TDLI RESEARCH ACHIEVEMENTS
近日,开云网页登录 李政道研究所谭先瑜副教授与加州大学圣克鲁斯分校张曦教授等合作者在《Science Advances》发表题为”Large-amplitude Variability Driven by Giant Dust Storms on a Planetary-mass Companion”的研究论文。该研究揭示超级木星拥有与木星截然不同的大气环流。
褐矮星是质量介于巨行星与恒星之间的天体,其内部无法维持氢核聚变。自形成以来,它们持续冷却,数十亿年后达到较低温度,与巨行星高度相似,因此常被用作研究巨行星热演化和大气物理的参照。部分褐矮星质量仅十余倍木星质量,被称为超级木星。
观测和统计研究表明,褐矮星普遍存在光变现象。其光变源于表面亮度不均匀,自转导致观测亮度周期性变化。部分褐矮星的光变曲线呈现显著非周期性变化,表明存在强烈的“天气”活动,为研究系外巨行星大气环流提供了理想场所。
天文学家认为褐矮星大气环流与木星类似,具有显著的纬向(东西向)多条带结构和类似大红斑的稳定涡旋。这种类木星环流构成了解释褐矮星光变的传统模型基础(图1左)。然而,由开云网页登录 李政道研究所谭先瑜副教授主导的研究挑战了这一观点。通过构建模型解释一颗超级木星的光变曲线和光谱,该研究提出这颗超级木星有与木星截然不同的大气环流(图1右)。
图1. 左图:艺术构想图示意天文学家长期持有的观点,即褐矮星的外表和木星相似,呈现显著的纬向条带和类似大红斑的稳定涡旋结构,这种类木星大气环流是解释褐矮星光变的传统模型基础。图片来源:NASA/JPL-Caltech。右图:该研究模拟的超级木星大气辐射通量图。
超级木星VHS 1256B是目前观测到光变幅度最大的目标星,其在近红外波段的光变幅度高达约40% (参考文献 2)。詹姆斯·韦伯空间望远镜(JWST)直接成像早期科学团队对其进行了全波段光谱观测 (参考文献 3),详细测绘了其大气特性,发现大气中布满尘埃(或称“云”)。研究团队进一步通过大气模拟,试图阐明其光变和光谱背后的环流机制。
研究团队为VHS 1256B构建了复杂的大气环流模型,模拟其大气动态演化以及相应的光变和光谱。模型包含了符合实际的辐射传输、云的形成与耗散,以及云、辐射和动力过程的耦合。通过对比模型与观测数据,该研究旨在揭示VHS 1256B可能的大气环流特征。如图2所示,模拟结果显示VHS 1256B的大气环流与木星完全不同,该模型定性解释了观测到的光变曲线和光谱。
其光变源于大尺度赤道波动。这种波动由低纬度区域云层引起的非均匀辐射加热所驱动,形成了动态的罗斯贝波(Rossby wave)和开尔文波(Kelvin wave)。这些波动反过来又促进了云层的增长和演化,形成纬向传播的大尺度“沙尘暴”。该机制不仅揭示了VHS 1256B大幅度光变的成因,赤道波动的传播和循环也定性地解释了观测到的光变曲线长周期变化。
在光谱方面,该模型也合理解释了数据特征,包括近红外波段的偏红颜色以及10微米处显著的硅酸盐吸收特征。同时解释光变和光谱要求模型参数恰好处于一个“合理”区间,该工作阐明了该参数区间的选择与云-辐射反馈机制的内在关联。
“我们此前提出了云-辐射反馈的环流机制(参考文献 4-6),但理论模型要真正落地,必须经受观测事实的检验,这甚至比提出理论更具挑战性,” 该研究负责人、论文第一兼通讯作者谭先瑜表示,“幸运的是,与VHS 1256B观测数据的成功比对提供了这一机会,验证了新环流机制的可行性。当然,我们尚不能完全排除其他潜在的重要机制。”
图2. 上排:大气模型在不同时期大气层顶(TOA)的辐射通量图,显示了赤道区域的全球尺度沙尘暴(深色低辐射区)及其长期演化。中排:对应赤道沙尘暴三个特征演化阶段的光变曲线。蓝线是HST/WFC3 G141波段的光线曲线,红线是Spitzer 4.5波段的光线曲线。下排:VHS 1256B的模型光谱(光谱分辨率为𝑅 ∼ 3000,黑线)与JWST观测光谱(彩色线)。图片来源:参考文献 1。
为何超级木星的大气环流与太阳系巨行星如此迥异?答案在于其温度差异。尽管经过长期冷却的超级木星类似巨行星(相对于恒星而言),但其表面温度大多仍超过1000开尔文,远高于木星的约100开尔文。超级木星大气的辐射响应效率远高于木星,大气动能通过云-辐射反馈被快速注入大尺度波动中,且此过程倾向于抑制类木星的多条带结构;而木星大气动能注入率较低,只能通过漫长的湍流反向串级过程将能量积累至大尺度结构,该过程则易形成类木星的高度有序条带。
“巨行星大气环流机制一直是行星科学中一个重要而未决的问题,” 该研究主要合作者、加州大学圣克鲁斯分校张曦教授指出,“超级木星上这些新颖的波动过程,为我们审视这一问题的基本理解提供了独特视角。”
在太阳系,火星的全球性沙尘暴广为人知,但那主要源于地表沙尘的参与。而本次研究发现,在完全没有固态表面的超级木星上,竟也出现了大尺度“沙尘暴”现象。这揭示了行星大气活动的惊人多样性,也为比较行星学提供了全新的视角。
开云网页登录 李政道研究所谭先瑜副教授为该论文的第一兼通讯作者,多名美国、欧洲学者为论文合作者。本工作得到了国家自然科学基金委员会、开云网页登录 高性能计算中心的支持。
参考文献
1, Tan et al., Science Advances, 11(48), (2025)
2, Zhou et al., The Astronomical Journal, 164, 239, (2022)
3, Miles et al., The Astrophysical Journal, 946, L6, (2023)
4, Tan & Showman, The Astrophysical Journal, 874, 111, (2019)
5, Tan & Showman, Monthly Notices of the Royal Astronomical Society, 502, 2198, (2021)
6, Tan & Showman, Monthly Notices of the Royal Astronomical Society, 502, 678, (2021)
论文原文
https://www.science.org/doi/10.1126/sciadv.adv3324
Do super-Jupiters look like Jupiter?
Not necessarily.
TDLI RESEARCH ACHIEVEMENTS
Recently, an article titled "Large-amplitude Variability Driven by Giant Dust Storms on a Planetary-mass Companion" was published in Science Advances by Prof. Tan Xianyu from the Tsung-Dao Lee Institute at Shanghai Jiao Tong University and collaborators, including Prof. Zhang Xi from the University of California, Santa Cruz. This study reveals that the super-Jupiter possesses an atmospheric circulation fundamentally different from that of Jupiter.
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
Fig. 1. Left: artistic conception illustrating the long-held astronomical view - brown dwarfs resemble Jupiter in appearance, displaying prominent multiple zonal bands and stable vortices similar to the Great Red Spot. This Jupiter-like atmospheric circulation formed the basic picture for explaining brown dwarfs’ variability. Image credit: NASA/JPL-Caltech. Right: modeled general circulation for super-Jupiter VHS 1256B.
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
Fig. 2. Top panels: Modeled top-of-atmosphere (TOA) bolometric flux maps at different epochs, showing global-scale dust storms (the dark regions) at the equator. Middle panels: synthetic light curves of the three epochs corresponding to three characteristic evolution stages of the equatorial dust storm. Bottom panel: model spectrum of VHS 1256B with a spectral resolution of 𝑅 ∼ 3000 (black line), and the observed spectrum obtained by JWST (colored lines).
"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.
Associate Professor Tan Xianyu from the Tsung-Dao Lee Institute at Shanghai Jiao Tong University is the first and corresponding author of this paper, with collaborators from the United States and Europe. This work received support from the National Natural Science Foundation of China and the High-Performance Computing Center of Shanghai Jiao Tong University.
References
1, Tan et al., Science Advances, 11(48), (2025)
2, Zhou et al., The Astronomical Journal, 164, 239, (2022)
3, Miles et al., The Astrophysical Journal, 946, L6, (2023)
4, Tan & Showman, The Astrophysical Journal, 874, 111, (2019)
5, Tan & Showman, Monthly Notices of the Royal Astronomical Society, 502, 2198, (2021)
6, Tan & Showman, Monthly Notices of the Royal Astronomical Society, 502, 678, (2021)
Paper Link
https://www.science.org/doi/10.1126/sciadv.adv3324
文稿 | 谭先瑜
编辑 | 孟闻卓
责任编辑 | 李姝姝 陆梦珠