JWST Detects Silane in a Cold Low-Metallicity World

Within 20 pc of the Sun there are 29 isolated sources with measured distances (Kirkpatrick et al. 2024) and an estimated temperature between that of Jupiter (170K) and∼500K – the cold brown dwarf regime. These objects are valuable laboratories for exploring the diversity of extrasolar atmospheres and comparing them to solar system gas giants. Here we report JWST observations of one such source, WISEA J153429.75-104303.3 (W1534), which we confirm is a substellar mass member of the Galactic halo with a metallicity <0.01×solar. Its spectrum reveals methane (CH4), water (H2O), and silane (SiH4) gas. Although SiH4 is expected to serve as a key reservoir for the critical cloud-forming element Si in gas giant worlds, it eluded detection in any substellar mass object (solar system or otherwise) until now because it is removed from observable atmospheres by the formation of silicate clouds at depth. These condensates are favored with increasing metallicity, explaining why SiH4 remains undetected on well studied, metal-rich solar system worlds like Jupiter and Saturn (Fegley & Lodders 1994). On the metal-poor world W1534, we detect a clear signature of SiH4 centered at∼4.55 µm with an abundance of∼20 parts per billion (ppb). Chemical models suggest it is likely quenched from regions just above the silicate cloud layers where moderate to strong vertical mixing from a pressure level of several hundred bar can transport it to the observable photosphere. The formation and detection of SiH4 – favoured by the low metallicity and short convective mixing timescales in the atmosphere of W1534 – demonstrates key coupled relationships between composition, cloud formation, and atmospheric mixing in cold brown dwarf and planetary atmospheres.