The James Webb Space Telescope (JWST) has given scientists the first detailed look at an unusual phenomenon on the distant exoplanet WASP-121b, known as Tylos. This “hot Jupiter” turned out to have two long helium tails, which somewhat puzzled scientists with the question of the physical processes that form the planet’s atmosphere. The results of the study were published in Nature Communications.

Unusual tails surprised astronomers

WASP-121b is located about 858 light years away. It is a massive gas giant that orbits its star in just 30 hours. At such proximity, the radiation from the parent star heats the planet’s atmosphere to 2300 ℃. Under the influence of this heat, light elements such as hydrogen and helium gradually escape into space. This process, called atmospheric leakage,can last millions of years and gradually change the mass, composition and structure of the planet.

Previously, observations of the leak were recorded only during transits, when the planet passed in front of the star.Now JWST has made it possible to monitor the process continuously throughout the full rotation of Tylos, which was a unique achievement for scientists.

“We were incredibly surprised to see how long the helium leak lasted. This discovery shows how complex the processes in the atmospheres of exoplanets and their interaction with the stellar environment are,” said study leader Romain Allard from the University of Montreal.

Double helium tail

Observations with the NIRSpec near-infrared spectrograph showed that WASP-121b’s gas envelope extends far beyond the planet. Surprisingly, the helium forms two separate tails. One is deflected back by radiation and stellar wind, the other follows the planet in its orbit, likely being pulled toward the star by gravity.

Collectively, these tails are a hundred times longer than the width of the planet and three times the distance between WASP-121b and its star. Scientists cannot yet explain the existence of double tails using existing models.

“Very often, new observations reveal the limitations of our numerical models and push us to study new physical mechanisms. Each discovery helps to better understand these distant worlds,” said Vincent Bourrier from the University of Geneva.

Why is this important

Helium is a key indicator of atmospheric leakage. Previously, astronomers recorded only short-term manifestations, but now scientists were able to track the process continuously over than half of the planet’s orbit.This creates the most complete picture of atmospheric leakage to date.

The new observation helps to understand how the atmospheres of ultra-hot Jupiters react to the extreme radiation of stars, and how their gaseous envelopes form and evolve. The discovery also highlights JWST’s unique role in studying distant planets, allowing astronomers to see details that other instruments cannot see.

“We are just beginning to discover the true complexity of these worlds. Each new discovery raises new questions about the physics of the atmosphere, interactions with radiation and gravity, and the evolution of planets outside the solar system,” Allar added.