Dusty Outflows in Planetary Atmospheres: Understanding “Super-puffs” and Transmission Spectra of Sub-Neptunes

"Super-puffs" are planets with anomalously low mean densities ($\lesssim {10}^{-1}\,{\rm{g}}\,{\mathrm{cm}}^{-3}$). With a low surface gravity, the extended atmosphere is susceptible to extreme hydrodynamic mass loss ("boil-off") on a timescale that is much shorter than the system's age. Even more puzzling, super-puffs are estimated to have a scale height of $\sim 3000\,\mathrm{km}$, 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 ($\dot{M}\gtrsim {10}^{-10}\,{M}_{\oplus }\,{\mathrm{yr}}^{-1}$) atmospheres that carry very small dust grains (∼10 Å in size, $\sim {10}^{-2}$ in mass fraction) to high altitudes ($\lesssim {10}^{-6}\,\mathrm{bar}$). Dust at high altitudes inflates the observed transit radius of the planet while flattening the transmission spectra. Previous static atmospheric models struggle 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 ($\lesssim {10}^{9}\,\mathrm{yr}$), low-mass ($\lesssim 10\,{M}_{\oplus }$) exoplanets, thereby limiting our ability to study the atmospheric composition in transmission, and inflate the observed transit radius of a planet, hence obscuring the underlying mass–radius relationship.