Named WASP-69b, scientists have studied this Jupiter-sized planet in the past, focusing on its escaping atmosphere and observing only a small trail of helium gas. But during a press conference today at the January 2024 Meeting of the American Astronomical Society, a University of California, Los Angeles (UCLA)-led team of researchers announced that new Keck Observatory data reveal the tail is at least 350,000 miles long.

"Previous observations suggested that WASP-69b had a modest tail, or no tail at all," says Dakotah Tyler, astrophysics doctoral candidate at UCLA and first author of the study. "However, we have been able to definitively show that this planet's helium tail extends at least seven times the radius of the giant planet itself."

Located 160 light-years away from Earth, WASP-69b is so close to its sun that one year on this alien world lasts only 3.9 Earth days. Their proximity subjects the planet to extreme radiation from its host star, causing the gas giant's atmosphere to burn off.

The team observed this using Keck Observatory's Near-Infrared Spectrograph (NIRSPEC) to capture sharp images of WASP-69b, which revealed the sequence of events showing its tail stretching out as the planet shed its atmosphere.

"The WASP-69b system is a gem because we are able to study its atmospheric mass-loss in real-time," says co-author, Erik Petigura, associate professor of astronomy and astrophysics at UCLA. "This makes for a rare opportunity to understand the critical physics that shapes thousands of other planets."

"What truly set Keck apart in our observations was the large collecting area of its mirror, which enabled us to detect far more light from the star. This, combined with the high-resolution capabilities of the NIRSPEC instrument, gave us extremely high sensitivity to the velocity structure and total absorption of the escaped atmosphere, which strong stellar winds have sculpted into a long, wispy tail," says Tyler.

Although WASP-69b is only about 30 percent the mass of Jupiter, it is 10 percent larger due to the extreme heat from its host star, which causes its atmosphere to expand before breaking free. The escaping atmosphere then produces wind that violently interacts with the wind from the planet's host star, forming WASP-69b's helium tail.

"These comet-like tails are really valuable because they form when the escaping atmosphere of the planet rams into the stellar wind, which causes the gas to be swept back. Observing such an extended tail allows us to study these interactions in great detail," says Petigura.

Studying atmospheric mass-loss directly is pivotal for understanding exactly how planets across our galaxy evolve over time with their stars.

WASP-69b is losing about 1 Earth mass every billion years, but with a total mass nearly 90 times that of the Earth, the planet is in no danger of losing all of its atmosphere during its lifetime.

"The resilience of this planet in such an extreme and hostile environment allows us to study the process of atmospheric mass-loss, which helps us understand how stars can cause their planets to evolve. But it also serves as a powerful reminder to us all," says Tyler. "Perspective is everything. Despite the multitude of challenges we may face, like WASP-69b, we have what it takes to continue on."

Research Report:WASP-69b's Escaping Envelope Is Confined to a Tail Extending at Least 7 Rp

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