‘Weird’ exoplanet has puffy atmosphere despite intense radiation

A rare exoplanet, nicknamed “Phoenix,” has defied expectations by developing a puffy atmosphere despite being bombarded by intense radiation from its nearby host star. This discovery challenges existing theories about how planets evolve and perish in extreme environments. 

The study, published in The Astronomical Journal, was led by Sam Grunblatt, a Johns Hopkins University astrophysicist. 

Hot Neptune planets

“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,” he said.

Phoenix, officially named TIC365102760 b, belongs to the rare “hot Neptunes” category, sharing characteristics with Neptune but being much closer to its star and significantly hotter. The planet, which is 6.2 times larger than Earth, orbits its star every 4.2 days, at a distance six times closer than Mercury is to the Sun. 

Defying all expectations

Despite its age and high temperatures, Phoenix has managed to retain a low-density atmosphere, which has perplexed scientists. They estimate that Phoenix is 60 times less dense than the densest known hot Neptune and predict it will only survive another 100 million years before spiraling into its host star.

“It’s the smallest planet we’ve ever found around one of these red giants,” Grunblatt said. “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.”

Detecting the exoplanet’s puffy atmosphere 

Grunblatt and his team used a new method to analyze data from NASA’s Transiting Exoplanet Survey Satellite, filtering out unwanted light and combining the data with additional measurements from the W.M. Keck Observatory in Hawaii. This approach allowed them to detect the planet’s puffy atmosphere and gain insights into its unusual characteristics.

Evolution of planetary atmospheres 

The discovery of Phoenix could improve our understanding of how planetary atmospheres, including Earth’s, might evolve. Scientists predict that the Sun will eventually expand into a red giant star, potentially engulfing 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 rare, with scientists estimating that only about 1% of stars host them. These planets are composed of gasses, ice, or other lighter materials, making them less dense than any planet in our solar system. 

The smaller size of exoplanets like Phoenix makes them harder to detect, but Grunblatt’s team is actively searching for more. They have 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 concluded.

Rare planets with puffy atmospheres

Puffy planets, often referred to as “inflated gas giants,” are a type of exoplanet notable for their extremely low densities relative to their sizes. 

These planets are generally larger than expected for their mass, akin to having the volume of Jupiter but a significantly lesser mass, which makes them less dense and hence “puffy.” This characteristic can sometimes result in these planets having radii much larger than standard gas giants under similar conditions.

The puffiness of these planets is primarily attributed to their proximity to their host stars. They orbit very close to their stars, and the intense stellar radiation they receive can heat their atmospheres to extremely high temperatures. 

This heat adds energy to the planet’s atmosphere, causing it to expand and bloat. In essence, the thermal energy overcomes the gravitational pull of the planet to an extent, leading to a larger radius than would typically be expected for a planet of similar mass.

Another factor contributing to the puffiness could be tidal heating, where gravitational interaction between the planet and its star (or other planets) generates additional internal heat, further causing the atmosphere to expand.

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