New research conducted by scientists at the University of Washington concludes that it is unlikely comets caused any more than one minor extinction event.

The Earth's fossil record is peppered with mass extinction events – when a large number of species died out in a very short period of time. The most well known is the event that caused the death of the dinosaurs 65 million years ago. It is well accepted that this catastrophe was associated with an asteroid some ten kilometres wide striking the Earth, but it is uncertain how many other extinctions might have resulted from similar asteroid or comet collisions.

The gas giants have long acted as planetary goal keepers, protecting the Earth and inner planets from comets, a point reinforced around ten days ago when a huge impact scar appeared on Jupiter. But new research from the University of Washington suggests that it is unlikely that comets have been responsible for any more than one minor extinction event in the Earth's history.

The work also shows that many long period comets that cross the Earth originate from a region of the Oort cloud that astronomers long believed could not produce observable comets. The Oort cloud is the remnant of the solar nebula that contains billions of comets, and stretches from a distance of 150 billion kilometres from the Sun out to three light years.

It was also believed that nearly all long-period comets that move inside Jupiter onto Earth-crossing orbits were nudged by the gravity of a passing star, and the same was thought to apply to short period comets from the inner Oort cloud too. But it turns out that some comets can slip past the safety barrier of Jupiter and Saturn without the aid of a passing star, instead influenced by the Milky Way's "tide".

"Basically, the Sun lies above the plane of the Milky Way disc, and for instance, if an Oort Cloud body lies 100,000 AU further above the plane it will feel a different downward pull toward the midplane than the Sun," lead author Nathan Kaib explains. "As a result, the comet feels an extra force in addition to the Sun's gravity. This causes its orbital elements to oscillate over time."

The team modelled the evolution of a comet cloud containing around one trillion bodies over a period of 1.2 billion years. By looking at the motions and number densities of stars in the stellar neighborhood, they calculated the most powerful stellar passage that has occurred in the last 500 million years.

"We ran a simulation of a stellar passage of this magnitude through the Oort Cloud model and measured how many comets were thrown into Earth-crossing orbits," Kaib tells Astronomy Now. "Then we scaled up these simulation results to correspond to an inner Oort Cloud with the maximum possible population. This then gave us the number of impacting bodies we would expect."

The results showed that just two or three comets would have struck the Earth during a single powerful comet show in the last 500 million years. The team marry their results to an extinction event 40 million years ago – during the time period that geologists call the Eocene epoch – which wiped out almost one-third as many species as the event that killed off the dinosaurs.

"They all fall within about a million years of each other," says Kaib. "Whether they are actually associated with this extinction event is not certain. Because they occur at roughly the same time and such a cluster of major impacts is very improbable, previous works have proposed that they are causally related."

Kaib also comments that if this particular extinction event was in fact caused by a comet shower, it most likely represents the most powerful comet shower that has occurred since the fossil record began. "Therefore, it's likely that any other comet shower that has occurred has had a weaker effect on the Earth and would be unlikely to cause an extinction event of equal or greater magnitude, if the late Eocene event is representative of a "typical" comet shower," he says.

As for the other extinction events, while an asteroid impact almost certainly caused the demise of the dinosaurs, environmental factors such as climate variation, sea level change or intense geologic activity could have been the trigger instead.