The origin of water on Earth is only barely known. All life on Earth needs water, and most types of life started in the sea. Earth is the only planet known with liquid water on its surface. The water on Earth may have come from the Solar System as it formed, or it may have formed on Earth. Both are possible. We do know that hydrogen and oxygen, the elements that constitute water come from stars. Oxygen is created through fusion of helium and carbon atoms inside the core of red giants.