Phoenix exoplanet’s puffy atmosphere survives red giant star

• Astronomers have discovered a new “hot Neptune” exoplanet, using the TESS space telescope and W.M. Keck Observatory telescopes. It is about 1,840 light-years away.
• Nicknamed Phoenix, it appears to have retained its puffy atmosphere, while orbiting close to its red giant parent star. That’s surprising; it should have lost its atmosphere due to its star’s intense radiation.
• It is the smallest and least dense exoplanet ever found orbiting a red giant star. It’s similar to other so-called “puffy” exoplanets, the least dense types of exoplanets known to exist, and might provide insights into these worlds.

Meet Phoenix

Astronomers have discovered many exoplanets that orbit close to their stars, much closer than our sun’s innermost planet, Mercury, orbits the sun. These distant exoplanets are in danger of losing their atmospheres, due to their stars’ intense radiation. But, apparently, not always. On June 5, 2024, a team of scientists, led by researchers at Johns Hopkins University in Baltimore, said they discovered a “hot Neptune” exoplanet orbiting a red giant star. Nicknamed Phoenix, it’s the smallest planet around a red giant ever found. Surprisingly, the planet’s puffy atmosphere has survived the radiation of the star.

And, like other puffy exoplanets, it is very low in density.

The researchers used data from NASA’s Transiting Exoplanet Survey Satellite (TESS) and the W.M. Keck Observatory in Hawaii to discover and analyze the odd new exoplanet.

The research team published its peer-reviewed discovery in The Astronomical Journal on June 5, 2024.

Phoenix – with the scientific name of TIC365102760 b – is a “hot Neptune” exoplanet orbiting a red giant star 1,840 light-years away. It is 6.2 times larger than Earth and orbits its star every 4.2 days. Astronomers call it a hot Neptune due to its size and its close proximity to its star. In fact, its orbit is about six times closer to its star than Mercury is to our sun. That means it is bathed in intense radiation from the red giant star. Because of this, astronomers thought it was unlikely to have any atmosphere left.

Exoplanet’s puffy atmosphere surprises astronomers

But it turned out it did have an atmosphere, and a “puffy” one at that. Sam Grunblatt, an astrophysicist at Johns Hopkins University, led the research study and said:

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

The paper stated:

TIC 365102760 b is an inflated, hot-Neptune-like planet and among the smallest and lowest-mass planets ever confirmed to be transiting a red giant star. Despite its low mass and high incident flux, the planet appears to be holding onto a significant atmosphere. This suggests that it has a relatively high mass fraction of hydrogen and helium.

Phoenix is smaller, older and hotter than scientists had thought was possible. Indeed, hot Neptunes are rare among the many exoplanets astronomers have discovered so far. While they share characteristics with ice giant planets like Neptune and Uranus in our solar system, they are scorching hot from being so close to their stars. So, in that regard, they are quite unlike the ice giants, or the gas giants, in our solar system.

Why so different from other hot Neptunes?

Even though Phoenix is a hot Neptune, it’s a weird hot Neptune. It may be losing its atmosphere, but if so, it’s at a much slower pace than usual. Other hot Neptunes are losing their atmospheres faster, and in environments less harsh than that of Phoenix. The planet is also about 60 times less dense than the densest known hot Neptune. Why? As Grunblatt explained:

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. 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.

In addition, astronomers say Phoenix’s life will be relatively short. In only about 100 million years, it will start to spiral into the red giant star where it will be consumed. The paper said:

Though this planet will not survive at its current orbit for much longer around its red giant host, it is unlikely to be experiencing runaway orbital inspiral at this time. The planet is expected to survive less than 100 million years before beginning the inspiral process, assuming it does not lose a significant amount of its mass over that time. This is one of the shortest inspiral timescales for Neptune-mass planets.

Data from TESS and W.M. Keck Observatory

The researchers combined data from the TESS space telescope and W.M. Keck Observatory. They removed unwanted light from the TESS images and then combined them with data from the observatory. TESS and Keck make a good team. TESS is good at finding low-density planets while Keck can track the tiny wobbles a star makes as planets tug on it with their gravity.

The new findings will help astronomers better understand the later stages of planetary evolution, including planets like Earth. Earth, however, has a lot more time left than Phoenix. In a few billion years from now, our own sun will become a red giant star, devouring our home planet. But scientists still aren’t sure exactly how Earth’s atmosphere will change in the time leading up to that event. Grunblatt said:

We don’t understand the late-stage evolution of planetary systems very well. This is telling us that maybe Earth’s atmosphere won’t evolve exactly how we thought it would. We still have a long way to go in understanding how planetary atmospheres evolve over time.

Puffy planets are rare

Puffy exoplanets like Phoenix are rare, based on current knowledge of exoplanets. Astronomers estimate that only about 1% of stars have such planets. They are less dense than most other planets, including any in our own solar system. Grunblatt’s team has found about a dozen other candidates so far.

But they are definitely out there. Just late last month, researchers said the James Webb Space Telescope solved the mystery of why another exoplanet, WASP-107 b, is so puffy. It is hotter on the inside and has a larger core than had been expected. Tidal heating explains both the excess heat and puffiness of the atmosphere.

Bottom line: Astronomers have discovered a Neptune-sized world orbiting close to a red giant star. And surprisingly, the exoplanet’s puffy atmosphere is still intact.

Source: TESS Giants Transiting Giants. IV. A Low-density Hot Neptune Orbiting a Red Giant Star

Via Johns Hopkins University

Read more: Webb solves mystery of puffy exoplanet WASP-107 b

Read more: A rare Neptune-sized planet orbiting a giant star