When our solar system's orbits all break down and planets are crashing into each other, we'll know exactly who's to blame: a tiny ball of rock called Mercury. Chaotic factors in Mercury's orbit could destabilize the whole solar system.
French astronomers at the Observatoire de Paris have run 2,501 mathematical models of our solar system's next 5 billion years, and 25 of them end in severely disrupted orbits, often planetary collisions. These disrupted orbits don't all result in collisions with Earth, but they are all pretty disastrous for us.
In one model, Mars passes within 500 miles of Earth (that's about as far away as San Diego is from San Francisco), both planets' gravity ripping each other to pieces. In other models, either Mars or Venus careens into Earth in a fiery explosion. In all models, though, the culprit for the orbital breakdown is Mercury.
That's right, all of this possible chaos and destruction can be traced back to "orbital chaos" in Mercury's path around the sun. The models were built using old modeling techniques, but factoring in relativity effects. In the models, Mercury's orbit would start to resonate with that of Jupiter, and the combined effect would severely distort the rest of the solar system's orbits.
The odds are small (under 1% of the models have any disruption), and it wouldn't happen for billions of years. But astronomical observation shows that collisions like this might have happened before. If it did happen here, and there was anyone left to do any blaming, they should blame our diminutive near-neighbor, Mercury.
Image artistic design: J Vidal-Madjar, provided by Nature