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Silicate precursor silane detected in cold low-metallicity brown dwarf

Nature volume 645, pages 62–66 (2025)Cite this article

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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.

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Fig. 1: The distance-calibrated JWST NIRSpec prism and G395H portion of the SED for W1534.
Fig. 2: The JWST G395H spectrum for W1534.
Fig. 3: The best-fit retrieved thermal profile and corresponding contribution function for W1534.

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Data availability

The JWST data in this paper are part of GO program 3558 (principal investigator A.M.M.) and are publicly available in the Barbara A. Mikulski Archive for Space Telescopes (MAST; https://archive.stsci.edu) under that program ID.

Code availability

The data reduction pipeline jwst can be found at https://jwst-pipeline.readthedocs.io/en/latest/. This study made use of v.1.15.1 of the pipeline, which is available at Zenodo43 (https://zenodo.org/records/12692459). The Brewster code is open source and available at GitHub (https://github.com/substellar/brewster). The SEDkit code is open source and available at GitHub (https://github.com/hover2pi/sedkit).

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Acknowledgements

J.K.F. acknowledges funding from the Heising–Simons Foundation as well as NSF (award nos. 2238468 and 1909776) and NASA (award no. 80NSSC22K0142) and support from JWST-GO-03558. B.B. acknowledges support from the UK Research and Innovation Science and Technology Facilities Council (ST/X001091/1). C.V. acknowledges support from JWST-AR-2232. A.J.B. acknowledges funding support from the Heising–Simons Foundation. E.L.M. acknowledges funding support from the European Research Council Advanced Grant Substellar (project no. 101054354). B.G. acknowledges support from the Polish National Science Center (NCN) under SONATA (grant no. 2021/43/D/ST9/0194). M.L. acknowledges support from JWST-GO-03558. V.J.S.B., N.L., E.L.M. and M.R.Z.O. acknowledge support from grant no. PID2022-137241NB-C4[1,2] funded by Agencia Estatal de Investigación of the Ministerio de Ciencia, Innovación y Universidades (MICIU/AEI/10.13039/501100011033) and ERDF/EU. For open access, we have applied a Creative Commons Attribution (CC BY) licence to any Author Accepted Manuscript version arising. Portions of this research were carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.

Author information

Authors and Affiliations

  1. Department of Astrophysics, American Museum of Natural History, New York, NY, USA

    Jacqueline K. Faherty, Genaro Suárez, Sherelyn Alejandro Merchan, Austin James Rothermich & Dan Caselden

  2. Department of Physics, The Graduate Center City University of New York, New York, NY, USA

    Jacqueline K. Faherty, Sherelyn Alejandro Merchan & Austin James Rothermich

  3. NSF National Optical-Infrared Astronomy Research Laboratory, Tucson, AZ, USA

    Aaron M. Meisner

  4. Department of Physics, Astronomy and Mathematics, University of Hertfordshire, Hatfield, UK

    Ben Burningham

  5. Chemistry and Planetary Sciences, Dordt University, Sioux Center, IA, USA

    Channon Visscher

  6. Center for Exoplanetary Systems, Space Science Institute, Boulder, CO, USA

    Channon Visscher

  7. School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA

    Michael Line

  8. Planétarium de Montreal, Montreal, Quebec, Canada

    Jonathan Gagné

  9. Département de Physique, Trottier Institute for Research on Exoplanets, Université de Montréal, Montreal, Quebec, Canada

    Jonathan Gagné

  10. Department of Astronomy and Astrophysics, University of California, San Diego, La Jolla, CA, USA

    Adam J. Burgasser

  11. Flagstaff Station, United States Naval Observatory, Flagstaff, AZ, USA

    Adam C. Schneider

  12. IPAC, Caltech, Pasadena, CA, USA

    J. Davy Kirkpatrick, Christopher R. Gelino & Federico Marocco

  13. Science Mission Directorate, NASA, Washington DC, USA

    Marc Jason Kuchner

  14. Department of Physics and Astronomy, Amherst College, Amherst, MA, USA

    Daniella Carolina Bardalez Gagliuffi

  15. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA

    Peter Eisenhardt

  16. Department of Physics and Astronomy, San Francisco State University, San Francisco, CA, USA

    Eileen C. Gonzales

  17. NSF NOIRLab, Gemini Observatory, Hilo, HI, USA

    Sandy Leggett

  18. Instituto de Astrofísica de Canarias, San Cristobal de La Laguna, Spain

    Nicolas Lodieu, Víctor J. S. Béjar, Jun-Yan Zhang & Eduardo L. Martin

  19. Departamento de Astrofísica, Universidad de La Laguna, San Cristobal de La Laguna, Spain

    Nicolas Lodieu, Víctor J. S. Béjar, Jun-Yan Zhang & Eduardo L. Martin

  20. School of Physics and Astronomy, University of Leicester, Leicester, UK

    Sarah L. Casewell

  21. Maison de la Simulation, Université Paris-Saclay, UVSQ, CNRS, CEA, Gif-sur-Yvette, France

    Pascal Tremblin

  22. Ritter Astrophysical Research Center, Department of Physics and Astronomy, University of Toledo, Toledo, OH, USA

    Michael Cushing

  23. Centro de Astrobiología, CSIC-INTA, Madrid, Spain

    Maria Rosa Zapatero Osorio

  24. Janusz Gil Institute of Astronomy, University of Zielona Góra, Zielona Góra, Poland

    Bartosz Gauza

  25. Department of Physics and Astronomy, University of California, Los Angeles, Los Angeles, CA, USA

    Edward Wright

  26. Institute for Astronomy, University of Edinburgh, Edinburgh, UK

    Mark W. Phillips

Authors
  1. Jacqueline K. Faherty
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  2. Aaron M. Meisner
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  4. Channon Visscher
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  5. Michael Line
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  6. Genaro Suárez
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  7. Jonathan Gagné
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  8. Sherelyn Alejandro Merchan
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  9. Austin James Rothermich
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  10. Adam J. Burgasser
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  11. Adam C. Schneider
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  13. J. Davy Kirkpatrick
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  15. Daniella Carolina Bardalez Gagliuffi
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  16. Peter Eisenhardt
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  17. Christopher R. Gelino
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  18. Eileen C. Gonzales
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  19. Federico Marocco
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  21. Nicolas Lodieu
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  22. Sarah L. Casewell
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  30. Jun-Yan Zhang
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  31. Eduardo L. Martin
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Contributions

A.M.M. was the principal investigator of the JWST proposal and observed the execution. J.K.F. oversaw all data reduction, analysis and modelling. B.B. completed the atmospheric retrieval. J.G. extracted the radial velocity of W1534. G.S., A.J.R. and S.A.M. contributed to data reduction and SED analysis. C.V. completed the chemistry models and quenching kinetics analysis. M.L. first identified the SiH4 feature and suggested the initial idea and motivation for the paper. A.J.B., E.L.M., A.C.S., D.C.B.G., J.D.K., P.E., E.C.G., F.M., S.L., N.L., S.L.C., P.T., M.C., M.R.Z.O., V.J.S.B., B.G., E.W., M.J.K., C.R.G., M.W.P. and J.-Y.Z. contributed to the interpretation of the results and editing of the paper.

Corresponding author

Correspondence to Jacqueline K. Faherty.

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The authors declare no competing interests.

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Nature thanks the anonymous reviewers for their contribution to the peer review of this work.

Additional information

Extended data

is available for this paper at https://doi.org/10.1038/s41586-025-09369-1.

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Extended data figures and tables

Extended Data Fig. 1 The Spitzer infrared color-magnitude diagram for cold brown dwarfs.

The sources in JWST GO programs with comparable colors or magnitudes, as well as publicly available G395H or G395M spectra, are labeled and indicated by stars. The position of W1534 is the largest of the five-point stars which are labeled. Error bars indicate 1σ uncertainties.

Extended Data Fig. 2 Toomre diagram of UVW space motions of nearby stars and brown dwarfs highlighting W1534.

Points indicate sources with 3D kinematics from the 100 pc Gaia Catalog of Nearby stars (GCNS) containing radial velocities from Gaia DR3. Thin disk (grey), thick disk (blue), and halo (black) sources are indicated by color; the location of W1534 is indicated by the large star.

Extended Data Fig. 3 Retrieved and derived parameters for W1534.

Diagonal panels display the distributions of parameters marginalized over all other quantities, with dashed lines indicating the median and ± 1σ range, labeled above each panel. Interior contour plots display parameter correlations. Abundances, surface gravity (\(\log g\)), radial velocity (vrad), and rotational velocity (\(v\sin i\)) are retrieved from the posterior distributions; metallicity ([M/H]), C/O ratio, radius (R), mass (M), luminosity (log(L/Lsun)), and effective temperature (Teff) are computed from these quantities.

Extended Data Fig. 4 C/O ratio vs metallicity for FGK stars in the disk and halo.

Plotted are the C/O values and metallicity [Fe/H] for FGK primarily disk stars as measured by72 and halo stars as measured by73. The retrieved values for W1534 are overplotted as a five point star and labeled.

Extended Data Fig. 5 The equilibrium abundance of SiH4 as a function of pressure and temperature with C/O=0.26 in low-metallicity (left) and solar-metallicity (right) gas.

In each panel the contours represent the mole fraction abundance of silane; the red dashed curve indicates the ~ 20 ppb obtained from the atmospheric retrieval of W1534. The condensation curves for forsterite and enstatite are overplotted as dashed lines. At thermochemical equilibrium, SiH4 is the most abundant Si-bearing gas at high temperatures and high pressures until it is removed at lower temperatures by silicate condensation, or replaced at lower pressures by other Si-bearing gases such as SiO (cf.14).

Extended Data Fig. 6 The retrieved thermal profile for W1534 extended to deeper pressures to extrapolate to the SiH4 quench region.

The black line with grey error bar represents the retrieved thermal profile for W1534. The colored contours represent the equilibrium abundance of silane calculated for an atmosphere with [M/H]=-2.22 and C/O=0.26, with the red dashed line corresponding to the  ~ 20 ppb obtained from the retrieval. The condensation curves of enstatite and forsterite are overplotted, and demonstrate the rapid decrease in the SiH4(g) abundance at lower temperatures (i.e., above the cloud base). The thermal profile was extended to deeper pressures in three ways: by extrapolation from the trend of the retrieved profile (lower dashed line), an adiabatic extension from the deepest point of the retrieved profile (middle dashed line), and an adiabatic extension from the  ~ 10 bar level of the retrieved profile (top dashed line). The white dashed curve indicates the SiO-SiH4 equal abundance boundary (cf.14). Highlighted as filled ovals are the vertical mixing rates parameterized by Kzz (labelled in units of cm2 s−1) that indicate where quenching will occur along each thermal profile.

Extended Data Fig. 7 Quenching of silane along the thermal profile of W1534.

Equilibrium abundances of SiH4 and SiO are shown along each of the extended thermal profiles in Fig. 6, from the lowest pressure profile (left) to the highest pressure profile (right). Note the different pressure scale for each panel (right axes). The red band indicates the abundance and approximate observed altitude of the retrieved SiH4. Under equilibrium conditions, the abundances of Si-bearing species rapidly decrease above the silicate cloud layer (horizontal dashed line). However, transport-induced quenching may deliver much higher SiH4 abundances into the upper atmosphere, depending upon the strength of atmospheric mixing. The circles along the SiH4 profile label the values of log10 Kzz (cm2 s−1) that lead to the quenched abundances extending vertically upward to lower pressures.

Extended Data Table 1 Parameters of interest for W1534
Full size table
Extended Data Table 2 Parameters and priors adopted for the retrieval analysis
Full size table
Extended Data Table 3 Retrieved Gas Abundances
Full size table

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Faherty, J.K., Meisner, A.M., Burningham, B. et al. Silicate precursor silane detected in cold low-metallicity brown dwarf. Nature 645, 62–66 (2025). https://doi.org/10.1038/s41586-025-09369-1

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