Yes, the existence of Neptune has been proven long before 2023. Neptune is one of the known planets in our Solar System and was officially discovered on September 23, 1846. Its existence was initially predicted based on gravitational perturbations observed in the orbit of Uranus, which could not be explained by known celestial bodies at the time. Using mathematical calculations, astronomers Urbain Le Verrier and John Couch Adams independently predicted the location of an unknown planet, later confirmed to be Neptune, based on these perturbations.
Soon after the predictions were made, astronomers Johann Galle and Heinrich d'Arrest observed and confirmed the existence of Neptune through telescopic observations. Since then, Neptune has been extensively studied and its presence in the Solar System has been well-established through various observations, including direct imaging, spacecraft missions like Voyager 2, and ongoing astronomical research.