![The top panel shows comparisons of the observed spectrum of JADES-GS-BD-11 (black with grey 1σ error bars) and the median retrieved spectrum for both the PH3 and non-PH3 petitRADTRANS (red and green, respectively, with 1σ central credible intervals shaded around the median). The bottom panel shows the difference between the retrieved and observed spectra, divided by the observed uncertainty. The two retrieved median spectra are generally within 1σ of each other everywhere except at 4.3 µm (the Q-branch of the PH3 feature).-- [astro-ph.SR]](https://i0.wp.com/astrobiology.com/wp-content/uploads/2026/08/JWSTNIRSpec.png?fit=1200%2C674&ssl=1)
[astro-ph.SR] We present JWST NIRSpec prism spectra for three ultracool brown dwarfs discovered in extragalactic survey data, two from the JWST Advanced Deep Extragalactic Survey (JADES), and one from Public Release IMaging for Extragalactic Research (PRIMER) survey observed as part of the Mirage or Miracle (MoM) program.
The spectra for these sources indicate that one is a T6 dwarf (JADES-GS-BD-11, Teff=∼700 K) and two are Y0-Y1 dwarfs (JADES-GS-BD-5, Teff=∼400 K, and MoM-239450, Teff=∼500 K). Model atmospheric fits with 𝙽𝙸𝙵𝚃𝚈 to the spectra are consistent with this classification, and indicate that JADES-GS-BD-5 is only ∼150 pc from the Sun, MoM-239450 is ∼700−800 pc from the Sun, and JADES-GS-BD-11 is ∼1 kpc from the Sun, with these latter two more distant sources being best fit at sub-solar metallicities.
JADES-GS-BD-5 has an observed spectrum with significantly weaker J and H band emission than Y dwarf atmospheric models, potentially indicating the presence of water ice clouds in the brown dwarf. The spectrum for JADES-GS-BD-11 has a feature at 4.3μm consistent with absorption from the rarely seen phosphine molecule at 2.6σ confidence.
Given the low metallicity for this source ([M/H] =−0.7), our finding supports the theory that detecting phosphine in brown dwarf atmospheres is tied to atmospheric metallicity. JWST/NIRSpec spectroscopy continues to be a powerful tool for understanding the properties of these distant, and very cold brown dwarfs.
Kevin N. Hainline, Samuel A. Beiler, Adam J. Burgasser, Evan S. Chen, Jakob M. Helton, Jarron Leisenring, Brittany E. Miles, Mark S. Marley, Sagnick Mukherjee, Stefi Baum, Andrew J. Bunker, Stefano Carniani, Francesco D’Eugenio, Eiichi Egami, Tobias J. Looser, Pierluigi Rinaldi, Christopher N. A. Willmer
Comments: 17 pages, 5 figures, supplemental data and figures here: this https URL, submitted to AAS Journals (August 2026)
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2608.14786 [astro-ph.SR] (or arXiv:2608.14786v1 [astro-ph.SR] for this version)
https://doi.org/10.48550/arXiv.2608.14786
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Submission history
From: Kevin Hainline
[v1] Fri, 14 Aug 2026 18:00:06 UTC (1,952 KB)
https://arxiv.org/abs/2608.14786
astrobiology, astrochemistry,
