The exoplanet Phoenix, discovered with NASA’s Transiting Exoplanet Survey Satellite (TESS), has put astronomers in crisis. Despite being relentlessly bombarded by radiation from its red giant parent star, Posneta remained, against all odds, clinging to its atmosphere. It is also smaller, older and hotter than scientists thought possible for such a planet.
The mysterious exoplanet Phoenix
Indeed, the exoplanet, or “ exoplanet ” Phoenix, is expected to be a bare shell of rock due to its proximity to the star TIC 365102760, located about 1,800 light-years from Earth. Yet the planet 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 Neptune.” these are worlds with radii smaller than Jupiter’s, but larger than Earth’s.
And, unlike the solar system’s ice giant namesake, hot Neptunes dwell relatively close to their host stars. Phoenix may be an incredible survivor, but the luck and resilience of this roughly 10 billion-year-old planet 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 diverse variety of exoplanets that exist in the universe and demonstrates that a planetary system can evolve in many ways.
“Phoenix is not evolving as 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 he managed to maintain that atmosphere despite being so close to such a large guest star is the big question.”
TIC 365102760 is a red giant star, meaning it has spent around 10 billion years converting hydrogen to helium in its core. When the hydrogen fuel for this nuclear fusion process ran out, so did the energy supporting the star against its own gravity.
This meant that the star’s core would collapse while its outer layers, where nuclear fusion was still taking place, would swell to 100 times the star’s original width.
Phoenix orbits this star at a distance of about 5.6 million miles, which is about 0.06 times the distance between us and the sun. This means that the peculiar exoplanet has a year that lasts just 4.2 Earth days. Furthermore, 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 exoplanet Neptune discovered so far.
Phoenix’s advanced age and low density mean that some process must have destroyed its atmosphere much more slowly than scientists previously thought possible for a world so close to its star.
“It is the smallest planet we have ever found around one of these red giants and probably the lowest mass planet orbiting a giant star [rossa] that we’ve ever seen,” Grunblatt said. “That’s why it seems really strange. We don’t know why it still has an atmosphere while other ‘hot Neptunes’ that are much smaller and much denser appear 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 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 therefore help scientists better predict what will happen to Earth’s atmosphere before our planet meets its final fate.
“We don’t understand very well the advanced stage of evolution of planetary systems,” Grunblatt said. “This tells us that perhaps Earth’s atmosphere won’t evolve exactly as we thought.”
Phoenix is a rare find. Planets this small are difficult to see because of the dips in light they cause when they cross, or “transit,” the faces of their stars. Because this is the technique TESS uses to find planets, NASA spacecraft is generally better at seeing large, dense planets.
Phoenix’s discovery validates space explorer’s ability to see smaller, puffier planets when data is handled correctly. Grunblatt and colleagues have already used their newly developed method to observe dozens of smaller worlds, and this hunt will continue.
“We still have a long way to go to understand how planetary atmospheres evolve over time,” he concluded.
The team’s research was published Wednesday on The Astrophysical Journal.
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