New Theory Suggests Comet, Not Asteroid, Caused Dinosaur-Killing Chicxulub KEY POINTS The long-period comet responsible for it could have originated from the Oort cloud It's possible that the impact was caused by a fragment of the comet that broke apart What really killed the dinosaurs millions of years ago? The researchers of a new study are proposing a new theory to explain what the Chicxulub impactor might have been and where it really came from. It is believed that the Chicxulub impact, which left a crater in modern-day Mexico, led to the dinosaurs' mass extinction event some 66 million years ago. But exactly what caused the impact and how it ended up hitting Earth remains a mystery.