JWST maps the weather on a hot gas giant 700 light-years away
Source: Ars Technica
Scientists using the James Webb Space Telescope have successfully mapped the weather patterns on WASP-94A b, a hot gas giant located roughly 690 light-years from Earth. Led by astrophysicist Sagnick Mukherjee of Johns Hopkins University, the team published their findings in the journal Science. The planet orbits one star in a binary system and is tidally locked, meaning the same side always faces its star, creating permanent day and night hemispheres with dramatic temperature differences between them.
WASP-94A b is an unusual planet: its mass is slightly less than half of Jupiter's, yet its diameter is over 70 percent wider, making it extremely low in density. This bloated structure causes its atmosphere to extend far into space, making it an ideal candidate for atmospheric study. Researchers had long wondered whether the atmospheres of such tidally locked gas giants are static or dynamic, and whether they contain winds, clouds, or other active weather phenomena worth investigating.
Traditionally, scientists study exoplanet atmospheres using transmission spectroscopy, which analyzes starlight filtered through a planet's atmosphere as it passes in front of its star. However, this method averages the light across the planet's entire silhouette, treating the atmosphere as a single uniform gas. For tidally locked planets, this approach is a significant oversimplification, since massive temperature differences between the day and night sides produce very different atmospheric conditions on each limb of the planet.
To overcome this limitation, Mukherjee's team employed a technique called limb-resolved spectroscopy, using JWST's Near Infrared Imager and Slitless Spectrograph. Because the leading edge of the planet—the morning limb—blocks starlight slightly before the trailing evening limb during a transit, scientists could isolate separate chemical signals from each side. This revealed that the morning limb contains thick clouds of silicate particles, while the evening limb is relatively clear, confirming predictions from atmospheric circulation models about equatorial super-rotation.
This discovery has significant implications for how scientists interpret exoplanet chemistry. Because previous transmission spectroscopy averaged signals from both limbs, chemical compositions of many exoplanets may have been incorrectly estimated. The morning clouds on WASP-94A b appear to form as cooler nightside air moves into the hot dayside and condenses, while the evening limb shows clear skies where heated gases flow back into the night. This asymmetry reshapes our understanding of atmospheric dynamics on tidally locked worlds throughout the galaxy.
Japanese translation
ジェームズ・ウェッブ宇宙望遠鏡を使った科学者チームが、地球から約690光年離れた高温の巨大ガス惑星「WASP-94A b」の天気パターンの解明に成功した。ジョンズ・ホプキンス大学の天体物理学者サグニック・マカジー氏が率いるこの研究は科学誌『サイエンス』に掲載された。この惑星は連星系の一方の恒星を公転しており、常に同じ面が星に向いている「潮汐固定」状態にある。
WASP-94A bは非常に特異な惑星だ。質量は木星の半分以下にもかかわらず、直径は70パーセント以上も大きく、密度が極めて低い。この膨張した構造により大気が宇宙空間へ広く広がり、観測に適した好条件を生み出している。研究者たちはこうした潮汐固定された巨大ガス惑星の大気が静的なのか動的なのか、また風や雲が存在するのかを長年疑問に思っていた。
従来、科学者は「透過分光法」を用いて系外惑星の大気を研究してきた。これは惑星が恒星の前を通過する際に大気を透過した星の光を分析する手法だが、惑星全体の輪郭を平均化してしまうという欠点がある。潮汐固定された惑星では昼側と夜側で大きな温度差があるため、この方法は大幅な単純化に陥っており、正確な大気状態の把握が困難だった。
この限界を克服するため、マカジー氏のチームはJWSTの近赤外線撮像スリットレス分光器を用いた「周縁分解分光法」という技術を採用した。惑星が恒星を通過する際、先行する「朝側の縁」が「夕方側の縁」よりもわずかに早く恒星光を遮るため、それぞれの化学的シグナルを分離することが可能になった。この結果、朝側の縁には厚いケイ酸塩の雲があり、夕方側は比較的晴れていることが判明した。
この発見は系外惑星の化学組成の解釈に大きな影響を与える。これまでの透過分光法では両側のシグナルが平均化されていたため、多くの系外惑星の化学組成が誤って推定されていた可能性がある。WASP-94A bの朝側では冷たい夜側の空気が高温の昼側へ流れ込む際に凝縮して雲を形成し、夕方側では加熱された気体が夜側へ流れ込んで晴天をもたらす。この非対称性は銀河全体の潮汐固定惑星の大気力学への理解を塗り替えるものだ。
Key vocabulary
- tidally lockedadjective
Describes a celestial body that rotates at the same rate as it orbits, so the same face always points toward the object it orbits.
和訳: 潮汐固定された
Because the Moon is tidally locked to Earth, we never see its far side from the surface.
- transmission spectroscopynoun
A scientific technique that analyzes the spectrum of starlight filtered through a planet's atmosphere during a transit to determine its chemical composition.
和訳: 透過分光法
Astronomers used transmission spectroscopy to detect water vapor in the atmosphere of the distant exoplanet.
- limb-resolved spectroscopynoun
An advanced observational technique that isolates and separately analyzes the spectra of the leading and trailing edges of a transiting planet's atmosphere.
和訳: 周縁分解分光法
Limb-resolved spectroscopy allowed the research team to distinguish cloud patterns on opposite sides of the exoplanet.
- equatorial super-rotationnoun
A meteorological phenomenon in which winds near a planet's equator blow eastward at speeds faster than the planet itself rotates.














