木星・土星の謎を明らかにする「天体の事故」発見（NASA Study: Celestial ‘Accident’ Sheds Light on Jupiter, Saturn Riddle）

2025-09-09 NASA

＜関連情報＞

• https://www.nasa.gov/missions/webb/nasa-study-celestial-accident-sheds-light-on-jupiter-saturn-riddle/
• https://www.nature.com/articles/s41586-025-09369-1
• https://www.nature.com/articles/s41586-025-09369-1.epdf

低温低金属量褐色矮星でケイ酸塩前駆体シランを検出 Silicate precursor silane detected in cold low-metallicity brown dwarf

Jacqueline K. Faherty,Aaron M. Meisner,Ben Burningham,Channon Visscher,Michael Line,Genaro Suárez,Jonathan Gagné,Sherelyn Alejandro Merchan,Austin James Rothermich,Adam J. Burgasser,Adam C. Schneider,Dan Caselden,J. Davy Kirkpatrick,Marc Jason Kuchner,Daniella Carolina Bardalez Gagliuffi,Peter Eisenhardt,Christopher R. Gelino,Eileen C. Gonzales,Federico Marocco,Sandy Leggett,Nicolas Lodieu,Sarah L. Casewell,Pascal Tremblin,Michael Cushing,… Eduardo L. Martin
Nature  Published:20 August 2025
DOI:https://doi.org/10.1038/s41586-025-09369-1

Abstract

Within 20 pc of the Sun, there are currently 29 known cold brown dwarfs—sources with measured distances and an estimated effective temperature between that of Jupiter (170 K) and approximately 500 K (ref. ¹). These sources are almost all isolated and are the closest laboratories we have for detailed atmospheric studies of giant planets formed outside the Solar System. 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 of less than 0.01 times solar. Its spectrum reveals methane (CH₄), water (H₂O) and silane (SiH₄) gas. Although SiH₄ is expected to serve as a key reservoir for the cloud-forming element Si in gas giant worlds, it has remained undetected until now because it is removed from observable atmospheres by the formation of silicate clouds at depth. These condensates are favoured with increasing metallicity, explaining why SiH₄ remains undetected on well-studied metal-rich Solar System worlds such as Jupiter and Saturn². On the metal-poor world W1534, we detect a clear signature of SiH₄ centred at about 4.55 μm with an abundance of 19 ± 2 parts per billion. Our chemical modelling suggests that this SiH₄ abundance may be quenched at approximately kilobar levels just above the silicate cloud layers, in which vertical atmospheric mixing can transport SiH₄ to the observable photosphere. The formation and detection of SiH₄ demonstrates key coupled relationships between composition, cloud formation and atmospheric mixing in cold brown dwarf and planetary atmospheres.