Helium in the Extended Atmosphere of the Warm Superpuff TOI-1420b

Superpuffs are planets with exceptionally low densities (rho less than or similar to 0.1 g cm-3) and core masses (M c less than or similar to 5M circle plus). Many lower-mass (M p less than or similar to 10M circle plus) superpuffs are expected to be unstable to catastrophic mass loss via photoevaporation and/or boil-off, whereas the larger gravitational potentials of higher-mass (M p greater than or similar to 10M circle plus) superpuffs should make them more stable to these processes. We test this expectation by studying atmospheric loss in the warm, higher-mass superpuff TOI-1420b (M = 25.1M circle plus, R = 11.9R circle plus, rho = 0.08 g cm-3, T eq = 960 K). We observed one full transit and one partial transit of this planet using the metastable helium filter on Palomar/WIRC and found that the helium transits were 0.671% +/- 0.079% (8.5 sigma) deeper than the TESS transits, indicating an outflowing atmosphere. We modeled the excess helium absorption using a self-consistent 1D hydrodynamics code to constrain the thermal structure of the outflow given different assumptions for the stellar XUV spectrum. These calculations then informed a 3D simulation, which provided a good match to the observations with a modest planetary mass-loss rate of 1010.82 g s-1 ( M p / M approximate to 70 Gyr). Superpuffs with M p greater than or similar to 10M circle plus, like TOI-1420b and WASP-107b, appear perfectly capable of retaining atmospheres over long timescales; therefore, these planets may have formed with the unusually large envelope mass fractions they appear to possess today. Alternatively, tidal circularization could have plausibly heated and inflated these planets, which would bring their envelope mass fractions into better agreement with expectations from core-nucleated accretion.