Sitchin claimed that a planet named Nibiru, will pass the earth in 2003, but that event remains unseen to this day. While his background lays not in astronomy but rather the study of ancient of ancient manuscripts, he believes that those who lived on earth several thousand years ago have left behind records that their origins are not of this earth, but are from a distant planet described by the ancient Sumerians. While this seems fantastic, the origins of the Annunaki appear even more improbable: that they are the Nephilim mentioned in the book of Genesis and epistle of Jude, who are the offspring of angels and female humans. The Annunaki may very well be the Judeo-Christian version of ancient myths where gods cavorted with humans on earth, created semi-divine creatures similar to those found in Greek and Roman mythology. Zeus, for example, is considered to have had a number of affairs with mortal women. These religious moral stories serve as teaching tools to believers, but there is no concrete evidence to prove that such events really happened. To further complicate matters, heaven, as described in Genesis chapter 6 which mentions the home of the angels who came to earth, is not designated as a physical location but is more likely to be a spirit realm, which makes sense, if that is where the angels, archangels, and God exist. Sitchin's expertise in ancient Sumerian has led him to write a series of books, with "The Twelfth Planet" being the first. His re-creation of Sumerian myths and the theory of extra-terrestrial origins of the human race has led to his being more popular with conspiracy theorists than with NASA scientists.