近日，美国约翰霍普金斯大学的David K. Sing及其研究团队取得一项新进展。经过不懈努力，他们发现温暖的海王星甲烷揭示核心质量和强烈的大气混合。相关研究成果已于2024年5月20日在国际权威学术期刊《自然》上发表。

据悉，凌日气态巨行星的大气中甲烷普遍处于耗尽状态，但近期在大气中再次检测到甲烷。这种耗尽现象被推测是由不平衡过程，如光化学作用或来自行星较热内部的混合作用所维持的。然而，随着垂直混合强度的增强，内部过程变得难以控制，我们只能确定CH4耗尽的上限。值得注意的是，系外行星中的温暖海王星WASP-107 b因其异常低的密度、低核心质量以及适宜甲烷存在的温度而引人注目，尽管之前的观测并未探测到甲烷分子的存在。

该研究团队展现了SO₂、CH4以及H₂O、CO₂和CO的特征。他们在1.0±0.5 ppm的丰度下检测到了具有4.2σ显著性的甲烷，这一数值比平衡预期少了3个数量级。这一发现对WASP-107 b的大气及其内部结构提出了严格的约束条件，揭示了该行星的包层具有43±8倍太阳的超太阳金属丰度，内部温度更高，固有温度为T_int=460±40 K。强烈的垂直混合导致CH4的显著耗尽，其扩散系数达到K_zz = 10^11.6±0.1 cm²/s。光化学对CH₄丰度的影响被认为可以忽略，但SO₂的影响不容忽视。基于这些研究，研究人员推断出WASP-107 b的核心质量为11.5^+3.0_-3.6 M_⊕，这一数值远高于之前的上限，释放了核心吸积模型的张力。

附：英文原文

Title: A warm Neptune’s methane reveals core mass and vigorous atmospheric mixing

Author: Sing, David K., Rustamkulov, Zafar, Thorngren, Daniel P., Barstow, Joanna K., Tremblin, Pascal, de Oliveira, Catarina Alves, Beck, Tracy L., Birkmann, Stephan M., Challener, Ryan C., Crouzet, Nicolas, Espinoza, Nstor, Ferruit, Pierre, Giardino, Giovanna, Gressier, Amlie, Lee, Elspeth K. H., Lewis, Nikole K., Maiolino, Roberto, Manjavacas, Elena, Rauscher, Bernard J., Sirianni, Marco, Valenti, Jeff A.

Issue&Volume: 2024-05-20

Abstract: Observations of transiting gas giant exoplanets have revealed a pervasive depletion of methane, which has only recently been identified atmospherically. The depletion is thought to be maintained by disequilibrium processes such as photochemistry or mixing from a hotter interior. However, the interiors are largely unconstrained along with the vertical mixing strength and only upper limits on the CH₄ depletion have been available. The warm Neptune WASP-107 b stands out among exoplanets with an unusually low density, reported low core mass, and temperatures amenable to CH₄ though previous observations have yet to find the molecule. Here we present a JWST NIRSpec transmission spectrum of WASP-107 b which shows features from both SO₂ and CH₄ along with H₂O, CO₂, and CO. We detect methane with 4.2σ significance at an abundance of 1.0±0.5 ppm, which is depleted by 3 orders of magnitude relative to equilibrium expectations. Our results are highly constraining for the atmosphere and interior, which indicate the envelope has a super-solar metallicity of 43±8× solar, a hot interior with an intrinsic temperature of T_int=460±40 K, and vigorous vertical mixing which depletes CH4 with a diffusion coefficient of Kzz = 10^11.6±0.1 cm²/s. Photochemistry has a negligible effect on the CH₄ abundance, but is needed to account for the SO₂. We infer a core mass of 11.5^+3.0_-3.6 M_⊕, which is much higher than previous upper limits, releasing a tension with core-accretion models.

DOI: 10.1038/s41586-024-07395-z

Source: https://www.nature.com/articles/s41586-024-07395-z