Exoplanet Volatile Carbon Content as a Natural Pathway for Haze Formation

We explore terrestrial planet formation with a focus on the supply of solid-state organics as the main source of volatile carbon. For the water-poor Earth, the water ice line, or ice sublimation front, within the planet-forming disk has long been a key focal point. We posit that the soot line, the location where solid-state organics are irreversibly destroyed, is also a key location within the disk. The soot line is closer to the host star than the water snow line and overlaps with the location of the majority of detected exoplanets. In this work, we explore the ultimate atmospheric composition of a body that receives a major portion of its materials from the zone between the soot line and water ice line. We model a silicate-rich world with 0.1% and 1% carbon by mass with variable water content. We show that as a result of geochemical equilibrium, the mantle of these planets would be rich in reduced carbon but have relatively low water (hydrogen) content. Outgassing would naturally yield the ingredients for haze production when exposed to stellar UV photons in the upper atmosphere. Obscuring atmospheric hazes appear common in the exoplanetary inventory based on the presence of often featureless transmission spectra. Such hazes may be powered by the high volatile content of the underlying silicate-dominated mantle. Although this type of planet has no solar system counterpart, it should be common in the galaxy with potential impact on habitability.

Automated summary

We focus on the supply of solid-state organics as the main source of volatile carbon in terrestrial planet formation. The soot line, where solid-state organics are irreversibly destroyed, is a key location within the planet-forming disk. Our study explores the atmospheric composition of a planet receiving its materials from the zone between the soot line and water ice line, and finds that such planets would have a mantle rich in reduced carbon and low water content, which can lead to the formation of obscuring atmospheric hazes powered by the high volatile content of the mantle. These planets, although not found in our solar system, are likely common in the galaxy and may have an impact on habitability.