Astronomers have found a planet with the average density of a marshmallow.

Along with being a big softie, scientists found that the Jupiter-sized exoplanet would also float if it were hypothetically put in a giant cosmic bathtub.

Astronomers using the Kitt Peak National Observatory telescope in Arizona, observed an unusual planet in orbit around a cool red dwarf star (more on that later).

Located approximately 580 light-years from Earth in the constellation of Auriga the Charioteer, this planet, identified as TOI-3757 b, is the lowest-density planet ever detected around a red dwarf star.

TOI-3757 b's average density was calculated as being 0.27 grams per cubic centimeter (about 17 grams per cubic feet), which would make it less than half the density of Saturn (the lowest-density planet in the Solar System), about one quarter the density of water, or in fact, similar in density to a marshmallow.

NASA's Transiting Exoplanet Survey Satellite observed the crossing of this planet TOI-3757-b in front of its star, which allowed astronomers to calculate the planet's diameter to be about 100,000 miles (150,000 kilometers) or about just slightly larger than that of Jupiter.

The planet finishes one complete orbit around its host star in just 3.5 days, 25-times less than the closest planet in our Solar System—Mercury—which takes about 88 days to do so. One might think this would be enough to "roast" our marshmallow planet.

However red dwarf stars can also be cool, or an M dwarf star.

Red dwarf stars are the smallest and dimmest members of so-called main-sequence stars—stars that convert hydrogen into helium in their cores at a steady rate. Though "cool" compared to stars like our Sun, red dwarf stars can be extremely active and erupt with powerful flares capable of stripping a planet of its atmosphere, making this star system a seemingly inhospitable location to form such a gossamer planet.

"Giant planets around red dwarf stars have traditionally been thought to be hard to form," says Shubham Kanodia, first author on a paper published in The Astronomical Journal.

"So far this has only been looked at with small samples… which typically have found giant planets further away from these red dwarf stars. Until now we have not had a large enough sample of planets to find close-in gas planets in a robust manner."

There are still unexplained mysteries surrounding TOI-3757 b, the big one being how a gas-giant planet can form around a red dwarf star, and especially such a low-density planet. Kanodia's team, however, thinks they might have a solution to that mystery.

They propose that the extra-low density of TOI-3757 b could be the result of two factors. The first relates to the rocky core of the planet; gas giants are thought to begin as massive rocky cores about ten times the mass of Earth, at which point they rapidly pull in large amounts of neighboring gas to form the gas giants we see today.

TOI-3757b's star has a lower abundance of heavy elements compared to other M-dwarfs with gas giants, and this may have resulted in the rocky core forming more slowly, delaying the onset of gas accretion and therefore affecting the planet's overall density.

The second factor may be the planet's orbit, which is tentatively thought to be slightly elliptical. There are times it gets closer to its star than at other times, resulting in substantial excess heating that can cause the planet's atmosphere to bloat.

"Potential future observations of the atmosphere of this planet using NASA's new James Webb Space Telescope could help shed light on its puffy nature," says Jessica Libby-Roberts the second author on the paper.

"Finding more such systems with giant planets, which were once theorized to be extremely rare around red dwarfs, is part of our goal to understand how planets form," adds Kanodia.

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