Comet from solar system’s edge triggered a winter that killed the dinosaurs 66 mn-years-ago: Harvard study

These long-period comets take about 200 years to orbit the Sun and we can expect similar impacts every 250 million to 750 million years.

from http://www.firstpost.com/tech/science/comet-from-solar-systems-edge-triggered-a-winter-that-killed-the-dinosaurs-66-mn-years-ago-harvard-study-9313851.html