Super-hot Neptune discovered around a red giant

A new paper published in the Astronomical Journal reports that the newly discovered hot-Neptunian exoplanet TIC 365102760b (commonly known as “Phoenix”) has retained an atmosphere despite relentless radiation from its parent star. TIC 365102760 is a red giant star located 555 parsecs (1,810 light-years) away in the constellation Cygnus. Also known as 2MASS J20232153+5423395 or Gaia DR2 2185044477033336064, the star is at least 6.3 billion years old. The newly discovered planet, officially named TIC 365102760b, orbits the star every 4.2 days. This alien world is surprisingly smaller, older, and hotter than astronomers thought. The planet belongs to a rare category of exoplanets called hot Neptunes because it shares many similarities with the solar system’s outermost frozen giants, despite being much closer to its host star and much hotter. TIC 365102760b is 6.2 times larger and 19.2 times more massive than Earth, and is about six times closer to its star than Mercury is to the Sun. Due to the planet’s age, scorching temperature, and unexpectedly low density, the process of atmospheric ejection must have been slower than scientists thought. They also estimated that the planet’s density is 60 times lower than the densest hot Neptune ever found, meaning it could not survive more than 100 million years before spiraling into the giant star and starting to die. “This Earth is not evolving the way we thought it would. It seems to have a much larger, less dense atmosphere than we expected for these star systems,” said Dr. Sam Grunblatt, an astrophysicist at Johns Hopkins University.

The big question is how it managed to maintain this atmosphere despite being so close to such a massive host star. ” “TIC 365102760b is the smallest planet ever found orbiting a red giant, and possibly the lowest-mass planet we’ve ever seen orbiting a red giant.” “So he looks really odd. We don’t know why he still has an atmosphere, when other hot Neptunes are much smaller and denser, and seem to lose their atmospheres in less extreme environments.” Dr. Grunblatt and his colleagues were able to gain such insight by developing a new way to fine-tune data from NASA’s Transiting Exoplanet Survey Satellite (TESS). The satellite’s telescope can detect low-density planets that dim their brightness as they pass in front of their parent stars. But the research team removed unwanted light from the images and combined them with additional measurements from the W.M. Keck Observatory, a facility that tracks tiny variations in stars caused by orbiting planets. “This discovery could help scientists better understand how Earth-like atmospheres evolve,” said Dr. Green Leaf.

source: https://iopscience.iop.org/article/10.3847/1538-3881/ad4149