Products and Models for "Early Release Science of the Exoplanet WASP-39b with JWST NIRCam"

Associated Publication: https://www.nature.com/articles/s41586-022-05590-4<br> <br> OVERVIEW: Measuring the metallicity and carbon-to-oxygen (C/O) ratio in exoplanet atmospheres is a fundamental step towards constraining the dominant chemical processes at work and, if in equilibrium, revealing planet formation histories. Transmission spectroscopyprovides the necessary means by constraining the abundances of oxygen- and carbon-bearing species; however, this requires broad wavelength coverage, moderate spectral resolution, and high precision that, together, are not achievable with previous observatories. Now that JWST has commenced science operations, we are able to observe exoplanets at previously uncharted wavelengths and spectral resolutions. Here we report time-series observations of the transiting exoplanet WASP-39b using JWST’s Near InfraRed Camera (NIRCam). The long-wavelength spectroscopic and short-wavelength photometric light curves span 2.0 – 4.0 µm, exhibit minimal systematics, and reveal well-defined molecular absorption features in the planet’s spectrum. Specifically, we detect gaseous H₂O in the atmosphere and place an upper limit on the abundance of CH₄. The otherwise prominent CO₂ feature at 2.8 µm is largely masked by H₂O. The best-fit chemical equilibrium models favour an atmospheric metallicity of 1–100× solar (i.e., an enrichment of elements heavier than helium relative to the Sun) and a sub-stellar carbon-to-oxygen (C/O) ratio. The inferred high metallicity and low C/O ratio may indicate significant accretion of solid materials during planet formationor disequilibrium processes in the upper atmosphere.关联论文：https://www.nature.com/articles/s41586-022-05590-4<br><br>概述：测量系外行星大气中的金属丰度与碳氧（C/O）比，是限定主导化学过程、并在化学平衡条件下揭示行星形成历史的核心步骤。透射光谱法（Transmission spectroscopy）可通过限定含碳、含氧物质的丰度，提供实现上述目标的可行路径；但该方法需要宽波长覆盖范围、中等光谱分辨率与高精度观测的协同支持，而此前的天文观测设施无法同时满足这三项要求。如今詹姆斯·韦布空间望远镜（James Webb Space Telescope, JWST）已启动科学观测任务，我们得以在此前未被探索的波长与光谱分辨率区间内开展系外行星观测。本研究报告了利用JWST的近红外相机（Near InfraRed Camera, NIRCam）对凌星系外行星WASP-39b开展的时序观测结果。本次观测的长波光谱与短波测光光变曲线覆盖2.0–4.0 µm波段，系统误差极低，并在行星光谱中清晰呈现出明确的分子吸收特征。具体而言，我们在该行星大气中探测到气态水（H₂O），并对甲烷（CH₄）的丰度设定了上限。原本应显著出现在2.8 µm处的二氧化碳（CO₂）吸收特征，在很大程度上被水（H₂O）所掩盖。最优化学平衡模型拟合结果显示，该行星大气的金属丰度为太阳的1–100倍（即相较于太阳，氦以外的重元素丰度得到显著提升），且碳氧（C/O）比低于恒星水平。本次研究推断得到的高金属丰度与低碳氧比，可能意味着行星形成阶段发生了大量固态物质吸积过程，或是高层大气中存在非平衡化学过程。