Astronomers Discover Planet Has Been Hiding 350,000-Mile-Long Tail

Astronomers have discovered that a gigantic planet has been hiding a 350,000-mile-long tail as its atmosphere seeps away.

The exoplanet known as WASP-69 is gargantuan—roughly the size of Jupiter, the largest planet in our solar system—and is about 160 light-years from Earth. It is sometimes dubbed "hot Jupiter" because the gas planet orbits incredibly close to its host star. It is so close, in fact, that it completes its orbit of the star in less than four days, by Earth's time frame. The closest planet to our sun, Mercury, completes its orbit in 88 days.

A new study published in the Astrophysical Journal found that its star is so boiling hot atmosphere is escaping into space, at a rate of 200,000 tons per second.

The surrounding stellar winds have turned the escaping atmosphere into a tail that stretches on for at least 350,000 miles. The tail consists mainly of hydrogen and helium, and flows in the direction of Earth

"This planet has been observed before on two other occasions. One suggested a modest tail (much shorter than we detected) and the other suggested no tail at all," Dakotah Tyler, a UCLA doctoral student and first author of the study, told Newsweek.

"However our observations definitively showed that there was a giant tail—extending at least 350,000 miles (over seven times the size of the planet). Tails like these have been seen trailing some other planets as well," Tyler said. "A big surprise here, however, was how long the tail we detected actually was in comparison to those earlier observations by other groups."

The star has such high radiation that it is stripping away the exoplanet's atmosphere. The atmosphere is then being funneled into its long tail. This finding helps astrophysicists learn how stellar winds are affected in planets that are uniquely close to their stars.

Findings like this also help them understand how all planets, not just WASP-69, evolve and grow with their host stars.

"Over the last decade, we have learned that the majority of stars host a planet that orbits them closer than Mercury orbits our sun and that the erosion of their atmospheres plays a key role in explaining the types of planets we see today," co-author and UCLA professor of physics and astronomy Erik Petigura said in a summary of the findings. "However, for most known exoplanets, we suspect that the period of atmospheric loss concluded long ago. 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."

WASP-69b was first discovered around a decade ago. Previous observations showed barely any evidence of the tail, however astrophysicists suspected it was there.

However, this study used a much larger telescope at the W. M. Keck Observatory in Hawaii. This longer telescope gave scientists an insight they had never observed before.

"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," Petigura said. "Observing such an extended tail allows us to study these interactions in great detail."

Despite the fact that its atmosphere is being sucked away by the star, the researchers said it won't disappear completely.

"There are other planets that orbit closely to their stars like this one, however, it is relatively rare for them to have these tails," Tyler said. "Most hot-Jupiters will be losing some of their atmospheres because of the high energy radiation coming from the nearby star. But you only get the tail when the star has an incredibly strong stellar wind (an actual wind of particles coming from the star itself), which can reshape all of that escaped atmosphere into a long 'comet-like' tail like we see here in our observations."

Tyler said that repeat observations of this planet are always interesting because each time it is observed "there seem to be different results."

"Repeat observations would be valuable to continue digging into what specifically is going on with this planet," he said. "But more generally, this allows us to investigate how planets evolve with their host stars in our galaxy."

Correction 01/10/24, 4:30 p.m. ET: This article was updated to correct the length of the planet's tail, which is 350,000 miles long.

Update 01/11/24, 4:05 a.m. ET: This article was updated to include comment from Dakotah Tyler.

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