The Everything Guide to 2012 has a very useful summary of "Planet X and Nemesis" Theories in Chapter 17. There are a handful of scientists that seriously consider a companion star to the sun. A new NASA infra-red scope telescope has recently been launched and has begun to search the entire night sky for red and brown dwarf stars, asteroids and the like.
The most likely scenario is that a red or brown dwarf star exists beyond the Oort cloud, and this is what disturbs the debris of the Oort cloud sending comets into our solar system, as well as causing the eccentric orbits of rogue planetoids such as Sedna that might also be a destructive force in our solar system or a periodic basis.
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