A giant fluffy planet orbiting a cool red dwarf star

Artistic impression of a very thin gas giant planet orbiting a red dwarf star. Gas giant outer planet [right] Density of marshmallows detected in orbit around a cool red dwarf star [left] by the NASA-funded NEID Radial Velocity Instrument on the 3.5-meter WIYN Telescope at Kitt Peak National Observatory, a program of NSF NOIRLab. The planet, called TOI-3757 b, is the thinnest gas giant planet ever discovered around this type of star. Credit: NOIRLab/NSF/AURA/J. da Silva/Spaceengine/M. Zamani

The National Observatory’s Kitt Peak Telescope helps determine this[{” attribute=””>Jupiter-like Planet is the lowest-density gas giant ever detected around a red dwarf.

A gas giant exoplanet with the density of a marshmallow has been detected in orbit around a cool red dwarf star. A suite of astronomical instruments was used to make the observations, including the NASA-funded NEID radial-velocity instrument on the WIYN 3.5-meter Telescope at Kitt Peak National Observatory, a Program of NSF’s NOIRLab. Named TOI-3757 b, the exoplanet is the fluffiest gas giant planet ever discovered around this type of 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.

Shubham Kanodia, a researcher at the Carnegie Institution for Science’s Earth and Planetary Laboratory and first author on a paper published in Astrological Journato. So far this has only been seen by small samples of Doppler surveys, which have usually found giant planets far from these red dwarf stars. Until now we haven’t had a large enough sample of planets to find nearby gas planets in a robust way.”

There are still unexplained mysteries surrounding TOI-3757 b, chief among them how a gas giant planet could form around a red dwarf star, especially a low-density planet. However, the Kanodia team believes they may have a solution to this mystery.

From Earth from Kit Peak National Observatory (KPNO), a program of NSF NOIRLab, the 3.5-meter Wisconsin-Indiana-Yale-Noirlab (WIYN) telescope appears to be observing the Milky Way as it spills out from the horizon. Reddish atmospheric glare, a natural phenomenon, also colors the horizon. KPNO is located in the Arizona Sonoran Desert in the Tohono O’odham Nation and this clear view of part of the plane of the Milky Way shows the favorable conditions in this environment needed to view faint celestial bodies. These conditions, which include low levels of light pollution, 20-degree darker skies, and dry weather conditions, have allowed researchers at the WIYN consortium to continue to observe galaxies, nebulae, and exoplanets as well as many other astronomical targets using the WIYN 3.5-meter telescope and its sister, the WIYN 0.9-meter telescope. . Credit: KPNO/NOIRLab/NSF/AURA/R. Sparks

They suggest that the extremely low density of TOI-3757 b could 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 with a mass about ten times the mass of Earth, at which point they rapidly pull in large amounts of nearby gas to form the gas giants we see today. TOI-3757b has a lower abundance of heavy elements than other M dwarfs with gas giants, and this may have resulted in the formation of the rocky core more slowly, delaying the onset of gas accumulation and thus affecting the planet’s overall density.

A second factor may be the planet’s orbit, which is tentatively thought to be slightly elliptical. There are times when it gets closer to its star than at other times, resulting in significant excess heating that can cause the planet’s atmosphere to swell.

NASA’s Transiting Satellite for Exoplanet Survey ([{” 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 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), or in fact, similar in density to a marshmallow.

“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 and the second author on this paper.

“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,” says Kanodia.

The discovery highlights the importance of NEID in its ability to confirm some of the candidate exoplanets currently being discovered by NASA’s TESS mission, providing important targets for the new James Webb Space Telescope (JWST) to follow up on and begin characterizing their atmospheres. This will in turn inform astronomers what the planets are made of and how they formed and, for potentially habitable rocky worlds, whether they might be able to support life.

Reference: “TOI-3757 b: A low-density gas giant orbiting a solar-metallicity M dwarf” by Shubham Kanodia, Jessica Libby-Roberts, Caleb I. Cañas, Joe P. Ninan, Suvrath Mahadevan, Gudmundur Stefansson, Andrea S. J. Lin, Sinclaire Jones, Andrew Monson, Brock A. Parker, Henry A. Kobulnicky, Tera N. Swaby, Luke Powers, Corey Beard, Chad F. Bender, Cullen H. Blake, William D. Cochran, Jiayin Dong, Scott A. Diddams, Connor Fredrick, Arvind F. Gupta, Samuel Halverson, Fred Hearty, Sarah E. Logsdon, Andrew J. Metcalf, Michael W. McElwain, Caroline Morley, Jayadev Rajagopal, Lawrence W. Ramsey, Paul Robertson, Arpita Roy, Christian Schwab, Ryan C. Terrien, John Wisniewski and Jason T. Wright, 5 August 2022, The Astronomical Journal.
DOI: 10.3847/1538-3881/ac7c20