Using images from the James Webb Space Telescope (Webb), an international research team including Western's Stanimir Metchev has discovered new answers to explain how some brown dwarfs form giant dust storms, contradicting previous assumptions. These storms may look similar to Jupiter's iconic Great Red Spot, but the new study, led by Shanghai Jiao Tong University, shows they actually form quite differently.
The findings were published in the high impact journal, Science Advances.
Stanimir Metchev
Brown dwarfs are celestial objects with more mass than a giant planet and less than a small star. Unable to sustain hydrogen fusion in their cores, they cool after formation. After billions of years, they reach low temperatures, becoming highly similar to giant planets.
Some brown dwarfs are 10 times heavier than Jupiter, so they're sometimes called "super-Jupiters." Because they resemble giant planets and demonstrate strong space weather activity, they make great test analogues for studying how the atmospheres of giant exoplanets (planets that orbit stars other than our Sun) behave.
"Astronomers have generally assumed that brown dwarfs behave like Jupiter, with strong east-west bands and stable storms shaping their skies," said Metchev, a physics and astronomy professor. "Our research challenges that idea, suggesting that some of these worlds don't follow Jupiter's pattern after all. By modelling a super-Jupiter's changing light patterns, we show its atmosphere may circulate in a fundamentally different way."
Findings build on previous brown dwarf research
Over the past decade, Metchev and his team have established the prevalence of atmospheric storms on brown dwarfs, and more recently, the ubiquity of dust (or "sand") clouds on warm brown dwarfs like VHS 1256B, the one studied in the latest research. Metchev previously teamed up with Xianyu Tan, senior author of the new study and a T.D. Lee Fellow at Shanghai Jiao Tong University, and others for a study that conducted Webb observations that directly detected the dust in the clouds of this super-Jupiter.
The previous study showed VHS 1256B produced extremely high 'large-amplitude variability,' meaning its perceived brightness rises and falls significantly over time. This brightness variance usually signals dramatic atmospheric features, such as the giant dust storms now discovered.
Based on these findings, Tan, Metchev and their collaborators simulated VHS 1256B's atmosphere to match its large-amplitude variability and comparing this new model with observational data from Webb. The simulated model shows VHS 1256B's variability is caused quite differently than Jupiter's, stemming from large-scale equatorial waves.
These waves, created by a temperature imbalance as clouds near the equator heat the atmosphere, produce large east-west-moving dust storms. This phenomenon, known as 'cloud-radiative feedback,' explains both the strong brightness changes seen on VHS 1256B and the slow, long-term shifts in its light curve caused by dust storms that move and change over time.
"The cloud-radiative feedback circulation mechanism has been proposed, but for a model to be truly validated, it must withstand the test of observations, which is perhaps even more challenging than proposing the theory itself," said Tan. "Fortunately, successful comparisons with the VHS 1256B observations provided this opportunity, verifying the feasibility of the new circulation mechanism. Of course, we cannot yet completely rule out other potentially important mechanisms."
Super-Jupiters: Why so different?
The study also shows that super-Jupiters like VHS 1256B behave very differently from Jupiter mainly because their atmospheres are much hotter and respond to radiation much more strongly. This rapid response creates large-scale equatorial waves and prevents Jupiter-like multiple zonal banding (or rings), while Jupiter's cooler atmosphere builds these rings by a slower turbulent process.
"The mechanism of giant planets' atmospheric circulation has long been an important and unresolved question in planetary science," said Xi Zhang, a professor at UC Santa Cruz and a key collaborator on the study. "These novel wave dynamical processes on super-Jupiters provide us with a unique perspective to examine our fundamental understanding of this problem."
