The Buddha spoke of beings in existence in different planes of existence in our world system. There are countless other world systems where beings may exist. The universe is endless, beyond our imagination and reckoning. The Buddha called the universe "loka dhatu" and the world system "cakkavala" spiral cosmos.
In our world system there exists 31 planes of existence (depending on classification, the number of planes differ; but all the planes are included). We as humans occupy one plane, sharing with animals which is considered a separate plane. There are ghosts plane which in itself is a form of existence. The higher planes will be the planes of gods whose life spans are unimaginably long. In our terminology "aliens" would be those beings from other world systems. All these planes of existence, although with very different conditions and life span are all subject to impermanence. All life forms come into life, exist for a period, and ultimately disintegrate. Apart from human and animal planes, the other planes exist in different dimensions. That is why only very few people could see ghosts.