Pointing to the Poles of Brown Dwarfs: Polar Vortex Possibilities

Somewhere between planets and stars is a cool, cloudy class of objects called brown dwarfs. A new study suggests that swirling clouds at the poles of these intriguing objects may provide clarity on their observed characteristics.

Trendy Brown Dwarfs

Not quite big enough to spark hydrogen fusion, but not quite small enough to lump in with gas giant planets, brown dwarfs sit in a perplexing in-between space and are not yet fully understood. As space-based observations of brown dwarfs have evolved, recent studies have revealed long-term variations in the brightness of brown dwarfs as well as unexplained trends in their color and spectral characteristics depending on from which angle we happen to view them. 

Previous studies have theorized that these variations are driven by changes in atmospheric properties of brown dwarfs. Changes in the rotation of clouds in a brown dwarf’s atmosphere can explain the observed short-term variability, but the observed long-term variations are not well explained by this same property. Could there be a single contributor driving both the observed short- and long-term variations in brown dwarfs? 

Polar Vortex Potential

Two astronomers at the University of Arizona, Nguyen Fuda and Dániel Apai, posit that the color– and spectral–inclination trends observed in brown dwarfs could be driven by polar vortices — an expanse of swirling air sitting atop the pole of a planet. To test this hypothesis, the authors explore three model atmospheres (no vortex, evolving vortex, and stationary vortex) and compare the simulated observations of each to understand how the presence or absence of a polar vortex may impact brown dwarf observations over time.

Depending on the viewing angle, we may see primarily equatorial cloud bands or we may see a significant portion or all of the brown dwarf’s pole. By making simulated observations across a series of inclinations, the authors find that both the evolving and stationary vortex models produce color–inclination variations that align with previous observations of brown dwarfs — polar vortices tend to have bluer infrared colors compared to the redder equatorial regions.

All three scenarios, over short-term monitoring, exhibit variability–inclination trends. As more and more active equatorial bands become visible, short-term variations in brightness increase. On the other hand, over long-term monitoring, the evolving vortex scenario produces a variability-inclination trend opposite to that seen in the stationary and no vortex scenarios.

Solar System Similarities

Brown dwarfs share similar characteristics to gas giants, like those in our own solar system. Many observations of Jupiter, Saturn, Uranus, and Neptune reveal clear vortex-dominated poles that produce similar color and brightness variability. These planets, though smaller than brown dwarfs, show variations from pole to equator that grant validity to the idea of polar vortices being present in brown dwarf atmospheres.

What does this mean moving forward? The authors suggest that, based on their results, brown dwarf atmospheres are more complex than simpler, non-changing brown dwarf atmosphere models that have traditionally been used. As more long-term space-based observations of brown dwarfs become available, the polar vortex hypothesis can be tested further, allowing astronomers to unwind some of the mysteries of brown dwarfs. 

Citation

“The Polar Vortex Hypothesis: Evolving, Spectrally Distinct Polar Regions Explain Short- and Long-term Light-curve Evolution and Color–Inclination Trends in Brown Dwarfs and Giant Exoplanets,” Nguyen Fuda and Dániel Apai 2024 ApJL 975 L32. doi: 10.3847/2041-8213/ad87e9