Our solar system is a rich mosaic of objects hurling around the sun in various orbits, all interacting with each other via gravity. The 2012 "theory" has postulated, among a great deal of other things that a rouge planet may enter the inner solar system and cause untold catastrophe. One name for this world is called Nibiru. Like most parts of this rough collections of ideas it lacks distinction and you will hear different descriptions of this heavenly body depending on who you talk to (or more likely, what website you read). I have heard it range from an undiscovered planet beyond Pluto to a "brown dwarf", a failed star.
There is evidence of mass extinctions happening every 25 million years or so, and some people have thrown out the idea that there may be a companion star that orbits around the Sun. This gravitational influence could perturb the orbits of comets and send them towards the inner solar system. There is nothing inherently implausible about this idea. Many star systems contain more than one star. The main problem is that we haven't seen it or observed evidence of it gravitationally. This rules out a bright star such as our sun but does leave the possibility of something called a brown dwarf. They are many times the mass of Jupiter but much smaller than the sun. They don't have the mass to increase the core temperature to support hydrogen fusion. They are very dim, mostly seen in the infrared (heat) part of the spectrum.
Such a large object would have a gravitational influence on other objects in the solar system. Researchers have constrained the orbit of planets to vast distances. We haven't observed them either directly or indirectly. I could go into a great deal more detail on this idea but suffice it to say there is no evidence of a large mass posing danger to the Earth.
Other more nonsensical ideas incorporate things like a planet swinging by and knocking the planet off axis. A flyby of a planet simply couldn't do it. It is wild speculation not bridled to things such as reality. It grossly misunderstands the mechanics of gravity. The only way a major shift in Earth's axis could happen would be if a large object of comparable size to the Earth collided with us. Needless to say a tilting axis would be the least of our problems.
Now lets step a few quadrillion kilometers away. It turns out that supernovae have an angrier cousin called gamma ray bursts. These are truly remarkable and devastating astronomical events, the largest explosions in the universe since the big bang. Most of them last only a few seconds but in this time they release more energy than our sun does during its entire 10 billion year life. Pause for a second to comprehend that.
So this sounds pretty scary, clearly an object with this amount of power could do tremendous damage to the Earth. In fact it may already have. Around 440 million years ago a gamma ray burst may have caused the Ordovician extinction where 70 percent of species were wiped out. Luckily for us we appear to be safe for at least thousands and probably millions of years.
In order for a gamma ray burst to threaten Earth a few things need to come into place. Either you need a massive star, around 100 solar masses rapidly rotating or binary neutron stars spiralling in towards each other. In the first case when the supernova occurs material begins to get pulled into the newly formed black hole. This forms a rapidly spinning accretion disk around it and jets of extremely energetic particles and radiation coming from the poles. This radiation is a gamma-ray burst. In the neutron star case a black hole forms from the two spinning neutron stars collide and the rest of the matter is pulled in not unlike the supermassive star collapse. This jet of radiation is emitted in a line similar to a laser (rough analogy). In order to be in danger you need to be directly in line with the jet. It is estimated that these events are dangerous up to a few thousand light years. Which is pretty far even in galactic terms.
Luckily for us our neighbourhood appears to be fairly safe for the time being. The closest star that may explode in this fashion is Eta Carinae at 7500 light years away. At around 100 solar masses it is certainly a candidate. Its rotational axis however is not pointed at the solar system and we appear to be safe from any ill effects. There is certainly no evidence that it will explode in three years. The current best estimates range from thousands to a few million years. We do not have the capability to predict supernovae with any reasonable accuracy.
In no way have I hit on everything that has been proposed on the 2012 theory. I just tried to go over a few of the ideas that were at least possible based on the laws of physics and show that we are not in any danger when 2012 rolls around. I would encourage anyone still curious to check out Universe Today, I have a link in my links section. They do a wonderful job of covering the ideas in more detail.
I will try to start blogging more regularly now after taking a few weeks off.