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

Superpuffs are planets with exceptionally low densities ( ρ ≲ 0.1 g cm ^−3 ) and core masses ( M _c ≲ 5 M _⊕ ). Many lower-mass ( M _p ≲ 10 M _⊕ ) 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 ≳ 10 M _⊕ ) 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 = 25.1 M _⊕ , R = 11.9 R _⊕ , ρ = 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 σ ) 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 10 ^10.82 g s ^−1 ( ${M}_{p}/\dot{M}\approx 70$ Gyr). Superpuffs with M _p ≳ 10 M _⊕ , 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.