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Abstract About 70%–80% of stars in our solar and Galactic neighborhood are M dwarfs. They span a range of low masses and temperatures relative to solar-type stars, facilitating molecule formation throughout their atmospheres. Standard stellar atmosphere models primarily designed for FGK stars face challenges when characterizing broadband molecular features in spectra of cool stars. Here, we introduce SPHINX —a new 1D self-consistent radiative–convective thermochemical equilibrium chemistry model grid of atmospheres and spectra for M dwarfs in low resolution ( R ∼ 250). We incorporate the latest precomputed absorption cross sections with pressure broadening for key molecules dominant in late-K, early/main-sequence-M stars. We then validate our grid models by determining fundamental properties ( T eff , log g , [M/H], radius, and C/O) for 10 benchmark M+G binary stars with known host metallicities and 10 M dwarfs with interferometrically measured angular diameters. Incorporating the Gaussian process inference tool Starfish , we account for correlated and systematic noise in low-resolution (spectral stitching of SpeX, SNIFS, and STIS) observations and derive robust estimates of fundamental M-dwarf atmospheric parameters. Additionally, we assess the influence of photospheric heterogeneity on inferred [M/H] and find that it could explain some deviations from observations. We also probe whether the adopted convective mixing length parameter influences inferred radii, effective temperature, and [M/H] and again find that may explain discrepancies between interferometric observations and model-derived parameters for cooler M dwarfs. Mainly, we show the unique strength in leveraging broadband molecular absorption features occurring in low-resolution M dwarf spectra and demonstrate the ability to improve constraints on fundamental properties of exoplanet hosts and brown-dwarf companions. 

Abstract Beyond our Solar System, aurorae have been inferred from radio observations of isolated brown dwarfs^1,2. Within our Solar System, giant planets have auroral emission with signatures across the electromagnetic spectrum including infrared emission of H₃⁺and methane. Isolated brown dwarfs with auroral signatures in the radio have been searched for corresponding infrared features, but only null detections have been reported³. CWISEP J193518.59-154620.3. (W1935 for short) is an isolated brown dwarf with a temperature of approximately 482 K. Here we report James Webb Space Telescope observations of strong methane emission from W1935 at 3.326 μm. Atmospheric modelling leads us to conclude that a temperature inversion of approximately 300 K centred at 1–10 mbar replicates the feature. This represents an atmospheric temperature inversion for a Jupiter-like atmosphere without irradiation from a host star. A plausible explanation for the strong inversion is heating by auroral processes, although other internal and external dynamical processes cannot be ruled out. The best-fitting model rules out the contribution of H₃⁺emission, which is prominent in Solar System gas giants. However, this is consistent with rapid destruction of H₃⁺at the higher pressure where the W1935 emission originates⁴. 

Abstract The coldest Y spectral type brown dwarfs are similar in mass and temperature to cool and warm (∼200–400 K) giant exoplanets. We can therefore use their atmospheres as proxies for planetary atmospheres, testing our understanding of physics and chemistry for these complex, cool worlds. At these cold temperatures, their atmospheres are cold enough for water clouds to form, and chemical timescales increase, increasing the likelihood of disequilibrium chemistry compared to warmer classes of planets. JWST observations are revolutionizing the characterization of these worlds with high signal-to-noise, moderate-resolution near- and mid-infrared spectra. The spectra have been used to measure the abundances of prominent species, like water, methane, and ammonia; species that trace chemical reactions, like carbon monoxide; and even isotopologues of carbon monoxide and ammonia. Here, we present atmospheric retrieval results using both published fixed-slit (Guaranteed Time Observation program 1230) and new averaged time series observations (GO program 2327) of the coldest known Y dwarf, WISE 0855–0714 (using NIRSpec G395M spectra), which has an effective temperature of ∼264 K. We present a detection of deuterium in an atmosphere outside of the solar system via a relative measurement of deuterated methane (CH₃D) and standard methane. From this, we infer the D/H ratio of a substellar object outside the solar system for the first time. We also present a well-constrained part-per-billion abundance of phosphine (PH₃). We discuss our interpretation of these results and the implications for brown dwarf and giant exoplanet formation and evolution.