Cool Stars 23 splinter session
This session aims to bring brown dwarf and exoplanet scientists together to capture a full synergetic picture of substellar atmospheres of both brown dwarf companions and exoplanets, as well as isolated brown dwarfs, to share what our current understanding is based on recent JWST results, and what developments are necessary in the future to address unanswered questions.
Since the last edition of Cool Stars, a wealth of new and exciting science results have emerged from the vast volume of data collected by the James Webb Space Telescope (JWST). We are receiving weather reports from cold substellar worlds, revealing unprecedented detail in the atmospheric physics that resides on these cool objects. This stream of observations with JWST has led the field of cool brown dwarfs and giant gaseous exoplanets into a data-rich environment, consistently challenging existing models, especially at the coldest end of this regime (<500 K).
Even within the past year, results from JWST include the direct detection of the coldest exoplanet yet, 14 Her c, with hints of carbon disequilibrium chemistry and water-ice clouds in its atmosphere (Bardalez Gagliuffi et al. 2025); the first direct detection of silicate clouds on an exoplanet (Hoch et al. 2025) within the multi-planet system YSES-1; and advanced data processing codes that have been developed to enhance JWST/NIRSpec and MIRI’s ability to extract high-contrast spectra of these cold objects directly for the first time in a wavelength range where the bulk of the luminosity of these cold objects resides (e.g., Ruffio et al. 2024).
There are many ties to be made since access to new direct spectra of mass-benchmark brown dwarfs can be compared and contrasted with high resolution direct spectra from isolated brown dwarfs, enabling a new area of brown dwarf science to be explored.
Furthermore, JWST has opened up a wavelength range (>3 micron) beyond what we are sensitive to from the ground that is fundamental in understanding the underlying properties of the coldest brown dwarfs and exoplanets, as never seen before. In the cases where ground-based observational facilities have the capability of observing in this wavelength range, they have been lacking in the spectral resolution and sensitivity required to deep dive into the atmospheric properties of the coldest substellar objects. The access to this wavelength range and spectral resolution continues to expose some of both the similarities and differences that exist in these cold substellar objects.