• Chemical evolution: The observed C-rich, O-poor chemistry is consistent with models of disk evolution around low-mass stars and may be driven by accretion, dust evolution, or initial conditions.
• Planet formation: The system's mass, separation, and ongoing accretion are not easily explained by core accretion or disk instability, motivating further theoretical work on planet formation in circumbinary and low-mass environments.
• Observational strategies: The results demonstrate the power of JWST/MIRI MRS for spatially and spectrally resolved studies of CPDs, and highlight the need for multi-epoch, multi-wavelength campaigns to disentangle atmospheric and disk contributions.

Future Directions

• Expanded CPD sample: Additional JWST/MIRI observations of CPDs around planetary-mass companions are needed to establish the generality of the observed chemical trends.
• High-resolution spectroscopy: Future facilities (e.g., ELT/METIS) will enable velocity-resolved studies of CPD kinematics and chemistry.
• Time-domain studies: Monitoring accretion variability and its impact on disk chemistry will constrain the interplay between accretion, disk winds, and chemical evolution.
• Integrated modeling: Joint atmospheric and disk retrievals, incorporating evolutionary and chemical models, are required to robustly constrain system parameters and formation histories.

Conclusion

The JWST/MIRI MRS observations of Delorme 1 AB b provide the first medium-resolution MIR spectrum of a circumplanetary disk, revealing a carbon-rich, oxygen-poor molecular inventory, evidence for grain growth and a dust cavity, and spatially extended H$_2$ emission consistent with a disk wind. These findings challenge standard models of disk dispersal and planet formation, and establish a new benchmark for the study of CPDs and their role in the final assembly of gas giants and their satellites. The results underscore the importance of MIR spectroscopy for probing the chemical and physical conditions of planet-forming environments at the lowest masses.