The size of the gas giant TOI-3757 b is slightly larger than that of Jupiter, but its density is many times lower and comparable to the density of a bottle cap. It is unclear how such a "loose" planet can survive in close proximity to the restless red dwarf, which can easily destroy its rarefied outer shell.
Observations by the U.S. National Kitt Peak Observatory have spotted a very unusual exoplanet. It is about 580 light-years away, in the constellation of Ascendant, orbiting the red dwarf TOI-3757. The planet itself is a gas giant, resembling our Jupiter and close to it in size. However, its density is almost five times less and is comparable to the density of a bottle cap. A new article published in The Astronomical Journal reports on the discovery.
Red dwarfs are the smallest and dimmest of the main-sequence stars. Despite this, they are characterized by highly unstable activity, now and then throwing out powerful bursts of radiation and particles. This makes their environment not the most favorable place for the formation and evolution of planets, especially gas giants, characterized by low density. If such exoplanets appear in red dwarfs, they usually appear in distant orbits, at a decent distance from the restless star.
The more unexpected was a new discovery made with the 3.5-meter telescope WIYN method of radial velocities - according to the weak deviations in the position of the star, which is caused by a planet orbiting it. Planet TOI-3757 b is in the red dwarf system, its diameter reaches 150 thousand kilometers (little more than Jupiterian) and mass - only 85 masses of Earth. It turns out that the density of TOI-3757 b is only 0.27 grams per centimeter cubic, which is comparable with the density of cork (0.24 grams per centimeter cubic).
Recall that the density of our Earth is about 5.5 grams per centimeter cubic, and, for example, Jupiter is only 1.3. Of course, there are known exoplanets of even lower density. However, TOI-3757 b is extremely close to its star - much closer than even Mercury to the Sun. It makes a complete annual orbit in just 3.5 of our days.
The authors of the article name two factors that may explain the appearance of such a "puffy" world in close proximity to the red dwarf. First, the star TOI-3757 is relatively poor in heavy elements, which begins the formation of gas giants. Such planets are thought to grow from a massive - larger than the entire Earth - rocky core, which collects the surrounding gas. If there are few heavy elements in the protoplanetary disk, the core may have formed much more slowly, and the gas may have accumulated longer, forming sparse shells. In addition, the orbit of TOI-3757 b is slightly elongated, and perhaps, as it approaches the star, its atmosphere may additionally heat up and expand, which leads to lower estimates of the planet's density.
