A Scorching Planet Grew Two Tails express
Planet WASP-121b orbits so near its star that the fierce heat boils its atmosphere away, streaming like a comet's tails. With the James Webb Space Telescope, astronomers continuously watched this process for the first time. They found that helium from the atmosphere is pulled into two enormous plumes, stretching across 60% of the orbit — hundreds of millions of kilometers. One tail, pushed by stellar radiation, surges ahead of the planet and falls onto the star. The other trails behind and escapes into space. The planet is morphing into a comet-like body, shedding its gaseous envelope. Such twin tails had never been seen in their full extent before. This helps us understand how swiftly gas worlds perish under alien suns. The observations relied on a special glow from helium, allowing light analysis to detect even the thinnest layers of the atmosphere, like peering deep into space weather.
Original abstract
as written by the authors arXiv:2510.09809
Atmospheric escape of planets on short orbital periods, driven by the host star's irradiation, influences their evolution, composition, and atmospheric dynamics. Our main avenue to probe atmospheric escape is through the near-infrared metastable helium triplet, which has enabled mass loss rate measurements for tens of exoplanets. Among them, only a few studies show evidence for out-of-transit absorption, supporting the presence of a hydrodynamic outflow. However, none of these observations precisely identified the physical extent of the outflow, either due to non-continuous or short-duration observations. This limits our measurements of accurate mass loss rates. Here we present the first continuous, full-orbit helium phase curve monitoring of an exoplanet, the ultra-hot Jupiter WASP-121b, obtained with JWST/NIRISS. It reveals helium absorption for nearly 60% of the orbit at >3sigma significance. Our results show that WASP-121b sustains a strong outflow, separating into two tails trailing and leading the planet. The persistent absorption from these tails, together with their measured radial velocity shifts, suggests that they remain in a collisional fluid regime at large distances from the planet and display very different dynamics. The leading trail has a higher density and moves toward the star, whereas the trailing trail is being pushed away from the star, with the latter being blue-shifted due to stellar irradiation pressure. While qualitatively agreeing with theoretical expectations, the observed structure of helium is not self-consistently reproducible by current models, limiting constraints on the mass loss rate. Furthermore, we show that while ground-based observations of the helium triplet are essential to measure the outflow dynamics precisely, they ideally should be combined with continuous JWST phase curves to constrain the absolute level of helium absorption.
The unique glow of helium lets us peek into the uppermost, bloated layers of exoplanet atmospheres — regions where ordinary light can't reach.