Gas giant exoplanet WASP-69b has captured the attention of astronomers, this time because of the trail of gas it's leaving behind – a tail that's some seven times longer than the planet's radius.

This tail is being created as the atmosphere of WASP-69b gets stripped away, making it a fascinating celestial object to study. Around the size of Jupiter, the planet is 160 light-years away from Earth, and sits unusually close to its star.

That close proximity is the main reason for the elongated tail, according to the team behind the discovery, led by researchers from the University of California, Los Angeles (UCLA): the stellar wind radiation is blasting away the atmosphere of WASP-69b, and shaping the trail of gas behind it.

"Work by previous groups showed that this planet was losing some of its atmosphere and suggested a subtle tail or perhaps none at all," says astrophysicist Dakotah Tyler from UCLA.

"However, we have now definitively detected this tail and shown it to be at least seven times longer than the planet itself."

WASP-69b illustration
An artist's illustration of a planet with a gas tail orbiting its star. (Adam Makarenko/WM Keck Observatory)

The new insight was made possible through observations from a 10-meter telescope at the WM Keck Observatory in Hawaii, and readings from its high-resolution spectrograph – capable of precisely mapping changes in light wavelength and frequency.

Through these fresh readings, the team was able to determine that WASP-69b was losing large amounts of hydrogen and helium, and could also study their interactions with the stellar wind in a high level of detail.

Data gathered about WASP-69b will give astronomers invaluable information about how planets and stars interact – and we've caught this phenomenon at a crucial, early stage.

"For most known exoplanets, we suspect that the period of atmospheric loss concluded long ago," says astrophysicist Erik Petigura from UCLA.

"The WASP-69b system is a gem because we have a rare opportunity to study atmospheric mass-loss in real time and understand the critical physics that shape thousands of other planets."

We've known about WASP-69b for over a decade, but as this latest study shows, there's still lots to discover. It's known as a hot Jupiter planet, one precariously close to its star, and it completes an orbit in less than four Earth days.

There's no risk of the exoplanet running out of steam just yet though, even with its massive tail. It's such a giant planet that its star will be gone long before all of its atmosphere disappears into space.

"At around 90 times the mass of Earth, WASP-69b has such a large reservoir of material that even losing this enormous amount of mass won't affect it much over the course of its life," says Tyler.

"It's in no danger of losing its entire atmosphere within the star's lifetime."

The research has been published in The Astrophysical Journal.