Draft:Soot Line

The soot line (also known as tar line in some literature)^[1][2] is a theoretical boundary in protoplanetary disks marking the radial distance from the central star where refractory organic carbon compounds, such as polycyclic aromatic hydrocarbons (PAHs) and other CHON (carbon, hydrogen, oxygen, nitrogen) materials (collectively referred to as "soot" or "tar"), are irreversibly destroyed by thermal processes and converted into gaseous species.^[3][4][5] First proposed in 2010 by Monika E. Kress and colleagues,^[3] the soot line is analogous to the snow line (or ice line) for volatiles like water, but occurs at higher temperatures closer to the star, typically around 300–1000 K (often ~500 K under typical disk conditions).^[6][1]

Interior to the soot line, planets form in a carbon-poor environment, explaining the low carbon content in terrestrial planets like Earth (~0.1% by mass compared to cosmic abundances).^[6][5] Exterior to the soot line but interior to the snow line, planets can accrete significant refractory carbon (up to 40% by mass in some models), leading to the formation of "soot planets" or "soot-water worlds."^[1][7] This concept has gained prominence in recent years (2021–2025) due to its implications for interpreting low-density exoplanets observed by the James Webb Space Telescope (JWST), suggesting many presumed "water worlds" may instead be carbon-rich with hazy, methane-dominated atmospheres.^[1][6][7]

The soot line was first introduced in 2010 by Monika E. Kress, Alexander G. G. M. Tielens, and Michael Frenklach as a mechanism to explain the destruction of presolar PAHs in the inner regions of protoplanetary disks.^[3] Building on this, subsequent studies in 2011 and 2017 refined the chemical models, incorporating kinetic reactions and disk dynamics.^[4][2]

The concept evolved significantly in the 2020s through work by Edwin A. Bergin, Jie Li, and collaborators at the University of Michigan. In 2021, Li et al. proposed an "inside-out" planet formation scenario linking the soot line to carbon depletion in inner planets.^[5] By 2023, Bergin et al. connected it to haze formation in exoplanet atmospheres.^[6] The 2025 paper by Li et al. introduced "soot planets" as an alternative to water worlds, integrating JWST observations of exoplanets like TOI-270d.^[1][7]

Protoplanetary disks exhibit radial temperature and pressure gradients, creating condensation/sublimation boundaries that dictate material availability for planet formation.^[8] The water snow line (~150–170 K) is where water and volatiles freeze, facilitating the formation of icy giants.^[8] Volatile carbon carriers sublimate by ~120 K, but refractory carbon persists until higher temperatures.^[3]

The soot line, at ~500 K, marks the irreversible conversion of solid carbon into gases like CO, CH₄, or C₂H₂ via reactions with H, OH, and O in oxygen-rich environments (C/O < 1).^[3][2] Unlike the snow line, where materials can re-condense, soot destruction is permanent due to kinetic barriers preventing reformation.^[5]

The soot line is defined as the disk radius where temperatures enable thermal destruction of PAHs and refractory carbon, typically at 300–1000 K, with a nominal value of ~500 K.^[6][1] Models assume chemical kinetics in warm, oxygen-rich disks, incorporating pebble drift, gas advection, and Inside-Out Planet Formation (IOPF).^[5][1] The line migrates inward over time, from tens of AU to inside 1 AU within ~1 million years as the disk cools.^[9]

Key reactions include oxidation and hydrogenation, leading to persistent C₂H₂ abundances due to inhibited conversion to CO/CO₂/CH₄.^[2]

Chemical destruction at the soot line can be modeled by reactions such as:

${\displaystyle {\text{PAH}}+{\text{OH}}\rightarrow {\text{products (e.g., }}C_{2}H_{2}{\text{, CO)}}}$^[3]

The radial position r of the soot line follows disk temperature profiles:

${\displaystyle r\propto T^{-2/\alpha }}$

where T is temperature and α is the flaring index (~1–1.5).^[6] More advanced models incorporate time-dependent migration:

${\displaystyle r(t)=r_{0}\left({\frac {t}{t_{0}}}\right)^{-\beta }}$

with β depending on accretion rates.^[9]

Direct observations of the soot line are challenging, but JWST mid-infrared spectroscopy has provided indirect evidence through carbon gas abundances in disks like d203-506.^[6] Exoplanet atmospheres, such as those of TOI-270d and K2-18b, show high C/O ratios and methane, consistent with soot-rich formation.^[1][7]

The soot line explains carbon deficits in inner planets and suggests low-density sub-Neptunes may be soot-rich rather than water-dominated, with mass-radius relations mimicking 50% rock-50% water compositions.^[1][7] Soot planets could have hazy atmospheres with hydrocarbons, affecting climate and detectability.^[6] This expands habitable zone concepts to include "soot zones" where carbon enhances potential for life.^[10]

• Snow line (astrophysics)
• Protoplanetary disk
• Exoplanet
• James Webb Space Telescope

Category:Astrophysics Category:Exoplanets Category:Astrochemistry