This huge exoplanet's comet-like tail is 350,000 miles long and scientists are thrilled

An alien planet trailing a long comet-like tail around red-orange star
The strange alien planet WASP-69b is trailing a huge comet-like tail 350,000 miles long as its atmosphere is blown off by its parent star. (Image credit: Adam Makarenko/W. M. Keck Observatory)

A comet-like planet beyond our solar system is losing a lot more atmosphere in its vast tail than previously thought, intriguing astronomers and sparking new questions about how planets evolve with their parent stars.

The exoplanet WASP-69b, a hot, puffy gas giant 160 light-years from Earth that circles its host star in a speedy 3.9 days, first rose to fame in 2018 when astronomers found a possible comet-like tail of gas leaking from the planet's atmosphere. That tail, which was thought to be just a tiny trail of helium particles, if it existed at all, is now estimated to be at least 350,000 miles long (563,270 kilometers) — about seven times the width of the planet —  as its  its atmosphere is blown away by a steady barrage of solar wind from its star.

The WASP-69b system is a gem because we have a rare opportunity to study atmospheric mass-loss in real time."

Erik Petigura, UCLA astronomer

"It's getting bathed in radiation," study co-author Dakotah Tyler of the University of California Los Angeles (UCLA), said during a Tuesday (Jan. 9) press briefing at the 243rd meeting of the American Astronomical Society in New Orleans. "If you're ever considering retiring, I would suggest that you do not consider retiring on this planet," he added. 

In Tuesday's briefing, Tyler shared new data of WASP-69 b's leaking atmosphere from the Keck Observatory in Hawaii, also described in a paper published in The Astrophysical Journal this week. The latest observations reveal the atmosphere is breaking free of the planet at a rate of 200,000 tons per second, forming an expansive comet-like tail not previously seen.

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The new findings are attributed mostly to the Keck Observatory's large telescope mirror, which collects more light than previous telescopes that observed WASP-69b. But it could also be changing the behavior of the WASP-69 star, which astronomers call stellar variability, Tyler said. "It's hard to get a handle on exactly what type of variability is going on within the star itself."

Thanks to its exuding atmosphere, WASP-69b is losing one Earth mass every billion years, which is "quite a bit," said Tyler, "but for a hot Jupiter, it's really not that much."

Observing the sweeping tail would reveal how WASP-69b's atmosphere interacts with its host star, shedding light on the evolution of planets along with their respective stars.

"For most known exoplanets, we suspect that the period of atmospheric loss concluded long ago," study co-author Erik Petigura of UCLA said in a statement. "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."

In addition to its scientific appeal, the planet's resilience in the face of incessant stellar wind also serves as a powerful reminder about perspective, Tyler said in the statement. 

"Despite the multitude of challenges we may face, like WASP-69b, we have what it takes to continue on."

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Sharmila Kuthunur contributor

Sharmila Kuthunur is a Seattle-based science journalist covering astronomy, astrophysics and space exploration. Follow her on X @skuthunur.