Did you notice that there is a major asteroid impact every 70 million years or so?
Why is it like a schedule?What is out there causing this?
Meet Nemesis.
Nemesis is known to be a brown/red dwarf star that is known to be on the outskirts of our solar system, further that the Oort Cloud.
It's orbit is known to make it crash into the Oort cloud every million years or so, which knocks them out of the orbit and towards the solar system.
Most of these comets would then hit the earth, which is the reason of extinction.
If it's out there, then we have a second sun.
Source:
http://www.lbl.gov/Science-Articles/Archive/extinctions-nemesis.html