In a paper recently published in The Astronomical Journal, astronomers have observed the lowest-density – or fluffiest – gas giant ever detected around a red dwarf star; its density is close to that of a marshmallow!
The planet – named TOI-3757 b – is approximately 580 light-years from Earth and orbits its host star in just 3.5 days. In comparison, Earth takes 365 days to orbit our star, the Sun. The host star of this planet is a red dwarf star: the smallest and dimmest of all stars. These stars are much cooler than the Sun, at temperatures of about half that of the Sun. Interestingly, these stars are much more active than Sun-like stars, emitting a lot of high-energy photons and particles in powerful flares. These kinds of conditions would make the star system seem like it would be rather inhospitable, with the flares capable of stripping the atmosphere off a planet entirely. Thus, gas giants around red giant stars have been thought to be hard to form given the extremely energetic and harsh environment. How then did such a delicate, low-density, planet form in such an environment?
The authors of the paper believe that there are two possible factors that have allowed such a fluffy planet to persist in such a hostile environment. The first factor has to do with the formation of the planet itself, while the second factor has to do with the orbit of the planet.
Gas giant planets are believed to begin as massive rocky cores, which are approximately 10 times the mass of the Earth. When the cores get massive enough, they begin to pull upon – or accrete – gas from their surroundings, which forms the gaseous exterior. The host star of this planet has been shown to have a lower abundance of heavy elements compared to other similar stars with gas giants in orbit around them. What does this mean? One, the rocky core of the planet could be made of less dense material. Two, maybe the rocky core of this planet formed more slowly, which would delay the onset of gas accretion. Thus, there would be less gas to accrete at this time since more gas would have gone into the formation of the star itself. Thus, the overall density of the planet would be affected.
On the other hand, the planet’s orbit is believed to be slightly elliptical. Thus, at times the planet is closer to the star, and at other times, the planet is further away from the star. When the planet is closest to its host star, the energy of the star would heat up the planet, causing the atmosphere to expand, increasing the overall size of the planet. Thus, decreasing the overall density of the planet.
Future observations of the atmosphere of this planet with NASA’s James Webb Space Telescope could help disentangle this mystery and help astronomers understand the nature of the extreme low density marshmallow world.
Source: NOIR Lab