An artistic impression of a very thin gas giant orbiting a cold red dwarf.

Astronomers have found a planet with an average density of marshmallows.

Besides being pretty fluffy, scientists have found that an exoplanet the size of Jupiter would also float if it was hypothetically placed in a giant cosmic bathtub.

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

Located about 580 light-years from Earth in the constellation of Auriga the Charioteer, this planet, known as TOI-3757 b, is the least dense planet ever discovered around a red dwarf star.

The average density of TOI-3757 b has been calculated as 0.27 grams per cubic centimeter (about 17 grams per cubic foot), making it less than half the density of Saturn (the least dense planet in the Solar System), about a quarter the density of water, or at Indeed, similar to the density of marshmallows.

NASA’s Transiting Exoplanet Survey satellite observed TOI-3757-b transiting in front of its star, allowing astronomers to calculate the planet’s diameter to be about 100,000 miles (150,000 km) or slightly larger than Jupiter.

The planet completed a full 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 would think that this would be enough to “roast” our planet marshmallow.

However, red dwarf stars can also be cold, or the M dwarf star.

Red dwarf stars are the smallest and fainter members of so-called main-sequence stars – stars that convert hydrogen into helium in their cores at a constant rate. Although “cool” compared to stars like our Sun, red dwarf stars can be very energetic and erupt with powerful flares capable of stripping a planet of its atmosphere, making this star system a seemingly inhospitable site for the formation of such a gossamer planet. .

“Giant planets around red dwarf stars are traditionally thought to be difficult to form,” says Shubham Kanodia, first author in a paper published in The Astronomical Journal.

“Until now, this has only been looked at with small samples… which have usually found giant planets far from these red dwarf stars. Until now we haven’t had a large enough sample of planets to find nearby gas planets in a robust way.”

There are still unexplained mysteries surrounding TOI-3757 b, chief among them how a gas giant planet could form around a red dwarf star, especially a low-density planet. However, the Kanodia team believes they may have a solution to this mystery.

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They suggest that the extremely 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 start out as massive rocky cores with a mass about ten times the mass of Earth, at which point they rapidly pull in large amounts of nearby gas to form the gas giants we see today.

TOI-3757b has a lower abundance of heavy elements than other M dwarfs with gas giants, and this may have resulted in the formation of the rocky core more slowly, delaying the onset of gas accumulation and thus affecting the planet’s overall density.

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

“Potential future observations of this planet’s atmosphere using NASA’s new James Webb Space Telescope can help shed light on its bloated nature,” says Jessica Libby Roberts, second author on the paper.

“Finding more such systems with giant planets, which were previously assumed to be very rare around red dwarfs, is part of our goal to understand how planets form,” Kanudia adds.

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