Understanding the Origins of Super-puff Planets: A New Mass-loss Regime Coupled to Planetary Evolution

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Abstract
Super-puffs are a class of low-mass, large-radius planets that have challenged planet formation and evolution models. Their high inferred H/He mass fractions, required to explain their physical sizes, would lead to rapid atmospheric escape, raising questions about their long-term retention. Recent modeling work indicates that low-mass planets typically require 50% less H/He mass to match their observed radius, due to the significant roles of the radiative atmosphere and interior heating from the rock/iron core. Here, through a new quantitative analysis of X-ray and EUV (XUV)–driven escape in sub-Neptunes, we find that previous studies overestimated mass loss, as scaling laws in low-gravity regimes deviate greatly from the widely used energy-limited regime. We define a new regime, thermal-energy-mediated photoevaporation, in which thermal-energy conversion critically sets the mass-loss rate. These effects make super-puffs more resilient to mass loss than previously thought. We develop a coupled evolution model integrating this updated thermal evolution framework with a 1D hydrodynamic photoevaporation model. Applying this novel, joint model to observed super-puffs and young low-density planets, we find that their masses, radii, and transit pressures align with predictions assuming either a clear or hazy atmosphere. This indicates that super-puffs have undergone a combination of boil-off and photoevaporative mass loss, with boil-off dominating the process. Our results indicate that low-density planets typically possess both a thick convective envelope and substantial radiative atmosphere, which contribute to their large radii. For this to occur, these planets must have intermediate masses of 5–10 M ⊕ and receive stellar insolation ≲30 F ⊕ , favoring FG-type stars over M dwarfs.

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