When extrasolar gas giants get too close to their star, it can tear parts of their atmosphere out into space. Sometimes this becomes visible as a long trail of gas trailing behind the affected planet. Now, for the first time, astronomers have discovered that an exoplanet has formed two such tails. The hot gas giant WASP-121b has massive helium tails that fill half of its orbit around the star. This is shown by observations of the hot gas giant WASP-121b with the near-infrared spectrograph NIRISS on the James Webb Space Telescope. This double tail sheds new light on the mechanisms by which planets can lose their atmosphere. However, it is still unclear whether WASP-121b is an isolated case with regard to this double tail or not.
Many of the exoplanets known to date are so-called hot Jupiters: gas giants that orbit their star in a very close orbit. They usually only need a few days to complete one orbit and always face the same side of their star. These planets are correspondingly very heated. On the dayside it is often hot enough to tear apart gas molecules and cause metals to vaporize. Liquid metal can rain on the night side. But the proximity to the star has another consequence: the intense, high-energy radiation inflates the atmosphere of these gas giants and pulls light gases out into space. Over millions of years, this can change a planet’s size, composition and future evolution. In particularly drastic cases, this gas loss even becomes visible: the affected planets trail long gas trails behind them, similar to the tail of a comet. Such planetary tails are shown, among other things, by subtle shadows during planetary transit – the planet’s passage in front of its star.
A hot Jupiter with loss of gas
But until now, astronomers have only ever been able to observe snapshots of such planetary gas tails. The transit observations also only allow the detection of a tail in the planet’s immediate vicinity. However, how far these gas flows extend and how they develop over time remained unclear. That’s why a team led by Romain Allart from the University of Montreal has now used the NIRISS spectrograph on the James Webb Telescope to take a closer look at a hot Jupiter. The target of their observations was the exoplanet WASP-121b, around 850 light-years away. This gas giant weighs around 1.2 Jupiter masses and is inflated to around 1.8 Jupiter radii. From previous observations it is known that the planet only needs around 1.27 days to orbit its star and that its dayside is heated to more than 2700 degrees. Transit observations also suggested that the gas giant is losing helium to space. “But it was not possible to narrow down the extent of this helium outflow more precisely,” explain the astronomers.
That’s why Allart and his team have now targeted WASP-121b over a period of a good 37 hours with the NIRISS spectrograph – and thus more than one complete revolution of the planet around the star. The spectral data obtained reveal, among other things, whether and where helium escapes from the planet’s gas envelope, but also how extensive and extensive the gas outflow from its atmosphere is. They represent the most complete continuous observation of a planet’s helium signature ever recorded. The absorption of helium atoms in the infrared wavelength region confirmed that the hot Jupiter WASP-121b is indeed losing large amounts of helium. The spectral helium signature was detectable in almost 60 percent of the planet’s orbit around its star, the astronomers report. This is the longest continuous evidence of atmospheric gas loss determined to date. “We were incredibly surprised at how long the helium outflow lasted,” says Allart.
Two tails instead of one
Closer analyzes showed that the hot gas giant WASP-121b does not just have a long helium tail behind it, as expected. Instead, the astronomers detected two different tails: One lies on the side of the planet facing away from the star, which is driven away from the star by radiation and stellar wind. The second, denser tail precedes the planet and points towards the star. It is apparently attracted by its gravity, as Allart and his colleagues explain. Together, the two helium outflows extend over 107 times the planet’s diameter. “This discovery reveals the complex physical processes by which exoplanet atmospheres interact with their stellar environments. We are only beginning to grasp the true complexity of these worlds,” says Allart. At the same time, this discovery reveals gaps in existing models of gas loss from planetary atmospheres. Such models can explain and reproduce individual, comet-like tails, but not the newly observed double structure, as astronomers explain.
“We now need to rethink how we simulate atmospheric mass loss – no longer as a simple stream, but as a 3D geometry that interacts with its star. This is crucial to understanding how planets evolve,” says Allart. Atmosphere loss is one of the key factors that determine whether a planet remains a gas giant, shrinks into a Neptune-like planet, or is eroded down to a rocky core. According to the researchers, the double loss of gas that has now been discovered could perhaps even explain how the rare “hot Neptunes” – gas planets orbiting close to their star – were formed. They may be the remnants of once much larger hot Jupiters that have lost most of their gas envelope. However, it is still unclear whether WASP-121b’s double tail is typical of such gas giants near stars or whether the exoplanet is an isolated case in this respect. The astronomers hope to clarify this through further observations of similar systems.
Source: Romain Allart (Université de Montréal) et al., Nature Communications, doi: 10.1038/s41467-025-66628-5