Yes, we are a little familiar with Zechariah Sitchin and his theories.
From the viewpoint of traditional or mainstream scholarship, his ideas represent an extreme. He is thought to have extrapolated much much more from the small amount of data that we have re the Sumerians. Most of his work is purest speculation, and it could be wished that it were more "grounded" in solid fact. But, often at crucial areas, his theories do lack some necessary factual data, and he goes very much "out on a limb."
This, of course, does not mean that he is "wrong.":) But Sitchin is not the very best basis for a cosmic philosophy, to which he aspires. Many of his concepts are attractive, but, again, it is difficult to justify building an entire philosophy on this rather tenuous basis.
As you can tell, we are not "true believers." In order for us to embrace any datum into our personal philosophy, we must be convinced that it is at least in harmony with the immense mountain of factual data in philosophy, history, science, and other related areas of knowledge.
But neither are we skeptics or cynics. Perhaps no one on earth has the data necessary to adopt or accept this position either. At present, we prefer to take a "wait and see" attitude towards these ideas re "the tenth planet," and related concepts, especially those regarding chronology and calendars.
One example is the wide, but not wise, spectrum of worldenders. It is still very, very fresh in our minds how, ten years ago, thousands of native American leaders, channelers, new-agers, and fundies of various stripes were anxiously holding their breath and predicting with utter certainty that the year twenty hundred would bring "the end of the world," through the "y2k" crisis.
It was one of the very rare years, in fact, specifically used by Nostradamus in his famous "prophecies," and Ed Cayce also had dire predictions for this "marked year." Worldenders were madly coming "out of the woodwork," and people thought it a bit "freaky" that we did not jump on this bizarre bandwagon. But now, we are very glad that we maintained a skeptical position.
When I was a fundy, we also made, in retrospect, the rather foolish error of predicting a certain specific year for the "end." That year, 1975, came and went without a single global disturbance, proving to any reasonable person that the fundy cult was wrong. This was no mere matter of interpretation. They were just plain wrong-- undeniably.
Sitchin has also predicted the return of Niburu. So far, no "tenth planet" has been discovered whose trajectory intersects with the orbit of the earth. So, this comes down to a remarkably simple truth that is not speculative, or even controversial: If he was right, this planet should have appeared, or else, it will do so shortly. And if it does not, then he was in error.
I am no specialist in either ancient Sumerian or astronomy. My specialty is spirituality. But, in Sitchin, the three areas overlap, and he claims to be an "expert" in all three. But almost no one in the academic or scientific community has been convinced either of his expertise or of the validity of his more radical theories. Only time, dear friend, will tell; and it will tell all.:)