Well.. Zacharia Zitchin is an author of several books. "The 12th Planet" is one in a series of 4 books.
About the book, he says that there are evidence of a 12th planet in our solar system ( Sun and moon counts ) The planet is closest to the sun every 3600 years, and it is likely that this planet are on its way back into our solar system in the next 10 to 20 years ( coming between Mars and Jupiter where the asteroid belt is)
He is an archeologist, and a number of other things, and is one of the few remaing people in the world who is able to read and understand Sumerian and those old languages.
The book is available everywhere, its very heavy reading, but if you manage to read through it, you should get amazed at his conclusions.
its not much of an explanation at the moment, but if you need some more info, i will try my best at explaining it...
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"All i say is keep looking".