Beyond the Fast Rotators: A JWST Multi-Cycle Treasury Survey of Substellar Weather

Natalia Oliveros-Gomez; Elena Manjavacas; Niall Whiteford; Caprice Phillips; Jared Bull; Kielan Hoch; Allison M. McCarthy; Beatriz Campos Estrada; Cian O'Toole; Evert Nasedkin; Stanimir Metchev; Gabriel-Dominique Marleau; Harshil Kothari; Antonia von Stauffenberg; Michael K. Plummer; Brianna I. Lacy; Fuda Nguyen; Xueqing Chen; Fei Wang; Genaro Súarez; Ben J. Sutlieff

AI summary · arXiv: 2611.13260v1 · astro-ph.IM

Beyond the Fast Rotators: A JWST Multi-Cycle Treasury Survey of Substellar Weather

Natalia Oliveros-Gomez, Elena Manjavacas, Niall Whiteford, Caprice Phillips, Jared Bull, Kielan Hoch, Allison M. McCarthy, Beatriz Campos Estrada,

and 13 more authors

Cian O'Toole, Evert Nasedkin, Stanimir Metchev, Gabriel-Dominique Marleau, Harshil Kothari, Antonia von Stauffenberg, Michael K. Plummer, Brianna I. Lacy, Fuda Nguyen, Xueqing Chen, Fei Wang, Genaro Súarez, Ben J. Sutlieff

Original paper Submitted to arXiv on 09 Nov. 2026 White paper submitted to the "Multi-Cycle Treasury Program Concepts with JWST" call, Nov 2026 License: CC BY 4.0 Also in: fr

How to read this summary? Illustrated guide

The essentials

The paper proposes a roughly 1,500-hour JWST survey to systematically study how rotation, viewing geometry, and atmospheric evolution shape brown dwarf and giant-exoplanet weather.

Question

How do rotation, viewing geometry, and atmospheric evolution shape substellar weather, and how can JWST distinguish the physical mechanisms behind variability across brown dwarfs? The paper asks how to address gaps left by heterogeneous observations of a limited range of targets.

Method

This is a proposed survey, not a report of completed observations. It outlines a stratified sample across L, T, and Y spectral types, rotation periods, and viewing geometries, using time-resolved NIRSpec/PRISM spectroscopy with complementary MIRI observations. For an illustrative sample of 30 objects with a mean rotation period of 9 hours, it estimates about 540 hours for initial two-rotation observations and another 540 hours to revisit half the sample at two additional epochs; the total allocation is estimated at about 1,500 hours including mid-infrared observations and overheads.

Findings

Result Existing JWST time-resolved spectroscopy shows wavelength-dependent variability that cloud heterogeneity alone may not explain.
See the passage in the paper
Recent JWST time-resolved spectroscopy has revealed complex wavelength- and pressure-dependent variability that heterogeneous clouds alone cannot always explain.
Result The proposed survey would revisit about half its sample at three epochs, including observations months and more than a year apart, to track changes in variability.
See the passage in the paper
Approximately half of the sample would be revisited at three epochs in total: initially, several months later, and more than one year after the first observation (following similar strategy of GO 8846).
Authors’ interpretation A consistent observing and processing strategy is intended to enable population-level comparisons and tests of atmospheric circulation models.
See the passage in the paper
This coordinated effort would provide a consistently designed and uniformly processed dataset of time-resolved spectra and variability diagnostics, enabling population-level comparisons, testing predictions from three-dimensional circulation models, and establishing a framework for interpreting the atmospheric diversity and evolution of brown dwarfs and directly imaged giant exoplanets.
Result The illustrative program estimates a total allocation on the order of 1,500 hours, with the final request dependent on observing assumptions and feasibility.
See the passage in the paper
Including these observations and operational overheads, the anticipated allocation would be on the order of 1500 hours under preliminary assumptions.

Limitations

The proposed allocation is preliminary and depends on the sample’s rotation-period distribution, sensitivity requirements, and observing feasibility.
See the passage in the paper
The final time request would depend on the rotational-period distribution, sensitivity requirements, and observing feasibility.
The survey proposal does not specify a finalized target list or a fully fixed observing-time request.
See the passage in the paper
The final time request would depend on the rotational-period distribution, sensitivity requirements, and observing feasibility.

Why it matters

The authors argue that a coordinated, multi-cycle survey would fill spectral-type and rotational-regime gaps and make variability measurements more directly comparable. Its uniformly processed public dataset would provide a benchmark for population studies, atmospheric models, and future exoplanet spectroscopy.

Written by AI from the paper. Passages are checked word for word against the text we analysed; the rest may contain errors.

AI-generated keywords: Brown dwarfs; JWST; atmospheric variability; substellar clouds; time-resolved spectroscopy

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Comprehensive Summary
Layman's Summary
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Original abstract

The paper proposes a JWST Multi-Cycle Treasury survey to study atmospheric variability in brown dwarfs and directly imaged giant exoplanets. It asks how rotation, viewing angle, and atmospheric evolution shape this variability, and how observations can distinguish among clouds, temperature changes, atmospheric circulation, and chemical disequilibrium. Rather than reporting results from a completed survey, the paper presents a proposed observing program. Its motivation comes from time-resolved observations showing that variability differs across wavelengths and atmospheric pressures. For example, WISE 1049B exhibits stronger variability in carbon monoxide and methane than in water, even when the signals probe comparable pressures. Studies of SIMP 0136 also find different variability patterns across atmospheric depths and spectral regions. Together, these findings suggest that cloud coverage alone cannot explain all observed variability. Mid-infrared observations could provide additional information about silicate clouds through absorption features around 7–11 micrometres. The paper also highlights the possible influence of viewing geometry. Objects viewed closer to equator-on tend to have larger near-infrared amplitudes, redder colours, and stronger silicate absorption than those viewed nearer pole-on. However, the evidence comes from small, heterogeneous samples, making it difficult to distinguish these trends from intrinsic differences between objects. The proposed survey would address gaps in spectral type, rotation rate, viewing geometry, and monitoring duration. It would use a stratified sample spanning L, T, and Y dwarfs, including fast, intermediate, and slow rotators. Where independent measurements are available, the sample would also cover a range of inclinations. Time-resolved NIRSpec/PRISM spectroscopy would provide a consistent core dataset, complemented by MIRI/LRS or MIRI/MRS observations. A selected subset, particularly objects near the L/T transition, could receive MIRI time-series observations to track changes in silicate absorption and cloud properties. Each target would be monitored over two rotations per visit, with about half the sample revisited at three epochs: initially, several months later, and more than a year afterward. The resulting data products would include uniformly reduced spectra, spectroscopic light curves, and standardized variability measurements for comparison with atmospheric models. The authors also propose analyses such as principal component analysis and time-series retrievals. For an illustrative sample of 30 objects with a mean rotation period of 9 hours, spanning 2–20 hours, the authors estimate approximately 540 hours for initial NIRSpec observations covering two rotations per target. Two additional epochs for half the sample would require another 540 hours. Complementary MIRI observations are estimated to take roughly 60–250 hours, depending on target brightness, instrument configuration, and observing strategy. Including overheads, the total is approximately 1,500 hours, though the final request would depend on target rotation periods, sensitivity requirements, and feasibility. The paper concludes that coordinated, repeated observations could enable population-level tests of how atmospheric structure varies with rotation, inclination, temperature, and time. The survey would also create a consistent public resource for studying substellar and exoplanet atmospheres. Its limitations stem mainly from its status as a proposal: it reports no new survey measurements, and its sample and observing strategy are illustrative rather than finalized. Time estimates remain preliminary, inclination constraints will be unavailable for some targets, and existing observations are too heterogeneous to establish the proposed physical relationships on their own.

Beyond the Fast Rotators: A Plan to Track Weather on Brown Dwarfs and Giant Exoplanets

Brown dwarfs and giant planets beyond our Solar System are not static balls of gas. Their atmospheres can change as clouds form and disperse, temperatures shift, and gases circulate or react. A new proposal, “Beyond the Fast Rotators: A JWST Multi-Cycle Treasury Survey of Substellar Weather,” outlines how the James Webb Space Telescope (JWST) could investigate these changes across a broad range of objects.

Written by Natalia Oliveros-Gomez, Elena Manjavacas, Niall Whiteford, Caprice Phillips, Jared Bull, Kielan Hoch et al., the paper presents a proposed observing program, not results from a completed survey. Its central goal is to understand how rotation, viewing angle, and atmospheric evolution shape observed variability—and to determine which physical processes are responsible.

Why look beyond changing clouds?

When a brown dwarf rotates, different parts of its atmosphere come into view. If those regions differ in brightness or composition, the object’s measured light changes over time. Watching that variability across wavelengths can reveal structures at different atmospheric depths.

Past observations suggest the story is more complex than a simple patchwork of clouds. For example, WISE 1049B shows stronger variability in carbon monoxide and methane than in water, even though the signals probe comparable atmospheric pressures. Studies of SIMP 0136 likewise find that variability patterns differ between spectral regions and atmospheric depths.

These differences matter because they point to several possible drivers: changes in cloud coverage, temperature variations, atmospheric circulation, or chemical disequilibrium. Cloud cover alone may not explain all the observed patterns. Mid-infrared measurements could add a useful clue by capturing silicate-cloud absorption features around 7–11 micrometres.

Does the viewing angle change the picture?

An object’s orientation may affect how much of its atmospheric pattern is visible. Objects viewed closer to equator-on have been associated with larger near-infrared variability, redder colours, and stronger silicate absorption than objects viewed nearer pole-on.

For now, those trends are suggestive rather than conclusive. Existing samples are small and combine observations made with different methods, so it is difficult to separate the effect of viewing geometry from natural differences between objects. The proposed survey aims to make comparisons more reliable by observing a more deliberately structured sample.

A coordinated survey across many kinds of worlds

The plan would cover L, T, and Y dwarfs, spanning a range of temperatures and atmospheric properties. It would include fast, intermediate, and slow rotators, and—when independent measurements are available—objects with different viewing inclinations. That breadth is intended to address gaps in earlier studies, which have often focused on a narrower set of targets or rotation rates.

Time-resolved NIRSpec/PRISM spectroscopy would provide a consistent foundation for the observations. MIRI/LRS or MIRI/MRS measurements would complement it, adding access to mid-infrared features. A selected group of targets, particularly objects near the L/T transition, could receive additional MIRI time-series observations to track changes in silicate absorption and cloud properties.

The observing plan calls for monitoring each target over two rotations per visit. About half the sample would be revisited at three stages: an initial visit, another several months later, and a further visit more than a year afterward. Repeated observations could help distinguish recurring rotational patterns from longer-term atmospheric evolution.

From telescope time to shared data

For an illustrative sample of 30 objects with a mean rotation period of nine hours, and periods ranging from two to 20 hours, the paper estimates about 540 hours for initial NIRSpec observations covering two rotations per target. Two additional epochs for half the sample would require roughly another 540 hours. Complementary MIRI observations could take around 60–250 hours, depending on target brightness, instrument setup, and observing strategy.

Including overheads, the total estimate is approximately 1,500 hours. This is a planning figure rather than a finalized request: the actual time needed would depend on the chosen targets, their rotation periods, sensitivity requirements, and feasibility assessments.

The expected products would include uniformly reduced spectra, spectroscopic light curves, and standardized measurements of variability. The researchers also propose using tools such as principal component analysis and time-series retrievals to compare observations with atmospheric models. Making the resulting resource consistent and public could support further work on brown dwarfs and directly imaged giant exoplanets.

A proposal with important open questions

The survey’s strength would be its coordinated, repeated approach. By examining objects across spectral type, rotation rate, viewing angle, and time, researchers could test how atmospheric structure relates to these factors at the population level.

But the proposal does not yet answer those questions. It reports no new survey measurements, and its sample and observing strategy are illustrative rather than final. Telescope-time estimates remain preliminary, while inclination information will be unavailable for some targets. Existing observations are also too heterogeneous to establish the proposed physical relationships on their own.

If carried out, the program could provide a much more consistent view of substellar weather—and a shared foundation for testing how clouds, chemistry, circulation, and temperature combine to shape atmospheric variability.

Created on 11 Nov. 2026

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Beyond the Fast Rotators: A JWST Multi-Cycle Treasury Survey of Substellar Weather

The essentials

Question

Method

Findings

Limitations

Why it matters

Read further

Definitions

Beyond the Fast Rotators: A Plan to Track Weather on Brown Dwarfs and Giant Exoplanets

Why look beyond changing clouds?

Does the viewing angle change the picture?

A coordinated survey across many kinds of worlds

From telescope time to shared data

A proposal with important open questions

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