No, it is not possible that the Milky Way Galaxy was originally one gigantic planet/star that went supernova. The Milky Way is a spiral galaxy consisting of billions of stars, along with planets, gas, dust, and other celestial objects. It formed through a process known as hierarchical galaxy formation, which involves the gravitational interactions and mergers of smaller structures like galaxies, gas clouds, and star clusters over billions of years.
Stars, including our Sun, are formed within massive clouds of gas and dust called nebulae. They condense under their own gravity and ignite nuclear fusion in their cores, becoming self-sustaining objects that emit light and heat. Supernovae, on the other hand, are incredibly energetic and catastrophic events that occur at the end of a massive star's life cycle. They result in the explosion and destruction of the star, releasing a tremendous amount of energy and producing heavy elements that are scattered into space.
The Milky Way Galaxy is thought to have formed from the gradual accumulation and aggregation of gas, dust, and stars over time. It has undergone numerous mergers and interactions with other galaxies, which have influenced its structure and composition. The stars and planets within the Milky Way are not remnants of a single supernova but rather the products of stellar formation processes occurring throughout its long history.
Our understanding of galaxy formation and evolution is based on extensive observational evidence and theoretical models. While the origin and evolution of galaxies are complex processes, they can be studied and understood through scientific investigation.