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. 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 from 1 to 18 µm. We introduce a method based on brightness temperature to generate a list of cloud species that are likely present in its atmosphere. We tested for the species' presence with petitRADTRANS retrievals. Using retrievals and grids from various climate models, we derived bulk parameters from 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 ̊m ^ C/^ C below ISM values. The atmospheric gravity proves difficult to constrain for both retrievals and grid models. Retrievals describing the flux of by mixing two 1D models (``two-column models'') appear favored over single-column models; this is consistent with 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 could be a more widespread phenomenon.<p></p>