In the phrase "tanto el disco de polvo como el planeta orbitan", I have translated as "both the disc of dust and the planet orbit", but this is simply to make it make sense - what does "como" actually mean in this sentence? Can it mean "and" or "as well as"? Also, why is "como" used instead of "y"? Could you not have "tanto el disco de polva y el planeta"? Or when you are using "tanto" do you need to use "como"?
"tanto" and "como" belong together here, forming what would be translated to "as well as", "both...and", "like" (in this context) etc depending on how literal you want to be. So yes, it means that both orbits...something.
No, you can't use "tanto...y". "tanto...como" is a fixed phrase, indicating equality. Another usage would, for example, be "tengo tantos libros como él" which means that I have as many books as he does.
Sorry I deleted my post with the response that "como" is used in the sense of like: "like in English we use the word like". I'm no expert...the languages use different words/phrases to express the same ideas in different subtle ways, which is why computers can't translate well. Look at Pablo's answer in this post: http://answers.yahoo.com/question/index?qid=20080627115033AAUD2qm