Grain Growth in Escaping Atmospheres: Implications for the Radius Inflation of Super-Puffs

Abstract

Super-puffs-low-mass exoplanets with extremely low bulk density-are attractive targets for exploring their atmospheres and formation processes. Recent studies suggested that the large radii of super-puffs may be caused by atmospheric dust entrained in the escaping atmospheres. In this study, we investigate how the dust grows in escaping atmospheres and influence the transit radii using a microphysical model of grain growth. Collision growth is efficient in many cases, leading to hinder the upward transport of dust via enhanced gravitational settling. We find that dust abundance in the outflow hardly exceeds the Mach number at the dust production region. Thus, dust formed at upper atmospheres, say P 10 −5 bar, are needed to launch a dusty outflow with high dust abundance. With sufficiently high dust production altitudes and rate, the dusty outflow can enhance the observable radius by a factor of ∼ 2 or even more. We suggest that photochemical haze is a promising candidate of high-altitude dust that can be entrained in the outflow. We also compute the synthetic transmission spectra of super-puff atmospheres and demonstrate that the dusty outflow produces a broad spectral slope and obscures molecular features, in agreement with recently reported featureless spectra. Lastly, using an interior structure model, we suggest that the atmospheric dust could drastically enhance the observable radius only for planets in a narrow mass range of ∼ 2-5M ⊕ , in which the boil-off tends to cause total atmospheric loss. This may explain why super-puffs are uncommon despite the suggested universality of photochemical hazes.

Cleaning our Hazy Lens: Exploring Trends in Transmission Spectra of Warm Exoplanets

“…We note that our haze model presented here is highly simplified. Further studies with detailed microphysical models (e.g., Lavvas & Koskinen 2017;Kawashima & Ikoma 2018;Gao & Zhang 2020;Ohno & Tanaka 2021) will be warranted to examine the haze trends that may appear in observed trends.…”

“…Aerosols are also suggested to impact the thermal structure of exoplanet atmospheres (e.g., Heng et al 2012;Morley et al 2015;Lavvas & Arfaux 2021), which potentially affects atmospheric chemical compositions (Molaverdikhani et al 2020) and dynamics (Roman & Rauscher 2017). Aerosols can also significantly inflate the observable transit radius from that expected for clear atmospheres, complicating the interpretation of the observed radius (Lammer et al 2016;Wang & Dai 2019;Kawashima et al 2019;Gao & Zhang 2020;Ohno & Tanaka 2021). Understanding the effect of these opacity sources are vital to unveil the atmospheric properties, such as composition and dynamics, of sub-Neptunes.…”