I've coded another disaster scenario (but not mentioned it prior to this post) where the Earth spirals inward or outward from its current orbit due to <insert your favourite technobabble reason here>.
Oh this is a setup for Nemesis if ever I've seen it:
2050 : Astronomers detect an earth sized body out near the orbit of Eris. While there is initial puzzlement as to how such a large body managed to be undetected, its trajectory and speed is quickly estimated, traveling an unheard of 500 kilometers per second the body will cross the inner solar system and presumably escape into deep space. This will take less than a year due to its extreme velocity.
As more powerful eyes are trained towards "planet x", its path through the system is more carefully scrutinized, but as each day passes the predicted path gets less and less certain, finally accurate radar images confirm the anomaly.
Planet X is slowing, and its path is describing an arc far too extreme then the suns gravity should capable of producing. A new prediction is made, "planet X" will slow to approximately 30 kilometers per second, and its slowly decreasing arc will put it into orbit somewhere near the the earth.
The world panics, a new name is given to the object "Nemesis".
While thus far I have described a hypothetical body spiraling towards the sun for "insert technobabble reason" when it arrives near earths neighborhood, its position relative to earth will dictate what happens to our world as due to gravitational attraction it changes our orbit.
