To some 880 light years from EarthAn extreme gas giant is slowly disintegrating in space. This is WASP-121 b, an ultra-hot Jupiter-like exoplanet that is losing part of its atmosphere and forming a new layer of gas. helium tails so long that they surround a good part of its orbit.
Thanks to james webb space telescopeAn international consortium of scientists—with European and Spanish participation—has managed to track this atmospheric escape for almost a complete orbit of the planet. The observations have revealed, for the first time in such detail, the presence of two distinct helium tailsOne that goes ahead of the planet and another that follows it, defying the classic models that predicted a single comet-like structure.
An ultra-hot Jupiter on the edge of its star
WASP-121 b belongs to the category of ultra-hot JupitersGas giants with masses comparable to or greater than Jupiter's, but in orbits extremely close to their star. In this case, the planet completes one orbit around its star in just 30 hours, or about 1,275 days, which implies a brutal proximity.
This distance is so small that it is equivalent to barely a 2,6% of the separation between the Earth and the SunAs a result, the planet is tidally locked: it always shows the same face to the star, just as the Moon does to Earth. The daytime hemisphere is subjected to temperatures of several thousand degrees, with estimates around 2350 kelvin or even more so in the upper layers of the atmosphere.
The planet's size is also extreme. WASP-121 b has a mass similar to or greater than Jupiter's, but its radius is almost that of Jupiter. double that of the giant of our Solar SystemThis results in a very inflated atmosphere. This gaseous envelope stretches beyond the so-called Roche lobe, the region where the planet's gravity can retain the gas against the star's attraction.
Under these conditions, light gases, especially hydrogen and heliumThey have a very easy time escaping. But the extreme heat and tidal forces don't just expel light elements: they also drag along heavier materials, including alkali metals and species such as silicon monoxide, profoundly altering the atmospheric structure.
A record-breaking track record with the James Webb
The discovery of helium tails is based on a prolonged observation campaign with the James Webb Space Telescope (JWST)For almost 37 consecutive hours, the telescope recorded variations in the host star's spectrum due to the absorption of helium in the planet's atmosphere, a period that covers more than one complete orbit of WASP-121 b.
The key has been the use of highly sensitive infrared instruments, among them NIRISSDeveloped by the Canadian Space Agency in collaboration with ESA and NASA, this spectrograph allows the detection of the signature of Helium in a metastable state, a kind of chemical fingerprint very useful for tracking gas that is escaping into space.
Until now, most studies of atmospheric leakage on exoplanets were limited to brief time windows, usually when the planet transited in front of its star's disk. However, in this case, it has been possible to track the atmospheric escape along approximately 60% of the orbitboth during and outside of transit.
This continuous approach has allowed for a much more detailed reconstruction of the three-dimensional geometry of gas flows around the planet. The data show that helium surrounds WASP-121 ba along much of its orbital path, resulting in a much more complex structure than a simple tail aligned with the stellar wind.
Two helium tails: one in front and one behind
Analysis of the observations has revealed that the escaping atmosphere of WASP-121 b is organized into two distinct helium tailsThe first, known as the leading stream, is located ahead of the planet in its orbital motion. The second, called the trailing stream, extends behind it and gradually disperses.
La front tail It appears to be strongly influenced by the star's gravity. Some of the helium that leaves the planet is literally "pulled" inward, channeled toward the gravitational equilibrium zone known as the Lagrange point L4Some models suggest that an accretion process of this gas towards the star could even begin.
For its part, the tail It is dominated by radiation pressure and stellar wind. Material escaping from the dayside hemisphere and the planet's terminator is pushed back, forming an extensive tail that gradually dilutes into the circumstellar medium.
The data indicates that these queues extend to colossal distances, on the order of more than 100 times the radius of the planetWe are talking about lengths that reach about 107 planetary radii, comparable to one-tenth of an astronomical unit, that is, around 0,1 times the average distance between the Earth and the SunIn visual terms, if we could observe the system closely, we would see the planet surrounded by a gaseous structure that covers a good part of its orbit.
This dual configuration has caught the scientific community off guard. classic hydrodynamic models Atmospheric escape patterns typically predict a single tail aligned with the stellar wind, similar to the tail of a cometHowever, in WASP-121 b a much more complex distribution is observed, which necessitates revising the simulations and taking into account the combined interaction of radiation, winds, gravitational tides, and rotation.
Hydrodynamic processes and extreme atmospheric leakage
At the heart of this phenomenon lies what experts call hydrodynamic exhaustUnlike milder gas losses, here the extreme heating of the upper layers of the atmosphere generates a massive flow that carries with it both light gases and heavier compounds.
The enormous radiation from the star heats the planet's thermosphere until it inflates to a colossal degree. oversized thermosphere It extends beyond the Roche lobe, where the planet's gravity ceases to be dominant. From there, the gas begins to flow outward as if it were a continuous planetary wind.
In addition to radiation, the tidal forces They play a crucial role. WASP-121 b's tight orbit results in intense gravitational interaction with the star, which warps the planet and facilitates gas escape through regions of weaker gravitational potential. This combination of heat and tidal forces makes atmospheric leakage particularly efficient.
Previous observations had already revealed that the atmosphere of WASP-121 b is anything but calm. Indications have been detected of stratosphere, calcium titanate clouds, vaporized metals and even processes that could lead to "rains" of exotic materials. Now, with the helium tails clearly visible, it is confirmed that the planet is losing a significant fraction of its gaseous envelope to space.
The results indicate that the leak persists persistent over timenot only at specific intervals. This suggests that, on scales of millions or billions of years, a planet like WASP-121 b could change radically, reducing its apparent size and transforming into an object of a different nature than the gas giant we observe today.
Lessons on the evolution of exoplanets
The WASP-121 b case has become a natural laboratory to study how giant planets evolve under extreme conditions. The continued loss of atmosphere raises the possibility that, over time, some ultra-hot Jupiters will eventually become smaller worlds, similar to Neptune or even... bare rock cores.
This type of process could help explain certain statistical patterns that astronomers observe in the exoplanet population. One of the most discussed examples is the so-called “desert of hot Neptunes”, a region in the mass and radius diagrams where intermediate-sized planets are rarely found very close to their stars.
One hypothesis is that many of these intermediate worlds have lost a large part of their atmospheres due to intense hydrodynamic exhaustThey are transformed into smaller, denser bodies, difficult to detect, or reclassified into other categories. Data from WASP-121 b provide key pieces for refining these planetary evolution models.
The new observations also underscore that atmospheric escape is not a simple unidirectional flow. Instead of a “jet” of gas moving away in a straight line, we find a complicated three-dimensional structurewhere the geometry of the orbit, the rotation of the planet, the inclination of the system, and the activity of the star combine to shape the tails.
This forces theorists to rethink their simulation tools. Two-dimensional or overly simplified models fall short of reproducing the dual configuration observed in WASP-121 b. From now on, more sophisticated models are needed. more sophisticated 3D simulations, capable of capturing the dynamics of the stellar wind, gravitational interactions and the response of the planet's atmosphere as a whole.
The role of the James Webb and European collaboration
The progress achieved with WASP-121 b is also a demonstration of the potential of James Webb Space Telescope for the study of exoplanet atmospheres. Launched in 2021 and operated by NASA, ESA and the Canadian Space Agency, the Webb has become the benchmark tool for observing very distant objects and subtle phenomena like these helium tails in the infrared.
In this case, the use of instruments such as NIRISS and other infrared spectrographs This has allowed scientists to decompose the light from the system and isolate the signature of the escaping helium. The telescope's stability and high sensitivity have been crucial for maintaining continuous observations for tens of hours, something very difficult to achieve from Earth.
The research is part of a coordinated international effort, with significant participation from European centers. Teams linked to the ESA already has research institutes in several European countries They have worked side by side in the design of observation campaigns, data processing, and the development of theoretical models.
Many of these results have been published in high-impact scientific journals, such as Nature Communications.The report details both the observations and their implications for exoplanet physics. The community hopes that future campaigns with the Webb telescope and other complementary telescopes will allow them to continue monitoring the evolution of WASP-121 ba over the coming years.
Beyond this specific case, the success of the observations is driving new programs aimed at other ultra-hot Jupiters, with the aim of verifying whether double tails are a rarity of this system or a relatively common phenomenon among giants very close to their stars.
The Spanish influence on the study of WASP-121 b
Within this international framework, the Spanish scientific community has also left its mark. Astronomers and astrophysicists from Spanish centers and universities They have participated in the analysis of the James Webb spectral data and in the construction of models that describe the gas flows and helium loss in WASP-121 b.
In recent years, Spain has been gaining ground in the field of European space researchThanks to collaboration with ESA, NASA, and other agencies, involvement in cutting-edge projects such as the Webb Space Telescope, exoplanet observation missions, and large ground-based facilities is consolidating a network of groups specializing in planetary atmospheres and exoplanet physics.
This type of work not only improves the country's position in international consortia, but also has a direct effect on the training of new generations of researchersThe opportunity to work with data from the James Webb Space Telescope and to publish articles in leading journals attracts young talent to scientific careers linked to astronomy and space technologies.
Looking ahead, it is expected that Spanish teams will continue to participate in extreme exoplanet observation campaignsThis has been observed both with the James Webb telescope and with telescopes yet to come, such as ESO's Extremely Large Telescope (ELT). WASP-121 b is just one of the systems where this presence has been noted, but all indications are that it will not be the last.
Taken together, the study of the helium tails in WASP-121 b exemplifies how the combination of world-class infrastructure, international cooperation and scientific talent It allows us to glimpse phenomena that until very recently seemed unattainable, and how Europe and Spain play a significant role in this new stage of exoplanet exploration.
The image that emerges from WASP-121 b is that of a gas giant subjected to a constant punishment by its star, with a swollen atmosphere that overflows and forms two colossal helium tails that extend along much of its orbit; the James Webb observations, supported by European and Spanish teams, not only allow us to follow live how a planet loses its gaseous envelopebut they help to rethink how the worlds that populate our galaxy are born, change and, in some cases, disintegrate.
