Schematic showing how a gap-opening planet triggers the C₂H ring associated with the continuum gap. (a) Before runaway growth, a small planet is embedded inside the gaseous disk and carbon-rich ice is frozen out on pebbles. (b) During runaway gas accretion, the planet grows rapidly, consumes the pebbles in its vicinity, and opens a gas gap in the disk. Since the accreting planet is hot, carbon-rich volatile ice (e.g., methane) with lower sublimation temperature is readily liberated from the ice. On the other hand, the sublimation of H₂O or CO₂, which are the major oxygen carriers, can only be reached in the immediate vicinity of the planet (<R_Hill), at which point the material is lost to the planet. The discrepancy in sublimation temperature renders the gas C/O>1. In addition, stellar UV photons penetrate the gas gap. The combination of C/O>1 and high UV fluxes trigger photochemistry reactions to form the C₂H ring. (c) After the gap opens, the supply of sublimated pebbles terminates due to the gap opening. The C/O ratio is smoothed out across the disk through radial mixing by diffusion.