It is commonly accepted that asteroids are capable of causing mass extinctions. The extinction of the dinosaurs, 65.5 million years ago, for instance, is widely considered to have been caused by an asteroid impact. However, the idea of similar mass extinctions being caused by comets is not very likely, according to the finding of a new study.
First, Jupiter and Saturn seem to do some shielding. They have much more gravitational force than the Earth does, and so can provide some protection from incoming comets. The scar found on Jupiter is evidence of this.
Also, according to the new study from the University of Washington, the probability of actually getting hit by a comet is very low. Most long-period comets, the sort that might cause an extinction event, are part of the Oort cloud (the remnants of the cloud from which our solar system formed). The study used a computer model to estimate the number of comets in the Oort cloud. From this prediction, they determined that only 2 or 3 comets could have hit the Earth during the most intense comet shower over the past 500 million years. These impact would have been nearly simultaneous, matching an extinction event occuring 40 million years ago. However, that extinction was relatively minor. Thus, it is highly unlikely that comet impacts caused any mass extinction events. The chances of comets getting past Jupiter and Saturn is small, and those that do aren't likely to hit our planet. Credit: Science Daily- Crashing Comets Not Likely the Cause of Earth's Mass Extinctions