TOI-3757b is the lowest-density transiting exoplanet known to orbit an M-dwarf (red dwarf) star.
An artist’s impression of TOI-3757b and its parent star. Image credit: 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 Dr. Shubham Kanodia, an astronomer at Carnegie Institution for Science.
“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 newly-discovered giant exoplanet is slightly larger than Jupiter but four times less massive.
Dubbed TOI-3757b, it has an average density of 0.27 g/cm3, which would make it less than half the density of Saturn (the lowest-density planet in the Solar System), about one quarter the density of water (meaning it would float if placed in a giant bathtub filled with water).
The planet finishes one complete orbit around its host star, TOI-3757, in just 3.5 days.
Also known as TIC 445751830, 2MASS J06040089+5501126, and UCAC4 726-038940, the star is located approximately 580 light-years away in the constellation of Auriga.
“The extra-low density of TOI-3757b could be the result of two factors,” the astronomers said.
“The first relates to the rocky core of the planet; gas giants are thought to begin as massive rocky cores about 10 times the mass of Earth, at which point they rapidly pull in large amounts of neighboring gas to form the gas giants we see today.”
“TOI-3757 has a lower abundance of heavy elements compared to other M-dwarfs with gas giants, and this may have resulted in the rocky core forming more slowly, delaying the onset of gas accretion and therefore affecting the planet’s overall density.”
“The second factor may be the planet’s orbit, which is tentatively thought to be slightly elliptical.”
“There are times it gets closer to its star than at other times, resulting in substantial excess heating that can cause the planet’s atmosphere to bloat.”
TOI-3757b was discovered using NASA’s Transiting Exoplanet Survey Satellite (TESS) and confirmed with the Habitable-zone Planet Finder (HPF) on the Hobby-Eberly Telescope and the NEID instrument on the WIYN 3.5-m telescope.
“Potential future observations of the atmosphere of this planet using the NASA/ESA/CSA James Webb Space Telescope could help shed light on its puffy nature,” said Dr. Jessica Libby-Roberts, a postdoctoral researcher at Pennsylvania State University.
“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,” Dr. Kanodia said.
The discovery is reported in a paper in the Astronomical Journal.
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Shubham Kanodia et al. 2022. TOI-3757b: A Low-density Gas Giant Orbiting a Solar-metallicity M Dwarf. AJ 164, 81; doi: 10.3847/1538-3881/ac7c20
