Astronomers Map Dual Helium Tails of Exoplanet WASP-121b During Full Orbital Monitoring

Astronomers have successfully concluded a comprehensive monitoring campaign focused on the atmospheric escape of WASP-121b, an "ultra-hot Jupiter" located roughly 858 light-years from Earth within the constellation of Corvus. This significant investigation, spearheaded by experts from the Trottier Institute for Research on Exoplanets (iREx) at the University of Montreal and the University of Geneva, uncovered a highly unusual structure: two distinct helium tails streaming away from the planet. The findings were formally published in the journal Nature Communications on December 8, 2025, following continuous observations conducted late in 2025.

To capture this atmospheric loss in action, the research team maintained uninterrupted observation of the exoplanet for nearly 37 hours, utilizing the NIRISS instrument aboard the James Webb Space Telescope (JWST). This extended surveillance allowed scientists to track the planet’s atmosphere being stripped away across an entire orbital revolution. WASP-121b is known for its extreme environment, boasting an orbital period of just 30 hours and surface temperatures soaring to approximately 2300 degrees Celsius (4200 degrees Fahrenheit). Such intense heat drives the evaporation of lighter gases, including hydrogen and helium, into space, fundamentally shaping the planet's size, composition, and long-term fate.

Romain Allard, the lead researcher from the University of Montreal, described this observation as setting a new benchmark for exoplanetary science. This data provides the most detailed snapshot yet of atmospheric evaporation, a phenomenon previously only glimpsed briefly during transit events. The most striking revelation was the confirmation that the escaping helium forms not one, but two massive tails, each extending over half the planet's orbital path. This discovery truly throws a wrench in the works of existing atmospheric models, as current simulations struggle to replicate this intricate three-dimensional geometry.

One of these dual tails trails behind the planet, pushed away by stellar radiation and the solar wind. Intriguingly, the second tail appears to be drawn out ahead of the planet, suggesting it might be gravitationally influenced by the host star. These twin tails dwarf the planet itself, exceeding its diameter by more than 100 times. Vincent Bourrier, a team member from the University of Geneva, pointed out that these observations clearly expose the limitations of current numerical models, necessitating the exploration of novel physical mechanisms to fully comprehend planetary evolution.

The implications of this discovery are far-reaching, particularly concerning the broader evolution of planets. Scientists are now considering whether such substantial mass-loss mechanisms could eventually transform gas giants into smaller, Neptune-like worlds, or perhaps even strip them down entirely to their rocky cores. WASP-121b was already famous for its bizarre characteristics, including clouds composed of vaporized metal and precipitation of rubies and sapphires, all due to its scorching proximity to its star. The continuous monitoring established a new record for detecting helium absorption, covering nearly 60% of the planet’s orbit.

This achievement underscores the JWST’s remarkable sensitivity for mapping atmospheric dynamics over vast distances and extended periods. The research team included astronomers from the University of Geneva (UNIGE), the National Centre of Competence in Research PlanetS, and iREx at the University of Montreal (UdeM). Professor Lisa Dang of Waterloo noted the exceptional clarity of the helium signal during data analysis. Capturing these dynamic processes across a full orbital cycle marks a significant technical leap in the field. It now prompts scientists to investigate whether this dual-tail structure is unique to WASP-121b or a common feature among other hot exoplanets.