HARPS-N and NEID radial velocities of TOI-1420

Using transit observations from TESS, LCOGT, Observatoire Prive 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 Rp=11.9{+/-}0.3R{Earth} and a mass of Mp=25.1{+/-}3.8M{Earth}.

TOI-1420b is the largest known planet with a mass less than 50M{Earth}, indicating that it contains a sizeable envelope of hydrogen and helium. We determine TOI-1420b's envelope mass fraction to be f~=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.