Dusty outflows in planetary atmospheres: Understanding "super-puffs" and transmission spectra of sub-Neptunes

Abstract

`Super-puffs' are planets with anomalously low mean densities ($\lesssim 10^{-1}~{\rm g\ cm}^{-3}$). With a low surface gravity, the extended atmosphere is susceptible to extreme hydrodynamic mass loss (`boil off') on a timescale much shorter than the system's age. Even more puzzling, super-puffs are estimated to have a scale height of $\sim 3000~{\rm km}$, yet recent observations revealed completely flat transmission spectra for Kepler 51b and 51d. We investigate a new scenario that explains both observations: non-static outflowing ($\dot{M}\gtrsim 10^{-10}~M_\oplus~{\rm yr}^{-1}$) atmospheres that carry very small dust grains ($\sim 10~{\rm A}$ in size, $\sim 10^{-2}$ in mass fraction) to high altitudes ($\lesssim 10^{-6}~{\rm bar}$). Dust at high altitudes inflates the observed transit radius of the planet while flattens the transmission spectra.Previous static atmospheric models struggles to achieve cloud elevation and production of photochemical haze at such high altitudes. We propose to test this scenario by extending the wavelength coverage of transmission spectra. If true, dusty atmospheric outflows may affect many young ($\lesssim 10^9~{\rm yr}$), low mass ($\lesssim 10~M_\oplus$) exoplanets, thereby limit our ability to study the atmospheric composition in transmission, and inflate the observed transit radius of a planet hence obscure the underlying mass-radius relationship.