A newly discovered exoplanet, nicknamed “Phoenix,” has left scientists puzzled by retaining a puffy atmosphere despite being bombarded by intense radiation from its nearby host star.
This rare planet challenges existing theories about how planets age and die in extreme environments.
The research findings were recently published in The Astronomical Journal.
Phoenix is an extraordinary example of the diversity and complexity of solar systems and planetary evolution, particularly near the end of a star’s life.
Sam Grunblatt, an astrophysicist at Johns Hopkins University who led the research, explained, “This planet isn’t evolving the way we thought it would. It has a much bigger, less dense atmosphere than we expected for these systems.
The big question is how it held on to that atmosphere despite being so close to such a large host star.”
Phoenix falls into a category of rare planets known as “hot Neptunes.” These planets share many characteristics with Neptune, the solar system’s outermost giant, but are much closer to their stars and far hotter.
Officially named TIC365102760 b, Phoenix is 6.2 times bigger than Earth, completes an orbit around its star every 4.2 days, and is six times closer to its star than Mercury is to the sun.
Given Phoenix’s age, high temperatures, and unexpectedly low density, scientists believe that the process of stripping its atmosphere has occurred much more slowly than previously thought. The planet is estimated to be 60 times less dense than the densest known “hot Neptune” and is expected to survive no more than 100 million years before it begins to spiral 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,” said Grunblatt. “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.”
To uncover these insights, Grunblatt and his team developed a new method for fine-tuning data from NASA’s Transiting Exoplanet Survey Satellite (TESS). TESS can detect low-density planets as they pass in front of their stars, dimming the starlight.
The team filtered out unwanted light from the images and combined the data with additional measurements from the W.M. Keck Observatory in Hawaii, which tracks the tiny wobbles of stars caused by orbiting planets.
The discovery of Phoenix could provide valuable information about the evolution of planetary atmospheres, including Earth’s.
In a few billion years, the sun is expected to expand into a red giant star, potentially engulfing Earth and the 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 like Phoenix are often composed of gases, ice, or other lighter materials, making them less dense than any planet in our solar system. These planets are so rare that scientists estimate only about 1% of stars host them.
Exoplanets like Phoenix are difficult to detect because of their small size compared to bigger, denser planets. However, Grunblatt’s team is searching for more of these elusive worlds and has already identified a dozen potential candidates using their new technique.
“We still have a long way to go in understanding how planetary atmospheres evolve over time,” Grunblatt said. The discovery of Phoenix is a step toward unraveling these mysteries and gaining a deeper understanding of the universe.
