Many people have come to believe that the Anunnaki, the ancient gods of Mesopotamia (now Iraq) were extraterrestrials. This is largely as a result of a series of books by Zecharia Sitchin, which have attained considerable popularity.
His argument is that there is a planet called Nibiru which circles our sun but has a 3600 year orbit and is somewhere out on the edge of the solar system. From this aliens arrived 450,000 years ago and in due course created the Sumerians. A planet called Tiamat was fragmented by a collision, forming the asteroid belt. This event is remembered in the epic Enuma Elish.
What played into Sitchin's hands is that the Sumerians, among whom the Anunnaki flourished, were a somewhat mysterious people. Archaeology tells us they appeared in Iraq about BC 4000. They seem to have been civilized from the start. Their language is an isolate - as far as can be determined, it is related to no other language. They had invented writing, in the form of the cuneiform script. Sitchin put myths+urban civilization+unrelated language together and claimed they were created by aliens.
However, if you look closely at the myths, you will see that there is no more reason to think their gods were aliens than there is the gods of other mythologies. The epic Enuma Elish (15th Century BC) tells that the goddess Tiamat, a sort of monster, determined to destroy the gods and Marduk, the god of Babylon, slew her, making out of her body the sky and the earth. This epic was written in Akkadian, a Semitic language that in time supplanted Sumerian. It looks more like sheer myth than the recollection of a planetary disaster.
In the earlier Sumerian myth contained in the Eridu Genesis we find that the gods An, Enlil, Enki and Ninhursag created the
With regard to these various beings, Tiamat started out as the salt water of the ocean, her consort Abzu being the fresh water of the rivers. Her eyes, when she was slain, exuded water forming the Tigris and Euphrates while her tail became the Milky Way. Marduk sliced her in two, making her upper part the sky and the lower part the earth.
As for the planet Nibiru, just what it signified has caused speculation. The word means 'crossing' in Akkadian and it may have designated whatever planet was most visible at the equinox. If it in fact spent most of its time beyond the currently known planets of the solar system, the climate would have been unendurably cold.
The chief of the Anunnaki was the god An, who was the sky. Ki signified the earth. This seems to be inconsistent with the notion that sky and earth were parts of the corpse of Tiamat. Inconsistency is a feature of mythhology. The various tales told about the Anunnaki in ancient Mesopotamia sound no more historical than the myths of other countries. The Anunnaki were only some of the gods and in later myths were the judges of the Underworld.
Of course, there are some mysteries associated with the Sumerians, such as their provenance and the origin of their civilization. Some historians think they are nothing more than a development of the pre-Sumerian Ubaid Culture, some that they may have come from the Zagros Mountains in Iran. In fact, nobody knows. There is nothing in their myths unequivocally indicating extraterrestrial involvement. Sitchin's knowledge of the Sumerian language has also given rise to unfavorable comment among experts.
I do not dismiss the possibiliy that aliens may have visited the earth at some time in antiquity or prehistory. I merely feel considerable doubt that the stories of the Anunnaki are more likely to contain a reminiscence of such an event than many other mythologies.