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Astronomers have discovered a gas giant planet with the density of a cotton candy orbiting a cool red dwarf star 580 light-years from Earth. This Jupiter-like exoplanet is the lowest-density planet ever observed orbiting a red dwarf.

Red dwarfs are the smallest and dimmest stars, and they convert hydrogen into helium in their cores through nuclear fusion, a phase in the life cycle of a star astronomers call the "main sequence." Although much cooler than the Sun, red dwarfs are known to be very active and can emit powerful flares capable of stripping away the atmosphere of any orbiting planet.

These violent eruptions make red dwarf systems an unlikely and inhospitable place to search for puffy exoplanets like the newly discovered TOI-3757 b. The planet is so close to its red dwarf star that it completes one orbit in just 3.5 Earth days, 25 times faster than Mercury, the planet closest to the sun. Its diameter is slightly larger than that of Jupiter, the largest planet in the solar system, at about 100,000 miles (150,000 kilometers).

"Giant planets around red dwarfs have traditionally been thought to form with difficulty," study leader Shubham Kanodia, an astronomer at the Carnegie Institution for Science in Washington, D.C., said in a statement from the National Science Foundation's NOIRLab, which operates some of the telescopes used in the study.

"Until now, we have only observed this from a small sample of Doppler surveys, which typically find giant planets farther out from these red dwarfs," he said. "Until now, we haven't had a large enough sample of planets to find gaseous planets close to red dwarfs in a robust way."

Kanodia and colleagues think they may be able to explain how low-density gas giants can form under such adverse and extreme conditions. They propose that TOI-3757 b's low density is due to two main factors.

The first has to do with how gas giants form: They start with a massive rocky core about 10 times the mass of Earth, quickly attracting large amounts of surrounding gas to form a Jupiter-like shape.

The research team believes that the low abundance of heavy elements in the red dwarf star that TOI-3757 b orbits means that the rocky core of this particular exoplanet may have formed more slowly. This would delay the process of gas being accreted into the rocky core and have an impact on the overall density of TOI-3757 b.

Second, astronomers think TOI-3757 b's orbit is slightly elliptical, like a flattened circle, so it sometimes gets closer to its red dwarf star. Such close proximity would overheat the planet, causing its atmosphere to expand.

TOI-3757 b was originally discovered by NASA's Transiting Exoplanet Survey Satellite (TESS) mission, which finds exoplanets by observing the tiny dips in light caused by planets transiting, or crossing, the face of their stars. Kanodia and his team then followed up on this initial discovery using a range of ground-based instruments.

TESS observations determined the diameter of TOI-3757 b, while scientists used instruments at Kitt Peak National Observatory in Arizona and McDonald Observatory in Texas to measure the exoplanet's radial velocity, or how fast it moves along the line of sight. From these results, astronomers calculated the planet's mass to be about one-quarter that of Jupiter or about 85 times the mass of Earth.

Knowing the diameter and mass of TOI-3757 b allowed the team to calculate the gas giant's density to be about 0.6 ounces (17 grams) per cubic foot. This puts TOI-3757 b's density at less than half that of Saturn, the least dense planet in the solar system, at about a quarter that of water.

This means that the exoplanet has a density similar to that of a marshmallow, and if there were a bathtub large enough to hold the giant planet, it would float on the water.

The research team hopes to use the James Webb Space Telescope (JWST) to further study the peculiar atmosphere of this puffy exoplanet and search for similar planets in other star systems.

"Discovering more of these giant planet systems, which were once thought to be extremely rare around red dwarfs, is one of our goals in understanding planet formation," Kanodia concludes.

The team's findings have been published in The Astrophysical Journal.

BY: Robert Lea

FY: Marshmallow Planet

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