Article by: Andacs Robert Eugen, on 28 October 2022, at 09:46 am PDT

Using the 3.5-meter WIYN telescope at Kitt Peak National Observatory in Arizona, astronomers have spotted an unusual Jupiter-like planet orbiting a cool red dwarf star. Located in the constellation Auriga, about 580 light-years from Earth, this planet, identified as TOI-3757 b, is the lowest-density planet ever detected around a red dwarf star and is estimated to have an average density similar to that of a marshmallow.

Red dwarf stars convert hydrogen into helium in their cores at a constant rate. Although they are "cool" compared to stars like our Sun, red dwarf stars can be extremely active and trigger powerful flares. This can affect the planets orbiting them, making this star system inhospitable to form such a planet.

"Giant planets around red dwarf stars have traditionally been thought to be difficult to form," says Shubham Kanodia, a researcher at the Carnegie Institution for Science's Earth and Planets Laboratory and first author of the paper published in The Astronomical Journal.

"This has only been looked at using small samples from Doppler studies, which usually find giant planets at a greater distance from these red dwarf stars. So far we haven't had a large enough sample of planets to find close gas planets".

There are still unexplained mysteries surrounding TOI-3757 b, the biggest being how a gas giant planet can form around a red dwarf star, especially one with such a low density. However, Kanodia's team believes they may have an explanation for this mystery.

They intuit that the very low density of planet TOI-3757 b could be the result of two factors.

The first concerns the rocky core of the planet - gas giants are thought to start as massive rocky cores about ten times the mass of Earth, at which point they rapidly attract large amounts of gas to form the gas giants we see today. TOI-3757b's star has a lower abundance of heavy elements compared to other gas giant dwarfs, and this could have led to the slower formation of the rocky core, delaying the onset of gas accretion and thus affecting the overall density of the planet.

The second factor may be the planet's orbit, which is tentatively believed to be slightly elliptical. At certain times it is closer to its star than at other times, leading to substantial overheating that can cause the planet's atmosphere to "swell".

The planet was initially observed by NASA's Transiting Exoplanet Survey Satellite (TESS).

Kanodia's team then made follow-up observations using ground-based instruments, including NEID and NESSI (NN-EXPLORE Exoplanet Exoplanet Stellar Speckle Imager), both hosted by WIYN's 3.5-meter Habitable-zone Planet Finder (HPF) telescope on the Hobby Telescope -Eberly, and the Red Buttes Observatory (RBO) in Wyoming.

TESS observed the passage of this planet TOI-3757 b in front of its star, which allowed astronomers to calculate that the diameter of the planet is about 150,000 kilometers, or slightly larger than that of Jupiter.

The planet completes a full orbit around its host star in just 3.5 days, 25 times less than the closest planet in our solar system - Mercury - which takes about 88 days to do so.

"Potential future observations of this planet's atmosphere with NASA's new James Webb Space Telescope could help clarify its nature," says Jessica Libby-Roberts, a postdoctoral researcher at Pennsylvania State University and second author of this paper.