Atmospheres & Climate

Exoplanet Volatile Carbon Content as a Natural Pathway for Haze Formation

By Keith Cowing
Status Report
May 9, 2023
Filed under , , , , , , , , ,
Exoplanet Volatile Carbon Content as a Natural Pathway for Haze Formation
Top: Abundance and temperature-pressure profiles for a 3 M⊕, 600 K equilibrium temperature planet, with 0.1% soot, orbiting an M-dwarf star. Even under the harsh UV irradiation environment of the host star, high abundances of methane (thick brown line) persist to high altitude and low pressure. A combination of methane photolysis in the upper atmosphere, vertical mixing, and thermochemistry give rise to significant quantities of higher-order hydrocarbons such as C2H2, C2H4, and C2H6, Subsequent photolysis and polymerization reactions result in the formation of hydrocarbon haze (thick dashed red line). Bottom: The resulting transmission spectrum (orange line) of this planet is shaped considerably by haze with some strong methane features permeating through the haze at longer wavelengths. The teal line shows the transmission spectrum of the same planet but with the haze opacity artificially removed, while the transparent colored line shows these data at a spectral resolution of R=1,000. — astro-ph.EP

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 snowline 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 (Kreidberg et al. 2014, Knutson et al. 2014, Libby-Roberts et al. 2020). 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.

E.A. Bergin, E. Kempton, M. Hirschmann, S.T. Bastelberger, D.J. Teal, G.A. Blake, F. Ciesla, J. Li

Comments: 15 pages, 6 figures, accepted by Astrophysical Journal Letters. ApJL version has 5 figures. We include an extra figure (Figure 6) in this submission which is an artist rendering of a young disk including the soot and water ice lines. Image Credit for Fig. 6: Ari Gea/SayoStudio
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2305.05056 [astro-ph.EP] (or arXiv:2305.05056v1 [astro-ph.EP] for this version)
Focus to learn more
Submission history
From: Edwin A. Bergin
[v1] Mon, 8 May 2023 21:27:46 UTC (4,527 KB)
Astrobiology, Astrochemistry

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻