## Abstract

^{-3}) transiting planet in a P = 6.96 day orbit around a late G dwarf star. Using transit observations from TESS, LCOGT, OPM, 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 ± 0.3 R⊕ and a mass of M

_{p}= 25.1 ± 3.8 M⊕. 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

_{env}= 82

^{+7}

_{-6}%, suggesting that runaway gas accretion occurred when its core was at most 4-5x 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.

Original language | English |
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Publisher | arXiv |

Number of pages | 18 |

Publication status | Submitted - 18 Sept 2023 |