Astronomers find a fluffy planet with the density of a marshmallow

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

Besides being a big softie, scientists found that the Jupiter-sized exoplanet would also float if hypothetically placed in a giant cosmic bathtub.

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

Located about 580 light-years from Earth in the constellation Auriga the Charioteer, this planet, identified as TOI-3757 bis the lowest-density planet ever observed around a red dwarf star.

TOI-3757 b’s average density was calculated as 0.27 grams per cubic centimeter (about 17 grams per cubic foot), which would make it less than half the density of Saturn (the lowest density planet in the solar system), about a quarter the density of water, or in fact comparable in density to a marshmallow.

NASA’s Transiting Exoplanet Survey Satellite observed this planet’s crossing TOI-3757-b in front of its star, allowing astronomers to calculate the planet’s diameter at about 100,000 miles (150,000 kilometers), or about just a little larger than Jupiter’s.

The planet completes a full orbit around its parent 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. You might think this would be enough to “roast” our marshmallow planet.

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

Red dwarf stars are the smallest and faintest members of so-called main sequence stars — stars that convert hydrogen at their cores into helium at a constant rate. While “cool” compared to stars like our sun, red dwarf stars can be extremely active and erupt with powerful outbursts that can strip a planet of its atmosphere, making this galaxy a seemingly inhospitable location to form such a gossamer planet. .

“Giant planets around red dwarf stars have traditionally been difficult to form,” say Shubham Kanodia, first author on a paper published in The Astronomical Journal.

“So far this has only been looked at with small samples … which have typically found giant planets further away from these red dwarf stars. So far we haven’t had a sample of planets large enough to robustly detect nearby gas planets.”

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. However, Kanodia’s team believes they have a solution to that mystery.

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They propose that the extra-low density of TOI-3757 b may be the result of two factors. The first relates to the rocky core of the planet; Gas giants are thought to start out as massive rocky cores about ten times the mass of Earth, then quickly pull in large amounts of neighboring gas to form the gas giants we see today.

The star of TOI-3757b has a lower abundance of heavy elements compared to other M dwarfs with gas giants, and this may have caused the rocky core to form more slowly, delaying the onset of gas accretion and thereby increasing the overall density of the planet is affected.

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The second factor may be the planet’s orbit, which is tentatively considered somewhat elliptical. Sometimes it gets closer to its star than at other times, resulting in significant excessive heating that can swell the planet’s atmosphere.

“Possible future observations of this planet’s atmosphere using NASA’s new James Webb Space Telescope could help shed light on the swollen nature,” said Jessica Libby-Roberts, the paper’s second author.

“Finding more such systems with giant planets, once believed to be extremely rare around red dwarfs, is part of our goal to understand how planets form,” Kanodia added.

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