There are a number of good critiques on the web showing the flaws in Sitchin’s discussion of astronomy from the Sumerian and Mesopotamian texts. Here are a few:
Chris Siren’s page
Ian Lawton’s analysis
Phil Plait’s Bad Astronomy website deals with “Planet X”
My own contribution to this subject was previously blogged. But here are links to my short paper on Nibiru (it isn’t a 12th planet) and Cylinder Seal VA243 (which doesn’t show a 12th planet). These files are old (written before I finished my doctorate) but still useful.