I have translated one of these names nibir, “wandering stars” or “planets,” but this is not the usual word for planet, and there is a star called Nibir near the place where the sun crossed the boundary between the old and new years, and this star was one of twelve supposed to be favourable to Babylonia.
It sure looks like Sitchin purposely distorted the original meaning to make his theory work, and it looks very much like he used George Smith's explanation of his early translation of the Enuma Elish to do so, something that seems clear when we see that Sitchin calls it the "Epic of Creation" most of the time rather than its scholarly name, implying that he used a source like Smith. Or, let's be more direct. Sitchin actually cites Smith's Chaldean Account of Genesis as a source in Twelfth Planet and takes from it everything he knows about the Greco-Roman discussion of Babylonian myth. I wonder if Smith's explanation of nibir doesn't lay behind Sitchin's wandering Nibiru.
Sitchin takes the fact that we know what planets are and projects it into the past, and he purposely conflates Smith's "wandering stars," which is actually a Classical Greek way of describing planets, with planets to invent the idea that the Babylonians and Sumerians viewed Nibiru as a wandering planet (i.e. chunk of rock) rather than a "wanderer," i.e. a light in the sky that does not follow the path of the fixed stars.
To make this work, Sitchin has to do another piece of fancy footwork. He has to discount the clear evidence that the Babylonian Nibiru (capitalized) was identified with Jupiter, not a wandering alien planet. His arguments are utterly juvenile and deserve no elaboration here. Suffice it to say that Nibiru is clearly meant to be Jupiter in the MUL.APIN astronomical compendium, set down around the time of the Enuma Elish (except, of course, when it is sometimes Mercury--but never a wandering planet):
As the text indicates, Marduk was identified with Jupiter and Nibiru. That Marduk and Jupiter were identified is quite clear to everyone except Zecharia Sitchin and the sources he critiques, which were out of date even in 1976. But if this weren't clear enough for Sitchin, the ancients themselves understood the connection. Alexander Polyhistor, citing the Babylonian priest Berossus, had written of Marduk (under his title of Bel, the Lord): "This Belus, by whom they signify Jupiter..." (he meant the god), thus again reaffirming a connection.
Everything else you've ever wanted to know about "nibiru" in Mesopotamian texts can be found in Michael Heiser's extensive discussion of it here.