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 discovered orbiting 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 ever discovered around this type of star. Credit: NOIRLab/NSF/AURA/J. da Silva/Spaceengine/M. Zmani
The National Observatory’s Kitt Peak Telescope is helping to determine that the Jupiter-like planet is the lowest-density gas giant ever discovered around a red dwarf.
A gas giant exoplanet with the density of a marshmallow has been discovered orbiting a cool red dwarf. An array of astronomical instruments, 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, were used to make the observations. Named TOI-3757 b, the exoplanet is the fluffiest gas giant ever discovered around this type of star.
Using the 3.5-meter WIYN telescope at Kitt Peak National Observatory in Arizona, astronomers have observed an unusual Jupiter-like planet orbiting a cool red dwarf. Located in the constellation Auriga the Chariot about 580 light-years from Earth, this planet, identified as TOI-3757 b, is the lowest-density planet ever discovered around a red dwarf star and is thought to have an average density similar to from marshmallows.
Red dwarf stars are the smallest and darkest members of so-called main-sequence stars—stars that 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 erupt in powerful flares. This can strip orbiting planets of their atmospheres, making this star system a seemingly inhospitable place for such a thin planet to form.
“Giant planets around red dwarf stars have traditionally been thought to be difficult to form,” said Shubam Kanodia, a researcher at the Carnegie Institution for Earth and Planetary Laboratory and first author of a paper published in The Astronomical Journal. “Until now, this has only been looked at with small samples of 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 unexplained mysteries surrounding TOI-3757 b, the biggest of which is how a gas giant planet could form around a red dwarf star, and especially such a low-density planet. However, the Kanodia team thinks there may be a solution to this 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 looks out over the Milky Way as it spills over the horizon. A reddish glow in the air, a natural phenomenon, also colors the horizon. KPNO is located in the Arizona-Sonoran Desert of the Tohono O’odham Nation, and this clear view of part of the Milky Way’s galactic plain shows the favorable conditions in this environment that are necessary for viewing faint celestial objects. These conditions, which include low levels of light pollution, skies darker than magnitude 20, and dry atmospheric conditions, have allowed researchers from the WIYN consortium to continue observations of galaxies, nebulae, and exoplanets, as well as many other astronomical targets, using 3 .5-meter WIYN telescope and its sister telescope, the 0.9-meter WIYN telescope. Credit: KPNO/NOIRLab/NSF/AURA/R. Sparks
They suggest that the extremely low density of TOI-3757 b may be the result of two factors. The first refers to the planet’s rocky core; Gas giants are thought to begin 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 than other M-dwarf gas giants, and this may have caused the rocky core to form more slowly, delaying the onset of gas accretion and therefore affecting the overall density of the planet.
The second factor may be the planet’s orbit, which is tentatively considered to be slightly elliptical. There are times when it is closer to its star than at other times, resulting in significant excessive heating that can cause the planet’s atmosphere to swell.
NASA’s Transiting Exoplanet Survey Satellite (TESS) first spotted the planet. The Kanodia team then made follow-up observations using ground-based instruments, including NEID and NESSI (NN-EXPLORE Exoplanet Stellar Speckle Imager), both located on the WIYN 3.5-meter telescope; the Hobby-Eberly Telescope’s Habitable Planet Finder (HPF); and Red Buttes Observatory (RBO) in Wyoming.
TESS probed 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 slightly larger than that of Jupiter. The planet completes one full orbit around its host star in just 3.5 days, 25 times less than the nearest planet in our solar system – Mercury – which takes about 88 days to do so.
Astronomers then used the NEID and HPF to measure the star’s apparent line-of-sight motion, also known as its radial velocity. These measurements give the planet’s mass, which is estimated to be about one-fourth that of Jupiter, or about 85 times the mass of Earth. Knowing the size and mass allowed Kanodia’s team to calculate the average density of TOI-3757 b as 0.27 grams per cubic centimeter (about 17 grams per cubic foot), which would make it less than half the density of Saturn ( the planet with the lowest density). in the Solar System), about a quarter the density of water (meaning it would float if placed in a giant tub full of water), or, in fact, similar in density to a marshmallow.
“Potential future observations of this planet’s atmosphere using NASA’s new James Webb Space Telescope could help shed light on its puffy nature,” said Jessica Libby-Roberts, a postdoctoral researcher at Pennsylvania State University and the second author of this article.
“Finding more such systems with giant planets – once thought to be extremely rare around red dwarfs – is part of our goal to understand how planets form,” says Kanodia.
The discovery underscores the importance of NEID in its ability to confirm some of the candidate exoplanets currently discovered by NASA’s TESS mission, providing important targets for the new James Webb Space Telescope (JWST) to track and begin characterizing their atmospheres. This in turn will inform astronomers what the planets are made of and how they formed, and for potentially habitable rocky worlds, whether they can 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 SJ 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 Harty, Sarah E. Logsdon, Andrew J. Metcalfe, 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 Aug 2022, The Astronomical Journal. DOI: 10.3847/1538-3881/ac7c20
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