2026ApJ...997L...2I

Atacama Large Millimeter/submillimeter Array observations suggest that the disk of HD 163296 is being actively shaped by embedded yet unseen protoplanets, as indicated by numerous gas and dust substructures consistent with planet-disk interaction models. We report the first detection of simple organic molecules, HCN and C₂H, tracing a candidate circumplanetary disk (CPD) in the HD 163296 system, located at an orbital radius of R = 88 ± 7 au and azimuth ϕ = 46^∘ ± 3^∘ (or $R=0\mathop{.}\limits^{^{\prime\prime} }75$, PA = 350^∘ in projected sky coordinates), and originating near the midplane of the circumstellar disk. The signature is localized but spectrally resolved, and it overlaps with a previously reported planet candidate, P94, identified through kinematic perturbations traced by CO lines. We propose a scenario in which the observed chemical anomalies arise from increased heating driven by the forming planet and ongoing accretion through its CPD, facilitating the thermal desorption of species that would otherwise remain frozen out in the disk midplane, and potentially triggering the activation barriers of chemical reactions that lead to enhanced molecular production. Based on a first-order dynamical analysis of the HCN spectrum from the CPD-isolated with a 7σ significance-we infer an upper limit on the planet mass of 1.8M_Jup, consistent with predictions from CO kinematics and constraints from direct imaging studies. By comparing the CPD sizes derived from our models with theoretical expectations where the CPD radius corresponds to roughly one-third of the planet's Hill radius, we favor CPD gas temperatures T > 150 K, planet masses M_p < 1.0M_Jup, and CPD radii R_CPD < 2 au.