Scorching Hot Planet the Size of Jupiter Is as Dense as a Marshmallow

Astronomers have discovered a new Jupiter-sized exoplanet that possesses less than one-third of the mass of the solar system's largest planet, giving it a density similar to a marshmallow.

The team from Pennsylvania State University (PSU) found the planet, designated TOI-3757 b, using NASA's Transiting Exoplanet Survey Satellite (TESS).

The planet has a radius of 1.09 times that of Jupiter, but a mass of just 0.27 times. It adds to a small catalog of gas giants found around M dwarfs, and it's the least dense exoplanet ever discovered around such a star.

The size and mass of TOI-3757 b give it a density of 0.27 grams per cubic centimeter. For comparison, Aqua Calc gives the density of marshmallow as just below this at 0.21 grams per cubic centimeter.

The astronomers were led by PSU Department of Astrophysical Sciences researcher Shubham Kanodia. Their discovery of the planet around the M dwarf star TOI-3757, which is about 37 percent smaller than the sun and located around 578 light-years from Earth, is detailed in a paper published on ArXiv.

Explaining the Planet's Low Density

The authors wrote in the paper: "We present the discovery of a new Jovian-sized planet, TOI-3757 b, the lowest density planet orbiting an M dwarf."

The discovery could reveal more about planetary formation as current models suggest that giant planets like this should be rare around small M dwarf stars as there should be less material in protoplanetary discs that surround them that's available for planet formation.

The team suggests potential explanations for the planet's low density. The first is connected to the lack of heavy elements, referred to as "metallicity" by astronomers, in its 7.1 billion-year-old host star.

TOI-3757 has the lowest stellar metallicity of all the M dwarf stars found hosting gas giants thus far. This could have caused a delay in the runaway accretion of gas that gives rise to planets, meaning the protoplanetary disk that formed TOI-3757 b had already begun to dissipate.

Alternatively, the researchers say that the planet may have been inflated by tidal heating by its star caused by its slightly flattened orbit. The authors suggest that could be tested by measuring the exoplanet's levels of methane and ammonia.

This would help determine the conditions within the interior of TOI-375 b, particularly its temperature, and could show if tidal heating had taken place.

Aside from its marshmallow-like density, TOI-3757 b is also remarkable for its proximity to its parent star. At a distance of just over 3.5 million miles from TOI-3757, slightly closer than Mercury is to the sun, the planet completes an orbit in around three and a half days.

This proximity also means that TOI-3757 b has a very hot temperature, which the team estimates to be around 907 degrees Fahrenheit.

Thus far TESS, which is currently conducting a survey of about 200,000 of the brightest stars near the sun with the aim of discovering worlds that cross, or transit, the face of their star, has identified over 5,400 exoplanet candidates. Around 200 of these have been confirmed, joining the 5,000 worlds in the NASA Exoplanet Archive.

Jupiter and Exoplanets
(Main) An artist's depiction of exoplanets orbiting their star. (Inset) An image of Jupiter captured by the Hubble Space Telescope. Astronomers have discovered a Jupiter-sized planet with under a third of its mass, giving it the density of a marshmallow. Instituto de Astrofísica de Canarias/ESA/NASA