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.