Artist’s impression of an ultra-fluffy gas giant planet orbiting a red dwarf star. A gas giant exoplanet [right] with the density of a marshmallow has been detected in orbit around a cool red dwarf star [left] by 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. The planet, called TOI-3757 b, is the fluffiest gas giant planet ever discovered around this type of star. Credit: NOIRLab/NSF/AURA/J. da Silva/Spaceengine/M. Zamani
Kitt Peak National Observatory’s telescope helps determine[{” 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.
“Giant planets around red dwarf stars have traditionally been thought to be difficult to form,” says Shubham Kanodia, a researcher in the Earth and Planets Laboratory at the Carnegie Institution for Science and first author of a paper published in the astronomical diaryI “Until now, this has only been analyzed with small samples of Doppler surveys, which have generally found giant planets further away from these red dwarf stars. So far 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, the most important being how a gas giant planet can form around a red dwarf star, and especially a low-density planet. The Kanodia team, however, thinks they might have a solution to that mystery.
From the ground at Kitt Peak National Observatory (KPNO), a program of NSF’s NOIRLab, the Wisconsin-Indiana-Yale-NOIRLab (WIYN) 3.5-meter Telescope apparently watches the Milky Way as it spills over the horizon . A reddish glow, 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 shows the favorable conditions in this environment that are needed to see faint celestial objects. These conditions, which include low levels of light pollution, a sky darker than magnitude 20, and dry atmospheric conditions, have enabled WIYN Consortium researchers to conduct observations of galaxies, nebulae, and exoplanets, as well as many other astronomical targets using the WIYN 3.5-meter telescope and its sister telescope, the WIYN 0.9-meter telescope. Credit: KPNO/NOIRLab/NSF/AURA/R. sparks
They propose that TOI-3757 b’s extra-low density could be the result of two factors. The first is related 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 which point they rapidly attract 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 gas giant M dwarfs, and this may have resulted in the rocky core forming more slowly, delaying the start of gas accretion and , therefore, affecting the overall density of the planet.
The second factor may be the planet’s orbit, which is tentatively believed to be slightly elliptical. There are times when it comes closer to its star than other times, resulting in substantial excess heat that can cause the planet’s atmosphere to swell.
NASA Transiting Exoplanet Survey Satellite ([{” 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
