Universe Today
When astronomers want to understand brown dwarfs, they often turn to WISE 1049AB. It's a benchmark brown dwarf in astronomy, and the closest and brightest brown dwarf we know of. The binary pair, which is also known as Luhman 16, is about 6.5 light-years away. Brown dwarfs are a crucial bridge between planets and stars, and understanding them helps astronomers understand the dynamics of both exoplanets and stars.
Understanding how exoplanets behave is a major goal in exoplanet science. Scientists need a solid understanding of atmospheres to understand potential habitability, atmospheric chemistry, and potential biosignatures. Since brown dwarfs are sub-stellar objects that fall between low-mass stars and high-mass planets, studying their atmospheres can help astronomers develop better General Circulation Models(GCM).
New research to be published in the Monthly Notices of the Royal Astronomical Society observed Luhman 16 with the JWST's NIRSpec and MIRI instruments, the second time researchers have done so. Combined with the previous epoch of observations, they create the longest baseline weather monitoring for a brown dwarf to date.
The research is titled "The JWST weather report from the nearest brown dwarfs II: Consistent variability mechanisms over 7 months revealed by 1-14 m NIRSpec + MIRI monitoring of WISE 1049AB." The lead author is Xueqing Chen from the Institute for Astronomy at the University of Edinburgh.
"Our 8-hour MIRI low-resolution spectroscopy (LRS) and 7-hour NIRSpec prism observations extended variability measurements for any brown dwarfs beyond 11 𝜇m for the first time, reaching up to 14 𝜇m," the authors write.
This is significant because it means the observations capture the lower and upper atmospheres and how small silicate grains behave in them. Silicate grains play an important role by forming cloud layers at specific temperatures and pressures. They're especially prominent in L-type brown dwarfs, and Luhman 16 consists of an L-type brown dwarf and one on the transition between L-type and T-type.
Brown dwarfs bridge the gap between the most massive gas giant planets and the lowest-mass stars. They have only a tiny percentage of the mass of our Sun. They lack the mass to fuse hydrogen into helium like main-sequence stars, and instead can only fuse deuterium. They emit very little radiation and are hard to spot. Fortunately, the JWST was built to spot and study objects like them. Image Credit: NASA, ESA, Joseph Olmsted (STScI)
"Occupying the mass range between the heaviest gas giant planets and the lightest stars, brown dwarfs offer a unique window to study atmospheric processes in ultracool environments," the authors write. Since they're fast rotators that complete rotations in hours, time-resolved studies like these let astronomers capture the inconsistency in their atmospheres and the underlying mechanisms responsible.
Previous observations of brown dwarfs in the optical, near-infrared, and mid-infrared showed that variability is common in their atmospheres. Scientists have proposed that patchy silicate clouds could be responsible, and by extending observations beyond 11 𝜇m and up to 14 𝜇m, the researchers were able to observe silicate particles for the first time.
This figure from the research shows the range of wavelengths observed in the study. Silicate particles were observed for the first time in a brown dwarf atmosphere. Image Credit: Chen et al. 2025, MNRAS.
Because of the highly complex and dynamic nature of these atmospheres, time-resolved spectroscopic observations with a wide simultaneous wavelength coverage are required to capture various effects at once," the authors write. "This approach allows us to obtain a comprehensive 3-D view of the atmosphere and disentangle the variability-driving mechanisms at all atmospheric depths.
To understand their meaning, the researchers compared their results to a general circulation model.
They found dramatic light curve shapes in the atmosphere's deepest layers (1–2.5 𝜇m) and concluded that patchy clouds cause these.
They found that the high-altitude levels between 2.5–3.6 𝜇m and 4.3–8.5 𝜇m are caused by hot spots from temperature/chemical variations of molecular species like carbon monoxide and methane.
They also found that small-grain silicates "potentially" contribute to the variability of WISE 1049A at 8.5-11 𝜇m. They found no indication of silicate in its partner.
This figure shows NIRSPec light curves for both brown dwarfs. Note the dramatic light curve shapes in the lower atmosphere from 1–2.5 𝜇m, especially for WISE1049B. The authors say that patchy clouds are responsible. Image Credit: Chen et al. 2025, MNRAS.
"While distinct atmospheric layers are governed by different mechanisms, we confirmed for the first time that each variability mechanism remains consistent within its layer over the long term," the authors write in their conclusion.
To determine whether these mechanisms are consistent over the long term on other brown dwarfs, the authors say that "expanded JWST variability surveys across the L-T-Y sequence" are necessary. This will allow them to "... trace and understand variability mechanisms across a wider population of brown dwarfs and planetary-mass objects," they conclude.
