Telescópio Espacial James Webb identified an unprecedented structure in the exoplanet’s atmosphereWASP-121b, located about 880 light years from Terra.
Astronomers observed two massive tails of helium escaping the planet, extending more than half its orbit around the host star.
The discovery occurred during continuous monitoring of almost 37 hours, covering more than one full orbit of the planet.
- The exoplanet is classified as an ultra-hot Júpiter.
- Its orbital period lasts only 30 hours.
- Proximity to the star heats its atmosphere to thousands of degrees, facilitating the loss of light gases.
Characteristics of the exoplanet WASP-121 b
WASP-121 b has a diameter approximately 1.8 times larger than that of Júpiter.
It orbits a star in the constellation Puppis, at a distance equivalent to about 2.6% of the separation between Terra and Sol.
The planet is tidal locked, with one side always facing the star, which generates extreme temperatures in the daytime hemisphere.
These conditions favor the expansion and escape of light elements such as helium and hydrogen.
流出した大気が形成する“二重の尾” 太陽系外惑星WASP-121 bをウェッブ宇宙望遠鏡が観測https://t.co/H0kiBEWY6W
— sorae 宇宙へのポータルサイト (@sorae_jp)December 15, 2025
WASP-121 こちらは、最新の研究成果にもとづb」の想像図。巨大ガス惑星の前後に帯状の雲のような尾が形成され、恒星を取り囲むように広がっている様子が描かれていますpic.twitter.com/J6oBs8dWPb
Observation details with JWST
Researchers used the NIRISS instrument, a Canadian contribution, to capture the signature of helium in the infrared.
Continued observation revealed that atmospheric escape persists for about 60% of the orbit.
Helium tails extend to distances greater than 100 times the diameter of the planet.
3D models indicate that one tail leads the orbital movement, attracted by stellar gravity, while the other follows behind, driven by stellar wind and radiation.
Formation of double tails
The front tail is formed by the star’s gravitational pull, pulling the escaped material inward.
The rear tail results from radiation pressure and stellar winds, pushing helium outward.
This dual configuration challenges current models of atmospheric escape, which generally predict a single cometary tail.
The observed structure covers almost 60% of the orbit, representing the most extensive detection ever recorded.
Implications for planetary evolution
Continued atmospheric escape can significantly alter the size and composition of gaseous planets over time.
In extreme cases, gas giants shrink to sizes similar to Netuno or lose their entire atmosphere, leaving only rocky cores.
WASP-121 b serves as a laboratory to study these processes in real time.
Future observations with JWST will help determine whether double structures occur on other similar exoplanets.
Instrument and research team
NIRISS has enabled the precise detection of metastable helium at interstellar distances.
The international team included astronomers from Instituto Trottier of Pesquisa in Exoplanetas of Universidade of
The results, published in Nature Communications, highlight the complexity of the atmospheres in ultra-hot Júpiteres.
Additional research will explore the role of radiation and stellar winds in shaping distant planetary atmospheres.
Theoretical models and challenges
Existing computer simulations explain single tails, but do not reproduce the observed double structure.
The discovery indicates the need for new three-dimensional models that incorporate gravitational interactions and wind dynamics.
The phenomenon can contribute to understanding rarities such as the “hot Netunos desert”, where intermediate planets are scarce.
Studies on other systems will test whether the double configuration represents an isolated case or a common pattern.
The atmospheric loss process observed in WASP-121 b occurs persistently and on a large scale.
This escape shapes the evolution of exoplanets close to their stars.
Continuous detection provides valuable data for refining theories of planetary formation and transformation.
Advances in JWST enable more detailed observations of previously inaccessible distant phenomena.
