2026-06-10 マックス・プランク研究所
◆WASP-121bは恒星に極めて近いホットジュピターで、自転と公転が同期しているため、一方の面が常に恒星を向いている。観測の結果、夕側では恒星から受けた熱が強い大気循環によって運ばれるため温度が高く、赤外線吸収も強いことが判明した。一方、高温の影響で水分子が分解されるため、夕側では水蒸気量が減少していた。これに対し朝側では比較的低温環境が維持され、水分子がより多く存在していた。
◆今回の研究は、系外惑星大気の三次元的な構造や熱輸送メカニズムを直接検証した重要な成果であり、極限環境下にある惑星の気象・化学過程の理解を大きく前進させるものである。JWSTの高感度観測能力が、系外惑星研究の新たな段階を切り開いたことを示している。
Top view of the orbit of the exoplanet WASP-121 b around its star. The planet’s rotation is synchronized to its orbit, both taking about 30 hours to complete. As a result, the planet constantly faces the star with the same side producing distinct day and night sides. The transition zones between those hemispheres are the morning and evening regions. Due to the planet’s proximity to the central star of only 1.9 stellar diameters, the planet rotates by about 30 degrees during its transit.
＜関連情報＞
JWSTで検出された自転トランジットにより明らかになったWASP-121 bの大気非対称性 Atmospheric asymmetries in WASP-121 b revealed by rotational transits detected with JWST
Cyril Gapp,Aurélien Falco,Thomas M. Evans-Soma,David K. Sing,Shashank Dholakia,Vivien Parmentier,Jérémy Leconte,Eva-Maria Ahrer & Guangwei Fu
Nature Astronomy Published:10 June 2026
DOI:https://doi.org/10.1038/s41550-026-02887-6
Abstract
Close-in exoplanets are tidally locked to their host star and thus exhibit extreme atmospheric temperature gradients. It has been theorized that the fraction of star light absorbed by such planets during transit changes as a function of orbital phase as progressively hotter or colder atmospheric gas rotates into view, but this effect has not been observed so far. Here we show that two transits of the ultrahot Jupiter WASP-121 b, acquired with JWST/NIRSpec and NIRISS, exhibit asymmetric light curves caused by the planet’s rotation during transit. We observe increasing CO absorption and slightly decreasing H2O absorption in the transmission spectrum as the planet rotates. These results are indicative of a stronger longitudinal temperature gradient across the evening than across the morning terminator, consistent with higher temperatures in the eastern half than in the western half of the dayside. The observed changes of the transmission spectrum with orbital phase are in line with the temperature increase causing thermal dissociation of H2O, while CO remains abundant. The observation of longitudinal gradients in atmospheric temperature and chemistry from the planet’s rotational transit provides a new probe for constraining atmospheric heterogeneity using JWST beyond differences between morning and evening terminators from limb asymmetries.
