The observed mass-radius relationship of low-mass planets informs our understanding of their composition and evolution .
Recent discoveries of low mass , large radii objects ( “ super-puffs ” ) have challenged theories of planet formation and atmospheric loss , as their high inferred gas masses make them vulnerable to runaway accretion and hydrodynamic escape .
Here we propose that high altitude photochemical hazes could enhance the observed radii of low-mass planets and explain the nature of super-puffs .
We construct model atmospheres in radiative-convective equilibrium and compute rates of atmospheric escape and haze distributions , taking into account haze coagulation , sedimentation , diffusion , and advection by an outflow wind .
We develop mass-radius diagrams that include atmospheric lifetimes and haze opacity , which is enhanced by the outflow , such that young ( \sim 0.1-1 Gyr ) , warm ( T _ { eq } \geq 500 K ) , low mass objects ( M _ { c } < 4M _ { \Earth } ) should experience the most apparent radius enhancement due to hazes , reaching factors of three .
This reconciles the densities and ages of the most extreme super-puffs .
For Kepler-51b , the inclusion of hazes reduces its inferred gas mass fraction to < 10 % , similar to that of planets on the large radius side of the sub-Neptune radius gap .
This suggests that Kepler-51b may be evolving towards that population , and that some warm sub-Neptunes may have evolved from super-puffs .
Hazes also render transmission spectra of super-puffs and sub-Neptunes featureless , consistent with recent measurements .
Our hypothesis can be tested by future observations of super-puffs ’ transmission spectra at mid-infrared wavelengths , where we predict that the planet radius will be half of that observed in the near-infrared .