As observation instruments become more precise, the Universe reveals unsuspected forms of violence. Some planets do not just suffer from their star, they disintegrate under its influence, slowly dismembered by heat and gravitational forces. This is the case of WASP-121 b, a gas giant whose fleeing atmosphere has just delivered a cosmic spectacle never before observed.
An atmospheric escape much more extensive than imagined
This isn't the first time astronomers have detected a fleeing atmosphere around an exoplanet. But never before had this leak created such a vast double trail of gas. The study published in Nature Communications in December reveals that WASP-121 b (a hot Jupiter-like gas giant) projects two tails of helium gas that extend more than 180 degrees from its orbit, or nearly 18 million kilometers.
Images obtained by JWST using its NIRSpec instrument confirm that this gas breakout is so large that it forms a structure comparable to that of a comet, with two opposing plumes. Their symmetry suggests a complex mass loss mechanism, influenced by the planetary rotation and tidal forces of its star.
© NASA, ESA and G. Bacon/STSci
Illustration of the exoplanet WASP-121b and its star.
What the case of WASP-121 b teaches us about unstable atmospheres
WASP-121 b is an extreme exoplanet. Its surface temperature exceeds 2,500°C, sufficient for certain metals to evaporate. It orbits its star in just 1.3 Earth days, at such a small distance that its atmosphere is continually bombarded by ultraviolet and X-ray radiation. This energetic cocktail destabilizes the upper layers of the atmosphere, which expand and escape into space.
This phenomenon is not simple evaporation. The researchers showed that the detected helium tails follow a dynamic pattern powered by gravitational interactions with the star. According to the authors of the study relayed by ScienceAlert, the extent of the atmospheric loss observed could lead to the complete disappearance of the atmosphere in just a few hundred million years.
Such an observation, made possible by the sensitivity of JWST, represents a turning point in the study of giant exoplanets close to their star. It demonstrates that the classic model of thermal exhaust is no longer sufficient to explain the data collected.
Observing these tails is like going back in time on a planet
Using the spectral signature of helium, researchers were able to map the speed and trajectory of particles escaping from the planet. This is a direct measurement of atmospheric dynamics in deep space, a technical feat which opens the way to a reconstruction of the stages of evolution of this type of celestial object.
By studying the shape, extent, and density of gas tails, scientists can now model the climate history of WASP-121 b, estimate the rate of mass loss, and predict how quickly this planet will become barren. This also allows us to better understand under what conditions a giant planet can retain its atmosphere, or on the contrary, be stripped in a geologically short period of time.
This type of analysis is not limited to WASP-121 b. It applies to many other exoplanets observed by JWST, including those located in equally extreme environments. Through this cosmic lens, astronomy questions not only the composition of distant worlds, but their future. WASP-121 b, in this sense, acts as a natural laboratory for examining the limits of planetary atmospheric stability.
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