The current consensus is a Manhattan-sized asteroid. [0] The science behind this is pretty strong. What's come open for question is how fast this happened. A new theory posits that the dinosaurs were wiped out in a matter of hours in the resulting heat wave [1].
Now where did this asteroid come from, and how did it get here? That's another story entirely for which I don't have a good answer.
I meant that the gravity from the hypothetical star could have caused a disturbance in one of the asteroid belts and that's why that massive asteroid hit the earth.
There is no need for a disturbance in asteroid belts for asteroids to hit the earth.
The orbits of asteroids are in general not stable. There are currently some >7000 known earth-orbit-crossers, which, if their orbits would not be modified by something else and given enough time, would all hit us eventually (except one which shares our orbital period). And that's really just scratching the surface, as we still haven't mapped but a small fraction of asteroids.
The sky up there is still really chaotic, as space is really big and ~5B years is simply not enough time to produce a stable equilibrium.
Which makes me wonder in line with that thought, whether the alien star in question from this article caused any serious asteroid strikes on earth circa 70,000 years ago.
Now where did this asteroid come from, and how did it get here? That's another story entirely for which I don't have a good answer.
[0] http://en.wikipedia.org/wiki/Cretaceous%E2%80%93Paleogene_ex...
[1] http://www.radiolab.org/story/dinopocalypse/