Unusual puffy exoplanet has the density of marshmallow

Exoplanets come in all sorts of sizes and all sorts of densities, from solid rocky planets like Earth or Mars to super-puff planets discovered by Hubble. Now, researchers using the WIYN 3.5-meter Telescope at Kitt Peak National Observatory have identified a puffy, low-density “marshmallow” planet orbiting a cool red dwarf star. TOI-3757 b, located 580 light-years away, is the lowest-density gas giant planet ever discovered orbiting this kind of star.

Red dwarfs are the most common type of star in the galaxy and are cooler than our sun, but they can give off powerful flares of radiation that would bombard nearby planets with ultraviolet light. These flares could strip the atmosphere off a planet, so it was thought that it would be unlikely to find puffy gas giants orbiting these stars.

A gas giant exoplanet [right] with the density of a marshmallow has been detected in orbit around a cool red dwarf star [left] by the NASA-funded NEID radial-velocity instrument on the 3.5-meter WIYN Telescope at Kitt Peak National Observatory, a Program of NSF’s NOIRLab. The planet, named TOI-3757 b, is the fluffiest gas giant planet ever discovered around this type of star. NOIRLab/NSF/AURA/J. da Silva/Spaceengine/M. Zamani

“Giant planets around red dwarf stars have traditionally been thought to be hard to form,” said lead researcher Shubham Kanodia of the Carnegie Institution for Science in a statement. “So far this has only been looked at with small samples from Doppler surveys, 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.”

The researchers think that this unusual planet was able to form because of the composition of its stars and its orbit. The star has an unusually low level of heavier elements which could have caused the core of the planet to form more slowly than is typical, making it acquire gas slowly and contributing to its low density. The planet’s orbit seems to be elliptical as well, meaning it comes closer to the star at some times than at others, which would have warmed the planet and caused its atmosphere to puff up further.

Current estimations put the density of the planet at an average of 0.27 grams per cubic centimeter, which is less than half the density of Saturn — “or in fact, similar in density to a marshmallow,” as NOIRLab writes.

The researchers plan to make more observations of the planet using the James Webb Space Telescope, looking at its atmosphere in particular. And they plan to search for other similar planets.

“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,” said Kanodia.

The research is published in The Astronomical Journal.

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