Its rocky core's massive gravity attracted more gas and particles from the nearby vacuum and became a Jovian ice giant itself around 4.555 billion years ago.
Because Fifth Giant hasn't made any contact with human technology yet, scientists aren't sure what kind of compounds the Fifth Giant was composed of.
But there's a theoretical explanation that the Fifth Giant was formed out of the same materials as Uranus and Neptune, like hydrogen and helium, methane, ammonia, rock, and a few traces of ice and water.
The Fifth Giant was ejected during the Migration of Neptune event that took place 4.4 billion years ago.
Because the Fifth Giant isn't yet found elsewhere in the Milky Way, there are many theories that suggest what happened to the missing planet. One theory suggests that the Fifth Giant was ripped apart because of the lack of the gravitational pull from a parent star a few thousand years after it was ejected. Another theory suggests that it was captured by a nearby star system and became an exoplanet in disguise, and many it's already discovered but we just don't know it yet.