A strange planet discovered by NASA’s Transiting Exoplanet Satellite (TESS) has baffled astronomers. Despite being relentlessly bombarded with radiation from its red giant parent star, the world, against all odds, hangs on to its atmosphere. It is also smaller, older and hotter than scientists thought possible for such a planet.
Indeed, the extrasolar planet, or “exoplanet,” must be a bare shell of rock because of its proximity to the star TIC 365102760, located about 1,800 light-years from Earth. However, the world, nicknamed “Phoenix”, has emerged from the flames of its host star with a nice, puffy atmosphere.
Phoenix, or TIC 365102760 b as the planet is officially designated, is part of a rare class of planets called “hot Neptunes.” these are worlds with radii smaller than Jupiter’s but larger than Earth’s. And, unlike the solar system’s ice giant of the same name, hot Neptunes reside relatively close to their host stars. Phoenix may be a remarkable survivor, but the fate and resilience of the roughly 10-billion-year-old planet won’t last forever. The team that discovered it predicts that it will pass to its giant star in about 100 million years.
The discovery of Phoenix shows the diverse variety of exoplanets that exist throughout the universe and shows that a planetary system can evolve in many ways.
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“This planet is not evolving the way we thought. It appears to have a much larger and less dense atmosphere than we expected for these systems,” team leader and Johns Hopkins University astrophysicist Sam Grunblatt said in a statement. “How it was maintained in that atmosphere, despite being so close to such a large star, is the big question.”
What Phoenix Can Tell Us About Earth’s Future
TIC 365102760 is a red giant star, meaning it has spent about 10 billion years converting hydrogen into helium in its core. When the hydrogen fuel for this nuclear fusion process ran out, the energy that supported the star against its gravity also ended. This meant that the star’s core would have collapsed while its outer layers, where nuclear fusion was still taking place, would have swelled to 100 times the star’s original width.
Phoenix orbits this star at a distance of about 5.6 million miles away, or about 0.06 times the distance between us and the sun. This means that the particular exoplanet has a year that lasts only 4.2 Earth days. Plus, with a width about 6.2 times that of Earth and a mass about 20 times that of our planet, Phoenix also has an unexpectedly low density. It is about 60 times less dense than the densest hot Neptune exoplanet discovered so far.
Phoenix’s advanced age and low density indicate that a process must have stripped its atmosphere much more slowly than scientists previously believed possible for a world so close to its star.
“It’s the smallest planet we’ve ever found around one of these red giants, and possibly the smallest-mass planet orbiting a [red] giant star we’ve ever seen,” Grunblatt said. “That’s why it looks really strange. We don’t know why it still has an atmosphere when other ‘hot Neptunes’ that are much smaller and much denser appear to be losing their atmospheres in much less extreme environments.”
The sun itself will undergo a similar red giant transformation in about 5 billion years, expanding to the orbit of Mars and consuming the rocky inner planets, including Earth.
Phoenix’s findings, made possible by filtering out unwanted starlight from TESS observations, could help scientists better predict what will happen to Earth’s atmosphere before our planet meets its ultimate fate.
“We don’t understand the late-stage evolution of planetary systems very well,” Grunblatt said. “This is telling us that maybe Earth’s atmosphere isn’t going to evolve exactly the way we thought it would.”
Phoenix is a rare find. Planets this small are difficult to see through the dips in light they cause when they pass, or “transit,” the faces of their stars. Since this is the technique TESS uses to find planets, NASA’s spacecraft is generally better at seeing large, dense planets.
The discovery of Phoenix proves the space explorer’s ability to see smaller, more bloated planets when the data is handled properly. Grunblatt and colleagues have already used their newly developed method to observe dozens of smaller worlds—and the hunt will continue.
“We still have a long way to go in understanding how planetary atmospheres evolve over time,” he concluded.
The team’s research was published Wednesday in The Astrophysical Journal.