Nicknamed "Phoenix," the newly discovered planet shows the vast diversity of solar systems and the complexity of planetary evolution, especially at the end of stars' lives.

The findings are published in The Astronomical Journal.

"This planet isn't evolving the way we thought it would, it appears to have a much bigger, less dense atmosphere than we expected for these systems," said Sam Grunblatt, a Johns Hopkins University astrophysicist who led the research.

"How it held on to that atmosphere despite being so close to such a large host star is the big question."

The new planet, classified as a "hot Neptune," shares similarities with Neptune despite being much closer to its host star and far hotter. Officially named TIC365102760 b, it is 6.2 times bigger than Earth, completes an orbit around its parent star every 4.2 days, and is about 6 times closer to its star than Mercury is to the Sun.

Due to Phoenix's age, high temperatures, and low density, the process of atmospheric stripping must have been slower than scientists thought possible. They estimated the planet is 60 times less dense than the densest "hot Neptune" found to date and predicted it will not survive more than 100 million years before spiraling into its giant star.

"It's the smallest planet we've ever found around one of these red giants, and probably the lowest mass planet orbiting a [red] giant star we've ever seen," Grunblatt said.

"That's why it looks really weird. We don't know why it still has an atmosphere when other 'hot Neptunes' that are much smaller and much denser seem to be losing their atmospheres in much less extreme environments."

Grunblatt and his team used a new method to fine-tune data from NASA's Transiting Exoplanet Survey Satellite. The satellite's telescope can detect low-density planets as they dim the brightness of their host stars when passing in front of them. The team filtered out unwanted light and combined this data with additional measurements from the W.M. Keck Observatory in Hawaii, which tracks tiny wobbles of stars caused by their orbiting planets.

These findings could improve understanding of how atmospheres like Earth's might evolve. Scientists predict that in a few billion years, the sun will expand into a red giant star that will engulf Earth and other inner planets.

"We don't understand the late-stage evolution of planetary systems very well," Grunblatt said.

"This is telling us that maybe Earth's atmosphere won't evolve exactly how we thought it would."

Puffy planets are often composed of gases, ice, or other lighter materials, making them less dense than any planet in the solar system. Scientists believe only about 1% of stars have them. Exoplanets like Phoenix are hard to spot due to their smaller sizes, but Grunblatt's team is searching for more of these smaller worlds, having already found a dozen potential candidates with their new technique.

"We still have a long way to go in understanding how planetary atmospheres evolve over time," Grunblatt said.

[Other authors are: Nicholas Saunders, Daniel Huber, and Ashley Chontos of the University of Hawaii at Manoa; Daniel Thorngren and Kevin Schlaufman of Johns Hopkins University; Shreyas Vissapragada and Stephanie Yoshida of Harvard University; Steven Giacalone, Emma Turtelboom, and Howard Isaacson of the University of California, Berkeley; Mason Macdougall of the University of California, Los Angeles; Corey Beard of the University of California, Irvine; Joseph M. Akana Murphy of the University of California, Santa Cruz; Malena Rice of Yale University; Ruth Angus of the American Museum of Natural History, Flatiron Institute, and Columbia University; and Andrew W. Howard of the California Institute of Technology.]

Research Report:TESS Giants Transiting Giants. IV. A low-density hot Neptune orbiting a red giant star

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