Origin of water on Earth
A long time ago, scientists thought that water on Earth did not come from the planet’s region of the protoplanetary disk. Instead, it was thought water must have been delivered to Earth from the outer Solar System later in its history. However, recent research suggests that hydrogen inside the Earth played a role in the formation of the ocean. The two ideas are not mutually exclusive: there is evidence water was delivered to Earth by impacts from icy planetesimals similar in composition to asteroids in the outer edges of the asteroid belt.