Free Astronomy Magazine January-February 2023

50 JANUARY-FEBRUARY 2023 ASTRO PUBLISHING on a paper published in The Astro- nomical Journal . “So far this has only been looked at with small sam- ples from Doppler surveys, which typically have found giant planets further away from these red dwarf stars. Until now we have not had a large enough sample of planets to find close-in gas planets in a robust manner.” There are still unexplained mysteries surrounding TOI-3757 b, the big one being how a gas-giant planet can form around a red dwarf star, and especially such a low-density planet. Kanodia’s team, however, thinks they might have a solution to that mystery. They propose that the extra-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 begin as mas- sive rocky cores about ten times the mass of Earth, at which point they rapidly pull in large amounts of neighboring gas to form the gas gi- ants we see today. TOI-3757b’s star has a lower abundance of heavy elements compared to other M- dwarfs with gas giants, and this may have resulted in the rocky core form- ing more slowly, delaying the onset of gas accretion and therefore af- fecting the planet’s overall density. The second factor may be the planet’s orbit, which is tentatively thought to be slightly elliptical. There are times it gets closer to its star than at other times, resulting in substan- tial excess heating that can cause the planet’s atmosphere to bloat. The planet was initially spotted by NASA’s Transiting Exoplanet Survey Satellite (TESS). Kanodia’s team then made follow-up observations using ground-based instruments, includ- ing 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 T his video presents the discovery of a new Jovian-sized planet, TOI-3757 b, the lowest-density transiting planet known to orbit an M dwarf. [NOIRLab/ NSF/AURA/J. da Silva/Spaceengine/M. Zamani, KPNO/P. Marenfeld, ESA/Hubble/ M. Kornmesser] Telescope; and the Red Buttes Ob- servatory (RBO) in Wyoming. TESS surveyed the crossing of this planet TOI-3757 b in front of its star, which allowed astronomers to calcu- late the planet’s diameter to be about 150,000 kilometers (100,000 miles) or about just slightly larger than that of Jupiter. The planet fin- ishes 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 al- lowed 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 Tel- escope 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 can- didate 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. !