Marshmallow World Discovered: Huge fluffy planet orbiting a cool red dwarf star

Using the WIYN 3.5-meter Telescope at Kitt Peak National Observatory in Arizona, astronomers have observed an unusual Jupiter-like planet in orbit around a cool red dwarf star. Located in the constellation of Auriga the Charioteer around 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 akin to that of a marshmallow.

Red dwarf stars are the smallest and dimmest members of so-called main-sequence stars — stars that convert hydrogen into helium in their cores at a steady rate. Although they are “cool” compared to stars like our Sun, red dwarf stars can be extremely active and erupt with powerful flares. This can strip orbiting planets of their atmospheres, making this star system a seemingly inhospitable location to form such a gossamer planet.

“Giant planets around red dwarf stars have traditionally been considered difficult to form,” says Shubham Kanodia, a researcher at the Carnegie Institution for Science’s Earth and Planets Laboratory and first author of a paper published in The Astronomical Magazinel. “So far, this has only been studied with small samples from Doppler surveys, which have typically found giant planets further away from these red dwarf stars. Until now, we have not had a large enough sample of planets to reliably find nearby gas planets.”

There are still unsolved mysteries surrounding TOI-3757 b, the big one, of how a gas giant planet can form around a red dwarf star, and particularly such a sparse planet. However, Kanodia’s team thinks they may have a solution to this mystery.

From the bottom of Kitt Peak National Observatory (KPNO), a program of NSF’s NOIRLab, the Wisconsin-Indiana-Yale-NOIRLab (WIYN) 3.5-meter telescope appears to be observing the Milky Way looming on the horizon. A reddish airglow, a natural phenomenon, also colors the horizon. KPNO is located in the Arizona-Sonoran Desert on the Tohono O’odham Nation, and this clear view of part of the Milky Way’s galactic plane demonstrates the favorable conditions in this area needed to see faint celestial objects. These conditions, which include low light pollution, skies darker than 20 orders of magnitude, and dry atmospheric conditions, have enabled researchers from the WIYN consortium to conduct observations of galaxies, nebulae, and exoplanets, as well as many other astronomical targets on the WIYN 3.5 meter telescope and its sister telescope, the WIYN 0.9 meter telescope. Credit: KPNO/NOIRLab/NSF/AURA/R. Spark

They suggest that TOI-3757 b’s particularly low density may be the result of two factors. The first relates to the rocky core of the planet; Gas giants are thought to start out as massive rocky cores about ten times the mass of Earth. At this point, they quickly pull in large amounts of neighboring gas to form the gas giants we see today. TOI-3757b’s star has a lower abundance of heavy elements compared to other M-dwarfs with gas giants, which may have caused the rocky core to form more slowly, delaying the onset of gas accretion and therefore affecting the planet’s overall density .

The second factor could be the planet’s orbit, which is tentatively thought to be slightly elliptical. Sometimes it gets closer to its star than other times, causing significant overheating that can bloat the planet’s atmosphere.

NASA’s Transiting Exoplanet Survey Satellite (

” data-gt-translate-attributes=”[{” attribute=””>TESS) initially spotted the planet. Kanodia’s team then made follow-up observations using ground-based instruments, including NEID and NESSI (NN-EXPLORE Exoplanet Stellar Speckle Imager), both housed at the WIYN 3.5-meter Telescope; the Habitable-zone Planet Finder (HPF) on the Hobby-Eberly Telescope; and the Red Buttes Observatory (RBO) in Wyoming.

TESS surveyed the crossing of this planet TOI-3757 b in front of its star, which allowed astronomers to calculate the planet’s diameter to be about 150,000 kilometers (100,000 miles) or about just slightly larger than that of Jupiter. The planet finishes one complete 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.

The astronomers then used NEID and HPF to measure the star’s apparent motion along the line of sight, also known as its radial velocity. These measurements provided the planet’s mass, which was calculated to be about one-quarter that of Jupiter, or about 85 times the mass of the Earth. Knowing the size and the mass allowed Kanodia’s team to calculate TOI-3757 b’s average density as being 0.27 grams per cubic centimeter (about 17 grams per cubic feet), which would make it less than half the density of

“Potential future observations of the atmosphere of this planet using NASA’s new