They might've been talking about Nemesis, Sol's hypothesized sister star. Apparently, every couple million years or so, Nemesis travels through the Oort Cloud, a vast region of space beyond Pluto and the Kuiper Belt that is filled with comets and such. It's supposed to perturb those comets, and send them flying towards the inner (and outer, obviously) solar system.

But don't worry, the likelihood of Nemesis existing is small, at best. If not miniscule.