TESS Spots a Super-puff: The Remarkably Low Density of TOI-1420b
López-Morales, Mercedes; Bieryla, Allyson; Latham, David W.; Berlind, Perry; Calkins, Michael L.; Esquerdo, Gilbert A.; Howell, Steve B.; Crossfield, Ian J. M.; Horne, Keith; Lissauer, Jack J.; Collins, Karen A.; Seager, Sara; Winn, Joshua N.; Jenkins, Jon M.; Barkaoui, Khalid; Ciardi, David R.; Dai, Fei; Logsdon, Sarah E.; Eastman, Jason D.; McLeod, Kim K.; Matson, Rachel A.; Schlieder, Joshua; Osborn, Hugh P.; Beichman, Charles; Gonzales, Erica J.; Matthews, Elisabeth C.; Striegel, Stephanie; Sozzetti, Alessandro; Stefánsson, Guđmundur; Srdoc, Gregor; Cosentino, Rosario; Ghedina, Adriano; Muñoz, Jose A.; Forés-Toribio, Raquel; Horta, Ferran Grau; Strakhov, Ivan A.; Laloum, Didier; Vissapragada, Shreyas; Goliguzova, Maria V.; Schmidt, Michael; Safonov, Boris; Klusmeyer, Jessica; Golub, Eli; Higuera, Jesus; Schweiker, Heidi; Thorngren, Daniel P.; Villaseñor, Joel; Malavolta, Luca; DiTomasso, Victoria; Yoshida, Stephanie; Buchave, Lars A.; Dowling, Nicholas; Hoch, Nora; Medina, Jennifer V.
United States, Belgium, Spain, Denmark, Germany, Russia, United Kingdom, France, Italy, Switzerland, Croatia
Abstract
We present the discovery of TOI-1420b, an exceptionally low-density (ρ = 0.08 ± 0.02 g cm-3) transiting planet in a P = 6.96 days orbit around a late G-dwarf star. Using transit observations from TESS, LCOGT, Observatoire Privé du Mont, Whitin, Wendelstein, OAUV, Ca l'Ou, and KeplerCam, along with radial velocity observations from HARPS-N and NEID, we find that the planet has a radius of R p = 11.9 ± 0.3R ⊕ and a mass of M p = 25.1 ± 3.8M ⊕. TOI-1420b is the largest known planet with a mass less than 50M ⊕, indicating that it contains a sizeable envelope of hydrogen and helium. We determine TOI-1420b's envelope mass fraction to be ${f}_{\mathrm{env}}={82}_{-6}^{+7} \% $ , suggesting that runaway gas accretion occurred when its core was at most four to five times the mass of the Earth. TOI-1420b is similar to the planet WASP-107b in mass, radius, density, and orbital period, so a comparison of these two systems may help reveal the origins of close-in low-density planets. With an atmospheric scale height of 1950 km, a transmission spectroscopy metric of 580, and a predicted Rossiter-McLaughlin amplitude of about 17 m s-1, TOI-1420b is an excellent target for future atmospheric and dynamical characterization.