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

TL;DR

The paper tackles the origins and survival of the H/He envelopes in super-puff planets by introducing a thermal-energy-mediated photoevaporation (TEMP) regime and by coupling a sub-Neptune interior-thermal evolution model to a 1D hydrodynamic photoevaporation solver. The authors show that for low-gravity planets, TEMP governs mass loss, yielding rates substantially lower than traditional energy-limited estimates and enabling larger radii to persist; boil-off dominates early evolution, while photoevaporation acts more gradually. Applying the model to Kepler-51, Kepler-87c, HIP 67522b, V1298 Tau b, and Kepler-223 system, they reproduce observed masses and radii with transit pressures ranging from 1 nbar to 20 mbar, and they infer hazy vs clear atmospheric outcomes alongside constraints on atmospheric metallicity. The framework resolves tensions between large radii and envelope retention, clarifies the role of host-star type and irradiation, and provides predictive power for atmospheric composition and observational signatures, highlighting that super-puffs most likely inhabit 5–12 M⊕ at modest insolation around FG-type stars with relatively metal-poor atmospheres. Overall, the study advances a physically grounded, self-consistent picture of how boil-off and TEMP together shape the diverse radii and densities of sub-Neptunes, with direct implications for formation scenarios and atmospheric characterization.

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 significant roles of the radiative atmosphere and interior heating from the rock/iron core. Here, through a new quantitative analysis of 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 (TEMP), 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$M_\oplus$ and receive stellar insolation $\lesssim 30F_\oplus$, favoring FG-type stars over M-dwarfs.

Figures (17)

• Figure 1: 1$\mu$bar radii calculated from the isothermal Parker wind (gray) and our non-isothermal, hydrostatic atmosphere (black), which serves as the fiducial model for determining optical radii. The transition between these two regimes (red) ensures a proper estimate of the lower-boundary radius for the photoevaporation model.
• Figure 2: Comparison of radial profiles for temperature (top left), Mach number (top right), density (bottom left), and ionization fraction (bottom right) between a super-puff (black, Kepler-51b) and a hot Jupiter (red, HD 209458b). The photoionization base is marked with circles. Structures are shown from the 1 $\mu$bar level (triangles) to the sonic point (crosses). See the main text for further discussion.
• Figure 3: Comparison of energy budgets for high-gravity (top) and low-gravity (bottom) planets, corresponding to the same planets shown in Figure \ref{['structure']}. Refer to the main text for a detailed discussion.
• Figure 4: In the top panel, we display the thermal-energy-mediated (red) and energy-limited (blue) mass loss rates, based on Eqs. \ref{['thermal-limited-mdot']} and \ref{['energy-limited']}, respectively. For comparison, the mass loss rate from our numerical model is shown with a black dashed line. The kinks in the mass-loss rate reflect the evolution of the EUV flux, which remains saturated during the early stages. In the bottom panel, we plot the evolution of the escape parameter at the sonic point ($R_s$, black) and at the wind base ($R_{\rm{base}}$, gray).
• Figure 5: Comparison of planetary evolution for Kepler-51b during the photoevaporation phase between our new framework (black) and the previous framework (gray), which assumes a constant scale height for the radiative atmosphere and energy-limited escape. The top panel presents the 20mbar (solid) and 1nbar (dashed) radii. For consistency in comparison, both models start with the same initial conditions, self-consistently calculated from a boil-off phase. In contrast, the previous framework treats initial conditions as free parameters, whose effects are not shown in the figure. See text for further discussion.