Evidence for SiO cloud nucleation in the rogue planet PSO J318

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
² Institute for Particle Physics and Astrophysics, ETH Zurich, Wolfgang-Pauli-Str 27, 8093 Zürich, Switzerland
³ SRON Space Research Organisation Netherlands, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands
⁴ Université Paris Cité, Université Paris-Saclay, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
⁵ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL The Netherlands
⁶ Department of Astrophysics, University of Vienna, Türkenschanzstr. 17, 1180 Vienna, Austria
⁷ Institute of Solid State Physics, Friedrich Schiller University, Jena, Germany
⁸ Department of Astronomy & Astrophysics, University of California, Santa Cruz, CA 95064, USA
⁹ Department of Physics & Astronomy, University of Rochester, Rochester, NY 14627, USA
¹⁰ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
¹¹ Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, UK
¹² Centre for Exoplanet Science, University of Edinburgh, Edinburgh EH9 3FD, UK
¹³ STAR Institute, Université de Liège, Allée du Six Août, 19C, 4000 Liège, Belgium
¹⁴ Centro de Astrobiología (CAB), CSIC-INTA, ESAC Campus, Camino bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
¹⁵ Université Paris-Saclay, CEA, IRFU, 91191 Gif-sur-Yvette, France
¹⁶ UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁷ Department of Astronomy, Stockholm University, AlbaNova University Center, 10691 Stockholm, Sweden
¹⁸ School of Physics & Astronomy, Space Park Leicester, University of Leicester, 92 Corporation Road, Leicester LE4 5SP, UK
¹⁹ LIRA, Observatoire de Paris, Université PSL, Sorbonne Université, Sorbonne Paris Cité, CY Cergy Paris Université, CNRS, 5 place Jules Janssen, 92195 Meudon, France
²⁰ Université Paris-Saclay, UVSQ, CNRS, CEA, Maison de la Simulation, 91191 Gif-sur-Yvette, France
²¹ Department of Astrophysics, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024, USA
²² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
²³ School of Cosmic Physics, Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin D02 XF86, Ireland

^★ Corresponding author: molliere@mpia.de

Received: 29 May 2025
Accepted: 24 July 2025

Abstract

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 petitRADTRANS 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.