UV-driven chemistry as a signpost of late-stage planet formation

The chemical reservoir within protoplanetary disks has a direct impact on planetary compositions and the potential for life. A long-lived carbon- nitrogen-rich chemistry at cold temperatures (≤ 50 K) is observed evolved planet-forming disks. This evidenced by bright emission from small organic radicals in 1–10 Myr aged systems that would otherwise have frozen out onto grains 1 Myr. We explain how of disk evolves cosmic-ray/X-ray-dominated regime to ultraviolet-dominated equilibrium. This, turn, will bring about temporal transition which planets accrete. photochemical dominated gas phase develops as dust via growth, settling drift, grain population depleted atmosphere. higher gas-to-dust mass ratio allows deeper penetration ultraviolet photons coupled with carbon-rich (C/O > 1) form carbon-bearing ions. further results formation molecules, then be accreted any actively forming present disk.