arXiv subjects

Sarah L. Casewell

Publications and source records attributed to Sarah L. Casewell.

At least 19 recordsLinked to original sources

arXiv2026

Dynamically Coupled Three-dimensional Cloud-Chemistry in Rapidly Rotating Irradiated Atmospheres: an Application to the Compact White Dwarf-Brown Dwarf system ZTFJ0038+2030

Clouds and disequilibrium chemistry shape the observable properties of substellar and planetary atmospheres, and JWST time-resolved spectroscopy now enables direct tests of their three-dimensional structures. We present an update to the 3D atmospheric framework ADAM, coupling the dynamical core to a miniature kinetic chemistry scheme (mini-chem) and a saturation-relaxation tracer cloud module. Using this framework, we investigate the atmosphere of the brown dwarf orbiting the white dwarf ZTFJ0038+2030 in a 10-hour orbit. The brown dwarf receives stellar irradiation comparable to its internal heat flux, placing the atmosphere in a regime where the circulation is characterized by a standing Matsuno--Gill wave response and equatorial superrotation. Rapid rotation confines these structures closer to the equator, producing more symmetric broadband light curves with smaller hotspot offsets than canonical hot Jupiters, in agreement with recent JWST observations of this system. Although most species are globally homogenized, rapid rotation limits global-scale horizontal mixing and allows circulation-driven vertical transport to maintain strong CO and CO$_2$ inhomogeneities in the photosphere. These chemical inhomogeneities produce asymmetric phase curves in the CO and CO$_2$ absorption bands, which could qualitatively explain the observed CO$_2$ light-curve asymmetry. When clouds are included, Na$_2$S clouds dominate the photosphere and weaken the large-scale circulation, lowering photospheric CO and CO$_2$ abundances, while MgSiO$_3$ and Fe clouds rain out at deeper levels. Because the dominant Na$_2$S clouds reside mostly above the quench region, cloud-driven convection has limited impact on the quenched abundances, which are instead regulated mainly by large-scale circulation.

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arXiv2026

Asymmetric nightside CO2 features, inefficient heat transport, and precise evolutionary constraints: Spectroscopic phase curves reveal the past and present of a white dwarf-brown dwarf binary

We present the first JWST phase curve of a white dwarf-brown dwarf binary, a NIRSpec PRISM observation of ZTFJ0038+2030. Short-period white dwarf-brown dwarf binaries provide unique laboratories to probe substellar atmospheres. Tidal locking drives hot Jupiter-like atmospheric dynamics in the brown dwarf. The system's formation history offers a window into planetary systems around post-main-sequence stars. We obtain a full-orbit phase curve of ZTF0038, including a total eclipse of the white dwarf, which enables us to separate the two components' emission throughout the entire orbit, and we model the brown dwarf's phase-resolved emission spectra using substellar atmosphere forward models and atmospheric retrievals. The PRISM spectrum covers ~80% of the brown dwarf's bolometric emission, enabling a nearly model-independent energy balance calculation, which yields a day-to-nightside heat transport efficiency of <10%. Inefficient heat redistribution is further supported by the phase curve shape and the nightside spectrum closely resembling non-irradiated mid-to-late T dwarfs. The spectroscopic phase curves reveal a stark nightside asymmetry associated with a strong CO2 absorption feature at 4.2 um, while the retrieved abundances indicate a longitudinally homogeneous distribution of CO2 as well as all other key species detected in the atmosphere. The precise internal luminosity measurement of the brown dwarf informs both the age of the WD-BD system (7.5-8.8 Gyr) and indicates a low common-envelope ejection efficiency. These data illustrate the exquisite opportunity to probe the three-dimensional processes of substellar atmospheres, connect substellar and exoplanet atmospheres, and probe the evolution of post-main-sequence planetary systems using WD-BDs.

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arXiv2026

NGTS clusters survey - VII. An Enigmatic Short-Period Circumsecondary Disk Candidate in Orion

Young stellar systems provide key insights into the processes governing star/planet formation, often exhibiting complex and evolving photometric variability. In this paper, we present an analysis of the object around NOI 105872, observed by the Next Generation Transit Survey (NGTS) to eclipse a young ($4.1\pm0.4$ Myr) M-type star in the Orion Nebula Cluster. The system displays deep, asymmetric eclipses with a period of $\sim0.69$ days, alongside variability in morphology. We combine photometric observations spanning approximately a decade from multiple ground- and space-based facilities to investigate the nature of this system. We explore a range of physical scenarios to reproduce the eclipse profiles and their evolution. After analysis, our preferred explanation is a circumsecondary disk surrounding a companion in orbit around the M-type primary. We adopt a six-parameter disk model and fit the eclipse profiles independently to each dataset. The best-fitting solution yields a disk radius of $\sim 0.005 \mathrm{au}$ or $0.98 R_{*}$, with broadly consistent orientations across epochs, but with variations that may be attributed to disk precession or intrinsic stellar variability. Constraints from the Hill radius imply a companion mass consistent with a very low-mass stellar companion. These results establish NOI 105872 as a candidate for a circumsecondary disk system in a young stellar environment. Continued photometric monitoring and spectroscopic follow-up will be essential to confirm this interpretation. If confirmed, systems such as NOI 105872 provide valuable benchmarks for understanding disk dynamics, companion formation, and the origin of complex eclipse phenomena in young stars.

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arXiv2026

The scarcity of white dwarf-brown dwarf binaries in the solar neighbourhood: A population synthesis study

White dwarf--brown dwarf (WD--BD) binaries are intrinsically rare systems that provide a unique opportunity to study binary evolution and the formation of substellar companions. Despite recent observational progress, only a small number are known in the solar neighbourhood. We investigate whether their scarcity can be explained by binary formation and evolution together with observational selection effects. We extend the MRBIN binary population synthesis code into the substellar regime by incorporating brown dwarf evolutionary models and an extended initial mass function. We simulate the stellar population within 100 pc of the Sun and compare the predicted WD--BD population with the observed sample. Our simulations predict 13+-5 WD--BD systems within 100 pc, consistent with observations. These systems represent only 0.1% of the local white dwarf population, confirming their rarity. The models reproduce the overall distributions of white dwarf masses, temperatures, and orbital periods, although the simulated brown dwarfs are systematically hotter than observed. Close WD--BD binaries are underproduced with the standard common-envelope efficiency 0.3, suggesting that higher efficiencies may be required to reproduce the observed short-period systems. The simulations also reproduce the observed deficit of systems at intermediate orbital periods between post-common-envelope binaries and wide non-interacting systems, as well as a weakened brown dwarf desert in main-sequence--brown dwarf binaries.

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arXiv2026

NGTS-39 b: A 58 d transiting warm Jupiter in an eccentric orbit

We report the discovery and characterisation of NGTS-39 b (TIC 453147896 b), a warm Jupiter transiting a Sun-like star on a 58.2 day, eccentric (e = 0.386 +/- 0.019) orbit. NGTS-39 b was first identified from a TESS single-transit event, and subsequently confirmed with NGTS photometry and radial-velocity measurements from CORALIE and HARPS. The host star is a bright (Tmag = 11.02) F9 dwarf with an effective temperature of Teff = 6053 +67/-30 K. NGTS-39 b is a Jupiter-sized gas giant with a radius of 1.088 +/- 0.012 RJ and a mass of 1.467 +/- 0.081 MJ. Its equilibrium temperature is 519 +6/-5 K, placing it between short-period hot Jupiters and cold, Jupiter-like giants. The high orbital eccentricity and intermediate equilibrium temperature of NGTS-39 b make it a valuable test case for formation and migration models, particularly in the poorly sampled regime of long-period gas giants. The RV data show a linear trend of gamma dot = -17.75 m s^-1 yr^-1, which indicates the presence of an outer companion. The discovery of NGTS-39 b contributes to the small but growing population of transiting warm Jupiters with P > 50 days orbiting bright stars.

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arXiv2026

TIC-65910228 b / NGTS-38 b, a 180 day transiting warm super-Jupiter

We present the discovery of TIC-65910228 b / NGTS-38 b, a giant exoplanet with a radius of $1.081\pm0.047$ R$_\text{J}$ and a mass of $4.78_{-0.37}^{+0.39}$ M$_\text{J}$ on a long-period ($180.52797\pm0.00036$ day), moderately eccentric ($e=0.3086\pm0.010$) orbit transiting a bright (V=$10.230\pm0.020$ mag) metal rich ([Fe/H]=$0.33\pm0.09$, 'dex') F6V-F7V type host star. The planet was initially detected from a single transit in TESS Sector 33. A photometric monitoring campaign of 228 nights with NGTS detected a transit egress of the planet, which together with spectroscopic radial velocity monitoring with CORALIE and HARPS identified an orbital period of ~180.5,d. These radial velocity measurements also showed the mass of the companion to be planetary. Additional transit observations coordinated by the TESS follow-up observing program allowed further confirmation and refinement of this period. With its relatively cool equilibrium temperature of $457\pm11$ K, NGTS-38 b joins a small but growing population of well characterised transiting warm-Jupiters and has one of the longest periods of any discovered to date. The target is situated in the LOPS2 field of the upcoming PLATO mission which will allow for greater refinement of the system parameters and potential for the discovery of additional companions too small and/or too long-period to be seen by TESS or NGTS. NGTS-38 b's bright host star and wide orbital separation make it an attractive target for further study, including potential measurement of its spin-orbit alignment or targeted exomoon/ring searches.

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arXiv2026

TOI-2155 b: A Massive Brown Dwarf or a Very Low-Mass Star?

We present TOI-2155\,b, a massive transiting companion, discovered using data from NASA's Transiting Exoplanet Survey Satellite (TESS) mission and confirmed with ground-based RV measurements from the Tillinghast Reflector Echelle Spectrograph (TRES). We also analyze ground-based follow-up photometric data from the Wendelstein Observatory (WST), Las Cumbres Observatory Global Telescope (LCOGT), and Wild Boar Remote Observatory (WBR). TOI-2155\,b is a short-period companion with {$P= 3.7246950 \pm{0.0000014}$}~days. The radius and mass of TOI-2155\,b are found to be $R_b = 0.972^{+0.009}_{-0.008} \,\mathrm{R_J}$ and $M_b = 80.6^{+1.0}_{-1.1} \,\mathrm{M_J}$, respectively, corresponding to a density of {$ρ_b= 109^{+3.1}_{-3.3}$ g cm$^{-3}$}. The F-type subgiant host star has an effective temperature of $T_{\rm eff} = 6085\pm 78$ K, a radius $R_{\thinstar} = 1.705^{+0.066}_{-0.064}$ $\mathrm{R_\odot}$ and a mass $M_\star = 1.33 \pm 0.008$~M$_\odot$. With a mass close to the hydrogen-burning minimum mass, TOI-2155\,b lies at the boundary between brown dwarfs and low-mass stars. Its measured mass, radius, and density place it in a transitional region, where distinguishing between a massive brown dwarf and a very low-mass star is not straightforward. TOI-2155\,b therefore provides a valuable benchmark for testing evolutionary models of stellar and substellar structure near the hydrogen-burning limit.

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arXiv2026

From Hubble to HWO: Bridging the Frontier of White Dwarf Exoplanet Science

White dwarf stars, the endpoint of stellar evolution for 97% of stars in our Milky Way, offer a unique and powerful window into the bulk elemental composition of rocky exoplanetary bodies. Up to 50% of single white dwarfs are observed with photospheric metal lines from accreted exoplanetary bodies (called 'polluted' white dwarfs), and spectroscopic observations reveal the bulk composition of this material. High-resolution (R>15,000) UV spectra are essential for detecting many elements present in the material, such as the volatile elements imperative for habitability studies (C, N, O, P, S) and key rock-forming elements required to constrain interior structure (e.g. Fe, Si, Mg, Al, Ni). HST, through its COS and STIS spectrographs, remains the only facility capable of performing this science in the near future. Looking to the next decade, the scientific case for continued HST UV observations of polluted white dwarfs is compelling on three fronts (i) as a standalone to enable the bulk composition of exoplanetary material to be measured in a statistically significant sample, (ii) as essential groundwork for the Habitable Worlds Observatory (HWO), and (iii) in a powerful synergy with JWST, to enable characterization of the bulk mineralogy and bulk elemental composition of exoplanetary material. This white paper argues that continued UV spectroscopic capabilities with HST is a high-return investment for white dwarf and exoplanet science, and preserving and prioritizing HST's UV capabilities through at least 2035 is crucial to maximize the scientific return from HST, JWST, and HWO.

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arXiv2026

Spectroscopic Characterization of WD J000801.25-350450 and its Two Co-Moving Companions

We present new spectroscopic data for Gaia DR3 2309499817384726016 (WD0008-350A) and its two wide, co-moving, low-mass companions. We confirm the white dwarf is a hydrogen-rich DA, with T$_{\rm eff}$=6200$\pm$90~K and a mass of 0.63$\pm$0.03~M${\odot}$, close to that of the average white dwarf. Near-infrared spectra of the two stellar companions to WD0008-350A reveal that the inner companion is an M dwarf, exhibiting a spectral type of M8. Furthermore, the outer companion is identified as a possible M6 + M9 binary. This paper examines the evidence which suggests the system may be quadruple.

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arXiv2026

Three Thousand Motion-Confirmed L and T Dwarf Candidates from the Backyard Worlds:~Planet 9 Citizen Science Project

The Backyard Worlds: Planet 9 citizen science project uses data from the Wide-field Infrared Survey Explorer to detect infrared objects with significant motion. In this work, we present the majority of the L and T dwarf candidates discovered through this effort. For each candidate, we provide proper motion measurements as well as optical, near-infrared, and mid-infrared photometry (when available), photometric spectral types and distance estimates. Three thousand and six new motion-confirmed discoveries are presented in this work, 2,357 with L-type photometric spectral types and 649 with T-type photometric spectral types. We also present an additional 80 objects as likely L or T dwarfs based on available photometry, but for which a significant motion measurement could not be obtained. We identify 28 objects in this sample as new comoving companions to higher-mass stars, and an additional 9 sources that are candidate binary systems made up of two ultracool dwarfs of L-type or later. Follow-up spectroscopic observations will be necessary to confirm spectral types and further characterize the sources discovered through this project. This work presents the largest single sample of motion-confirmed L and T dwarf discoveries to date, which would more than double the number of known L and T dwarfs, if confirmed. We wish to sincerely thank our citizen scientist collaborators for their monumental efforts that have directly impacted this project's success.

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arXiv2026

Unraveling the Brown Dwarf Desert: Four New Discoveries and a Unifying, Period-Coded Picture

We present four newly validated transiting brown dwarfs identified through TESS photometry and confirmed with high-precision radial velocity measurements obtained from the FEROS and PLATOSpec spectrographs. Notably, three of these companions exhibit orbital periods exceeding 100 days, thereby expanding the sample of long-period transiting brown dwarfs from two to five systems. The host stars of long-period brown dwarfs show mild subsolar metallicity. These discoveries highlight the expansion of the metal-poor, long-period distribution and help us better understand the brown dwarf desert. In our comparative analysis of eccentricity and metallicity demographics, we utilize catalogues of long-period giant planets, brown dwarfs, and low-mass stellar companions. After accounting for tidal influences, the eccentricity distribution aligns with that of low-mass stellar binaries, presenting a different profile than that observed within the giant planet population. Additionally, the metallicity of the host stars reveals a noteworthy trend: short-period transiting brown dwarfs are predominantly associated with metal-rich stars, whereas long-period brown dwarfs are more often found around metal-poor stars, demonstrating statistical similarities to low-mass stellar hosts. This trend has also been previously observed in studies of hot and cold Jupiters and points to a period-coded mixture of channels. A natural explanation is that most brown dwarfs originate from fragmentation at wider separations, with long-period systems retaining this stellar-like imprint, while only those embedded in massive, long-lived, metal-rich protoplanetary discs are efficiently delivered and stabilised to short orbits.

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arXiv2026

Transiting exoplanets as the immediate future for population-level atmospheric science

The transit method, during which a planet's presence is inferred by measuring the reduction in flux as it passes in front of its parent star, is a highly successful exoplanet detection and characterization technique. During transit, the small fraction of starlight that passes through a planet's atmosphere emerges with the fingerprints of atmospheric gases, aerosols and structure. During eclipse, the relatively small contribution of light from the planet itself may be observed as the planet is occulted by the star. For planets in near-edge-on, short-period orbits, observing the system throughout an entire orbit allows the varying flux from the planet to be extracted as the illuminated `dayside' rotates in and out of view. With spectroscopic observations, we can characterize not only the overall composition of the atmosphere, but also glean insights into atmospheric structure and dynamics. With over 6,000 transiting planets now discovered, such observations are currently our only window into a consistent sample of planetary atmospheres large enough to attempt a population-level study, a critical next step for our understanding of atmospheric science. The vast exoplanet population provides a laboratory for atmospheric physics, including chemistry, dynamics, cloud processes and evolution; extending these results to a larger number of targets will allow us to explore the effects of equilibrium temperature, gravity, mass and parent star type on atmospheric properties, as well as map observable trends to formation scenarios. These findings are critical for addressing STFC's Science Vision Challenge B: how do stars and planetary systems develop and how do they support the existence of life?. Here we outline how the UK must fit within this strategic context, suggest approaches and development for the future, and outline the unique capabilities and leadership of scientists across the UK.

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arXiv2025

A search for late-type brown dwarfs in the Euclid Quick Data Release 1

We present the identification and characterization of 15 mid-to-late T dwarf candidates in the Euclid Quick Release 1 (Q1) dataset, based on a combined photometric and spectroscopic analysis. Candidates were initially selected via color-based cuts in the Euclid $Y_E - J_E$ and $J_E - H_E$ color-color space, targeting the region occupied by ultracool dwarfs in synthetic photometry from the Sanghi et al. (2024) sample. From an initial pool of 38,845 sources, we extracted low-resolution near-infrared spectra from the Euclid NISP instrument and applied a two-stage validation procedure that included spectral template fitting followed by visual inspection. Eight of the 15 validated candidates are newly identified objects with no prior literature association. We examined their morphological and photometric properties and compared them with established spectral standards. Photometric distances were derived using band-averaged distance modulus estimates. We discuss the limitations and promise of the Euclid survey for ultracool dwarf studies, and demonstrate the potential for discovering substellar populations beyond the reach of current wide-field surveys.

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arXiv2025

A 43 day transiting Neptune and two 25 day Saturns from TESS, NGTS and ASTEP

Beyond orbital periods of 10 days, there is a dearth of known transiting gas giants. On longer orbits, planets are less affected by their host star, and become ideal probes of planet formation, migration and evolution. We report the discovery of a long period Neptune and two Saturns, each initially identified as single transits in the TESS photometry, and solved through additional transits from ground-based follow-up photometric observations by NGTS and ASTEP. High-resolution radial velocity mass measurements using CORALIE and HARPS confirm their planetary nature. From joint modelling of the photometric and spectroscopic data, we determine an orbital period of $43.12655_{-0.00017}^{+0.00012}~$days, radius of $3.65\pm0.22~\mathrm{R_{\oplus}}$, and mass of $19.1_{-4.5}^{+4.9}~\mathrm{M_{\oplus}}$ for NGTS-34b, making it one of the longest period well-characterized transiting Neptunes. Orbiting a late F-type star, bright in the K-band (Kmag$~\simeq7.9$), it is amenable for cool atmosphere studies using JWST or Ariel. TOI-4940b is a small Saturn on a $25.867811_{-0.000056}^{+0.000058}~$day orbit with a radius of $6.61\pm0.37~\mathrm{R_{\oplus}}$ and an upper mass limit $<89~\mathrm{M_{\oplus}}$. NGTS-35b(=TOI-6669b) is a larger Saturn on a $25.241192\pm0.000022~$day, moderately eccentric orbit ($e = 0.192_{-0.033}^{+0.037}$), with a radius of $10.90\pm0.65~\mathrm{R_{\oplus}}$ and a mass of $152_{-19}^{+22}~\mathrm{M_{\oplus}}$. With an assumed albedo $A=0.3$, each of these planets has an equilibrium temperature below 700K, with NGTS-35b especially cold at $450~$K. These three giants add to the small but growing population of long period planets that can further our understanding of planet formation mechanisms.

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arXiv2025

TOI-333b: A Neptune Desert planet around a F7V star

Observations have shown that planets similar to Neptune are rarely found orbiting Sun-like stars with periods up to ~4 days, defining the so-called Neptune desert region. Therefore, the detection of each individual planet in this region holds a high value, providing detailed insights into how such a population came to form and evolve. Here we report the detection of TOI-333b, a Neptune desert planet with a mass, radius, and bulk density of 20.1 $\pm$ 2.4 M$_{\oplus}$, 4.26 $\pm$ 0.11 R$_{\oplus}$, and 1.42 $\pm$ 0.21 \gccc, respectively. The planet orbits a F7V star every 3.78 d, whose mass, radius and effective temperature are of 1.2 $\pm$ 0.1 \msun, 1.10 $\pm$ 0.03 \rsun, and 6241$^{+73}_{-62}$ K, respectively. TOI-333b is likely younger than 1 Gyr, which is supported by the presence of the doublet Li line around 6707.856 textup{~Å} and its comparison to Li abundances in open clusters with well constrained ages. The planet is expected to host only 8.5$^{+10.9}_{-8.3}\%$ gas-to-core mass ratio for a H/He envelope. On the other hand, irradiated ocean world models predict 20$^{+11}_{-10}\%$ H$_2$O mass fraction with a core fraction of 35$^{+20}_{-23}\%$. Therefore, we expect that TOI-333b internal composition may be dominated by a pure rocky composition with almost no H/He envelope, or a rocky world with almost equal mass fraction of water. Finally, TOI-333b is more massive and larger than 77$\%$ and 82$\%$ of its Neptune desert counterparts, respectively, while its host ranks among the hottest known for Neptune Desert planets, making this system a unique laboratory to study the evolution of such planets around hot stars.

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arXiv2025

NGTS-11 c: a transiting Neptune-mass planet interior to the warm Saturn NGTS-11 b

We report the discovery of NGTS-11 c, a transiting warm Neptune ($P \approx 12.8$ d; $M_{p} = 1.2^{+0.3}_{-0.2} M_{\mathrm{Nep}}$; $R_{p} = 1.24 \pm 0.03 R_{\mathrm{Nep}}$), in an orbit interior to the previously reported transiting warm Saturn NGTS-11 b ($P \approx 35.5$ d). We also find evidence of a third outer companion orbiting the K-dwarf NGTS-11. We first detected transits of NGTS-11 c in TESS light curves and confirmed them with follow-up transits from NGTS and many other ground-based facilities. Radial-velocity monitoring with the HARPS and FEROS spectrographs revealed the mass of NGTS-11 c and provides evidence for a long-period companion ($P > 2300$ d; $M_{p} \sin i > 3.6 M_{\mathrm{Jup}}$). Taking into account the two additional bodies in our expanded datasets, we find that the mass of NGTS-11 b ($M_{p} = 0.63 \pm 0.09 M_{\mathrm{Sat}}$; $R_{p} = 0.97 \pm 0.02 R_{\mathrm{Sat}}$) is lower than previously reported ($M_{p} = 1.2 \pm 0.3 M_{\mathrm{Sat}}$). Given their near-circular and compact orbits, NGTS-11 c and b are unlikely to have reached their present locations via high-eccentricity migration. Instead, they probably either formed in situ or formed farther out and then underwent disk migration. A comparison of NGTS-11 with the eight other known systems hosting multiple well-characterized warm giants shows that it is most similar to Kepler-56. Finally, we find that the commonly used 10-day boundary between hot and warm Jupiters is empirically well supported.

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arXiv2025

NGTS-EB-8: A double-lined eclipsing M+M binary discovered by citizen scientists

We report the identification and characterization of a new binary system composed of two near-equal mass M-dwarfs. The binary NGTS-EB-8 was identified as a planet candidate in data from the Next Generation Transit Survey (NGTS) by citizen scientists participating in the Planet Hunters NGTS project. High-resolution spectroscopic observations reveal the system to be a double-lined binary. By modeling the photometric and radial velocity observations, we determine an orbital period of 4.2 days and the masses and radii of both stars to be $M_A=0.250^{+0.005}_{-0.004}$ M$_{\odot}$, $M_B=0.208^{+0.005}_{-0.004}$ M$_{\odot}$, $R_A=0.255^{+0.004}_{-0.005}$ R$_{\odot}$, $R_B=0.233^{+0.006}_{-0.005}$ R$_{\odot}$. We detect Balmer line emission from at least one of the stars but no significant flare activity. We note that both components lie in the fully convective regime of low-mass stars ($<0.35$ M$_{\odot}$), therefore can be a valuable test for stellar evolutionary models. We demonstrate that the photometric observations, speckle imaging and initial radial velocity measurements were unable to identify the true nature of this system and highlight that high-resolution spectroscopic observations are crucial in determining whether systems such as this are in fact binaries.

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arXiv2025

Silicate Precursor Silane detected in Cold Low-Metallicity Brown Dwarf

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 (170K) and ~500K. 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 <0.01xsolar. 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 eluded detection 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. On the metal-poor world W1534, we detect a clear signature of SiH4 centered at ~4.55 microns with an abundance of 19+/-2 parts per billion (ppb). Our chemical modelling suggests that this SiH4 abundance may be quenched at ~kilobar levels just above the silicate cloud layers, whereupon 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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