2026-05-22 ジョンズ・ホプキンス大学（JHU）
An artistic rendition of exoplanet WASP-94A b. Credit:Hannah Robbins/Johns Hopkins University
＜関連情報＞
- https://hub.jhu.edu/2026/05/21/astronomy-exoplanet-atmospheres-detecting-clouds/
- https://www.science.org/doi/10.1126/science.adx5903
巨大ガス惑星における曇りの朝と晴れた夕方 Cloudy mornings and clear evenings on a gas giant exoplanet
Sagnick Mukherjee, David K. Sing, Guangwei Fu, Kevin B. Stevenson, […] , and Maria Zamyatina
Science Published:21 May 2026
DOI:https://doi.org/10.1126/science.adx5903
Editor’s summary
When an exoplanet transits (passes in front of its host star, as seen from Earth), some of the background starlight passes through its atmosphere, where it can be absorbed or scattered. Transit observations can therefore determine the exoplanet’s atmospheric spectrum averaged over the morning and evening sides. Mukherjee et al. have observed a transit of a gas giant exoplanet orbiting a bright star with sufficient signal-to-noise to extract separate spectra for the morning and evening. They found that the morning atmosphere is cloudy, but the evening atmosphere is clear and has water absorption features. This asymmetry can bias atmospheric composition measurements that use averaged spectra. —Keith T. Smith
Abstract
The spectra of exoplanet atmospheres are affected by aerosols (clouds and hazes) of uncertain origin. Proposed aerosol formation mechanisms include gas condensation or photochemical reactions. We measured the transmission spectrum of the tidally locked gas giant exoplanet WASP-94A b and identified asymmetry in its atmosphere. The morning limb is cooler and cloudy, whereas the evening limb is hotter and exhibits gaseous water absorption features. We interpret this difference as being due to the formation of cloud droplets near the morning limb, which evaporate during circulation to the evening limb. The dominant aerosols are clouds cycling between the day and night sides of the atmosphere, not photochemical hazes. The resulting asymmetry can severely bias chemical abundance measurements, unless limb-resolved spectroscopy is available.
