A strange planet discovered by NASA’s Transiting Exoplanet Survey Satellite (TESS) is confusing astronomers. Despite being mercilessly bombarded with radiation from its red giant parent star, the world has, against all odds, held on to its atmosphere. It is also smaller, older and hotter than scientists thought possible for such a planet.
In reality, the extrasolar planet, or “exoplanet,” should be a bare shell of rock due to its proximity to the star TIC 365102760, which is about 1,800 light-years away from Earth. Yet the world, nicknamed ‘Phoenix’, has emerged from the flames of its host star with a beautiful and puffy atmosphere.
Phoenix, or TIC 365102760 b as the planet is officially called, is part of a rare class of planets called “hot Neptunes.” these are worlds with a radius smaller than that of Jupiter, but larger than that of Earth. And, unlike our solar system’s namesake ice giant, hot Neptunes are relatively close to their host stars. Phoenix may be an incredible survivor, but the roughly 10 billion-year-old planet’s luck and resilience won’t last forever. The team that discovered it predicts that it will spiral into its giant star in about 100 million years.
The discovery of Phoenix shows the wide variety of exoplanets that exist in the universe, and shows that a planetary system can evolve in many ways.
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“This planet is not evolving as we thought it would. It appears to have a much larger, less dense atmosphere than we expected for these systems,” Sam Grunblatt, team leader and astrophysicist at Johns Hopkins University, said in a statement. “How it kept that atmosphere despite being so close to such a big guest star is the big question.”
What can Phoenix tell us about the future of the Earth
TIC 365102760 is a red giant star, meaning it has spent about 10 billion years converting hydrogen to helium in its core. When the hydrogen fuel for this nuclear fusion process ran out, the energy supporting the star against its own gravity also ended. This meant that the star’s core would have collapsed while the outer layers, where nuclear fusion was still taking place, had swelled to as much as 100 times the star’s original width.
Phoenix orbits this star at a distance of about 6 million miles, or about 0.06 times the distance between us and the Sun. This means that the special exoplanet has a year that lasts only 4.2 Earth days. Furthermore, with a width of about 6.2 times that of Earth and a mass of about 20 times that of our planet, Phoenix also has an unexpectedly low density. Its density is about 60 times smaller than that of the densest hot exoplanet Neptune yet discovered.
Phoenix’s old age and low density indicate that some process must have stripped its atmosphere much more slowly than scientists previously thought possible for a world so close to its star.
‘It’s the smallest planet we’ve ever found orbiting one of these red giants and probably the lowest-mass planet orbiting one of these red giants. [red] gigantic star we have ever seen,” Grunblatt said. “That’s why it looks really weird. We don’t know why it still has an atmosphere, while other ‘hot Neptunes’, which are much smaller and denser, seem to lose their atmospheres in much less extreme environments.”
The Sun itself will undergo a similar red giant transformation in about 5 billion years, expanding into the orbit of Mars and swallowing the inner rocky planets, including Earth.
Phoenix’s findings, made possible by filtering out unwanted starlight from TESS observations, could therefore help scientists better predict what will happen to Earth’s atmosphere before our planet meets its ultimate fate.
“We don’t understand the evolution of late-stage planetary systems very well,” Grunblatt said. “This tells us that Earth’s atmosphere may not evolve exactly as we thought.”
Phoenix is a rare find. Planets of such small sizes are difficult to see through the dips in light they produce as they cross or ‘cross’ the surfaces of their stars. Because this is the technique TESS uses to find planets, the NASA spacecraft is generally better at seeing large and dense planets.
Phoenix’s discovery confirms the space explorer’s ability to see smaller and fatter planets if the data is processed correctly. 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 if we want to understand how planetary atmospheres evolve over time,” he concluded.
The team’s research was published Wednesday in The Astrophysical Journal.