Evidence for SiO cloud nucleation in the rogue planet PSO J318

Silicate clouds have long been known to significantly impact the spectra of late L-type brown dwarfs – with observable absorption features at ~10 μm. The James Webb Space Telescope (JWST) has reopened a window to the mid-infrared with unprecedented sensitivity, bringing the characterization of silicate clouds into focus again. Using JWST, we aim to characterize the planetary-mass brown dwarf PSO J318.5338-22.8603, concentrating on any silicate cloud absorption the object may exhibit. PSO J318’s spectrum is extremely red, and its flux is variable, both of which are thought to be hallmarks of cloud absorption. We present JWST NIRSpec PRISM, G395H, and MIRI MRS observations of PSO J318 from 1 to 18 μm. We introduce a method based on PSO J318’s brightness temperature to generate a list of cloud species that are likely present in its atmosphere. We tested for the species’ presence with <ns0:monospace>petitRADTRANS</ns0:monospace> retrievals. Using retrievals and grids from various climate models, we derived bulk parameters from PSO J318’s spectra, which are mutually compatible. Our retrieval results point to a solar to a slightly super-solar atmospheric C/O, a slightly super-solar metallicity, and a 12C/13C below ISM values. The atmospheric gravity proves difficult to constrain for both retrievals and grid models. Retrievals describing the flux of PSO J318 by mixing two 1D models (“two-column models”) appear favored over single-column models; this is consistent with PSO J318’s variability. The JWST observations also reveal a pronounced absorption feature at 10 μm. This absorption is best reproduced by introducing a high-altitude cloud layer of small (<0.1 μm) amorphous SiO grains. The retrieved particle size and location of the cloud is consistent with SiO condensing as cloud seeding nuclei. High-altitude clouds comprised of small SiO particles have been suggested in previous studies. Therefore, the SiO nucleation we potentially observe in PSO J318 could be a more widespread phenomenon.