A marshmallow-like planet: Astronomers discover an incredibly low-density gas giant

Follow-up observations from James Webb could shed new light on the mysterious exoplanet.
Chris Young
An artist's impression of the marshmallow-like planet orbiting its star.
An artist's impression of the marshmallow-like planet orbiting its star.

Source: NOIRLab/NSF/AURA/J. da Silva/Spaceengine/M. Zamani 

Scientists just discovered the squishiest planet ever observed.

The planet, dubbed TOI-3757 b, stood out to astronomers due to the fact that it has the density of a marshmallow, a press statement reveals.

The 150,000-kilometer-diameter planet orbits a cool red dwarf roughly 580 light-years from Earth in the constellation of Auriga the Charioteer. It is slightly larger than Jupiter and orbits its host star once every 3.5 days.

Astronomers don't quite understand how the marshmallow-like planet formed, but future observations by the James Webb Space Telescope could shed new light on the mysterious exoplanet.

A marshmallow-like planet

The research team investigating TOI-3757 b calculated its average density as 0.27 grams per cubic centimeter. That means the planet has roughly a quarter the density of water, and it would float if placed in a planet-sized, cosmic bathtub.

Astronomers first observed TOI-3757 b using the WIYN 3.5-meter Telescope at Kitt Peak National Observatory in Arizona, a program of NSF’s NOIRLab. The researchers determined that the planet is the lowest-density gas giant ever detected around a red dwarf.

Red dwarf stars are colder than our Sun, but they can erupt and emit powerful explosions meaning scientists traditionally believed they are not ideal for planetary formation.

“Giant planets around red dwarf stars have traditionally been thought to be hard to form,” said Shubham Kanodia, a researcher at Carnegie Institution for Science’s Earth and Planets Laboratory.

“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."

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James Webb could shed new light on the planet's composition

Astronomers are unsure how TOI-3757 b could have formed with such a low density. One theory is that it has fewer heavy elements in its rocky core compared with other gas giants, like Jupiter. This could have delayed the accumulation of gases, leading to the lower density observed on the planet.

The planet's slightly elliptical orbit also brings the planet very close to its host star, which may have caused the planet's atmosphere to bloat. This recalls a marshmallow hovering over a campfire, though with an atmosphere and a rocky core — scratch that, they're not very similar at all.

“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, a postdoctoral researcher at Pennsylvania State University.

Upcoming James Webb observations could allow astronomers to understand better the planet's composition and how it formed in the first place.

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