Fig. 2. Schematic illustration of the soot line in a protoplanetary disk.

The soot line (red parabola) delineates the phase boundary between solid and gaseous carbon carriers. In the accretion-dominated disk phase, it is located far from the proto-Sun and divides carbon-poor dust and pebbles (green dots) from carbon-rich ones (dark blue dots). Within 1 Ma, as a result of the transition to a radiation-dominated, or passive, disk phase, the soot line migrates inside Earth’s current orbit. Note that the Si-rich and C-rich solids do not represent distinct reservoirs because carbonaceous material is likely associated with silicates. They are provided for ease in illustration.