Twenty-five low-lying isomers of C9H2 lying within 2 eV (∼46.12 kcal mol−1) have been theoretically investigated using high-level quantum chemical calculations. Among them, the linear triplet isomer (1), the cyclopropenylidene derivatives (2 and 3) and the cumulene carbene isomer (11) have already been detected in the laboratory. The present theoretical study proposes fourteen cyclic structures and four open-chain structures that have not been investigated before. On the basis of relative energies obtained from coupled-cluster calculations, the current theoretical study reveals that six isomers (4 and 6–10), which energetically lie below 11, and fourteen isomers (12–25), which energetically lie above 11, are remaining elusive in the laboratory to date. Considering the astronomical importance of cyclopropenylidene and cumulene carbenes, the current quantum chemical data may be of relevance to molecular spectroscopists in identifying new C9H2 isomers in the laboratory. Consequently, such studies may benefit radioastronomers in confirming or neglecting C9H2 isomers in the interstellar medium. © 2019 Elsevier B.V.