成果发布 | 超级木星是否和木星长得一样？不一定。

Brown dwarfs are celestial objects between giant planets and stars. 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. Consequently, they are often used as proxies to study giant planets. Some brown dwarfs have masses of only about ten times that of Jupiter and are termed “super-Jupiters”.

Most brown dwarfs are variable. These variabilities originate from their patchy surfaces, with rotation causing periodic changes in observed brightness. Some brown dwarfs show significant non-periodic variations, indicating intense "weather" activity, making them ideal laboratories for studying the atmospheric circulation of extrasolar giant planets.

Astronomers have long assumed that brown dwarfs’ atmospheric circulation resemble Jupiter's, characterized by prominent, multiple zonal (east-west) banding and stable vortices analogous to the Great Red Spot. This Jupiter-like circulation has been the traditional picture for explaining brown dwarfs’ variability (Fig. 1). However, a recent research challenges this view. By constructing models to explain the light curves and spectrum of a super-Jupiter, the team reveals that the super-Jupiter may possess an atmospheric circulation patten fundamentally different from that of Jupiter

VHS 1256B is a super-Jupiter with the largest observed variability, reaching approximately 40% in the near-infrared (Ref. 2). The James Webb Space Telescope (JWST) Direct Imaging Early Release Science team conducted full-spectrum observations (Ref. 3), characterizing its atmospheric properties and finding its atmosphere filled with dust (or "clouds"). The research team further simulated its atmosphere to elucidate the circulation mechanism behind its variability and spectrum.

The team developed a comprehensive atmospheric circulation model for VHS 1256B to simulate its atmospheric dynamic, light curves and spectrum. By comparing the model with observational data, the study aimed to identify the possible atmospheric circulation pattern of VHS 1256B. As shown in Fig. 2, the simulated atmospheric circulation of VHS 1256B is completely unlike Jupiter's, and the model qualitatively explains the observed light curve and spectrum.

The variability stems from large-scale equatorial waves. These waves are driven by radiative heating caused by clouds at low latitudes, forming Rossby waves and Kelvin waves. These waves, in turn, regulate the growth and evolution of clouds, generating large-scale, zonally propagating "dust storms". This mechanism not only explains the cause of VHS 1256B's large-amplitude variations but the propagation and recycling of equatorial dust storms also account for the observed long-period changes in the light curve.

Spectroscopically, the model also provides a reasonable explanation for the data features, including the redness in the near-infrared and the prominent silicate absorption feature at 10 microns. 

Simultaneously explaining both the light curve and the spectrum requires the model parameters to lie within a specific "sweet spot." This work clarifies the relationship between this “sweet spot” and the cloud-radiative feedback mechanism

"The cloud-radiative feedback circulation mechanism has been proposed (Refs. 4-6)," said Prof. Xianyu Tan, the leading author of this work. "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. 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."

Why is the atmospheric circulation of super-Jupiter so distinct from that of Solar System giant planets? The answer lies in their different temperatures. Although super-Jupiters are more planet-like (relative to stars), most still possess surface temperatures exceeding 1000 Kelvin, far higher than Jupiter's ~100 Kelvin. The radiative response efficiency of super-Jupiter atmospheres is much stronger than Jupiter's. Energy is rapidly injected into large-scale waves via cloud-radiative feedback, a process that also tends to suppress Jupiter-like multiple zonal banding. In contrast, Jupiter has a much lower kinetic energy injection rate; energy can only accumulate into large-scale structures through a prolonged turbulent inverse cascade process, which tends to produce Jupiter's highly organized banded structure.

"The mechanism of giant planets’ atmospheric circulation has long been an important and unresolved question in planetary science," said Professor Xi Zhang, a key collaborator on the study from UC Santa Cruz. "These novel wave dynamical processes on super-Jupiters provide us with a unique perspective to examine our fundamental understanding of this problem."

In our solar system, the global dust storms on Mars are well-known, but these primarily arise from interactions of winds and the surface. This study, however, reveals that large-scale "dust storms" could also exist on a super-Jupiter—a celestial body completely lacking a solid surface. This discovery highlights the astonishing diversity of planetary atmospheric activities and offers a new perspective for comparative planetology.

References

Paper Link