I think I finally understand how it is… This makes sense. Watch this video and don't mind the 'music' or just disable the sound.
Summary: 'Comet' Elenin is Nibiru. Nibiru is a planet orbiting the brown dwarf star, the dark twin sun of our sun. Once in 3600 years, Nibiru's orbit is closer to our sun than it's own brown dwarf star and so it gets sucked into the suns orbit. On it's way back it returns to its normal orbit.
Earlier I had trouble into believing that the 'comet' Elenin is a brown dwarf star, the twin of our sun, because twin suns should keep their distance. They are orbiting each other so they can't get that close. This explanation dismiss the possibility that Elenin is a brown dwarf star. Instead it's a planet (the planet Nibiru). The leaked NASA video designated Elenin as Nibirus Minoris. Nibirus Majoris is probably the brown dwarf star.
So the right answer to the previous poll is: 'It's a planet (maybe Niburu)'.
Now I have a satisfying and logically sound explanation what 'comet' Elenin really is, the question remains what will happen when it comes close to earth. It will happen very soon. The closest distance will be reached in mid September 2011. And what will the Annunaki (the inhabitants of Nibiru) do? According to Sheldan Nidle they had already switched to the side of the light. I hope this is true. It shouldn't be long before we hear something from them. Or are they only revealing themselves to their descendants? Time will tell.