Detection of Hydrocarbons in the Disk around an Actively Accreting Planetary-mass Object

We present the 0.6-12 μm spectrum of Cha 1107-7626, a 6-10 Jupiter-mass free-floating object in the ∼2 Myr-old Chamaeleon-I star-forming region, from observations with the NIRSpec and MIRI instruments on board the James Webb Space Telescope. We confirm that Cha 1107-7626 is one of the lowest-mass objects known to harbor a dusty disk with infrared excess emission at wavelengths beyond 4 μm. Our NIRSpec data and prior ground-based observations provide strong evidence for ongoing accretion through hydrogen recombination lines. In the mid-infrared spectrum, we detect unambiguously emission lines caused by methane (CH4) and ethylene (C2H4) in its circumsubstellar disk. Our findings mean that Cha 1107-7626 is by far the lowest-mass object with hydrocarbons observed in its disk. The spectrum of the disk looks remarkably similar to that of ISO-ChaI 147, a very low-mass star with a carbon-rich disk that is 10-20 times more massive than Cha 1107-7626. The hydrocarbon lines can be accounted for with a model assuming gas temperatures of a few hundred kelvin in the inner disk. The obvious similarities between the spectra of a low-mass star and a planetary-mass object indicate that the conditions in the inner disks can be similar across a wide range of central object masses.