By Griet Scheldeman
Science reporter, BBC News
There has been uncertainty about the role of comets in mass extinctions
Comet strikes are an unlikely cause of past mass extinctions on Earth, according to computer simulations.
Scientists used the simulations to model the paths of long-period comets, to determine the likelihood of these "dirty snowballs" striking our planet.
The University of Washington, Seattle, research appears in Science journal.
How many mass extinctions in Earth's history were caused by these icy bodies crashing into our planet has been a subject of considerable debate.
Many scientists agree that an asteroid strike 65 million years ago wiped out the dinosaurs. But there is uncertainty about how many other such events were triggered by asteroid or comet colliding with Earth.
Comets are composed of dust, rock, water ice and frozen gases. When their orbits bring them into close proximity with the Sun, parts of them warm up, causing material to sublimate (turn directly from a solid to a gas state) and form a "fuzzy" envelope around the comet nucleus.
Asteroids are distinguished from comets precisely because they lack this envelope, or "coma".
Halley's comet - which reappears in our sky every 75 years - is described as "short period". This type of comet originates in a region of our Solar System called the Kuiper Belt.
So-called "long period" comets are thought to come from the more distant Oort Cloud. This is a large area of debris left over from the formation of the Solar System some 4.5 billion years ago.
The outer part of this Oort Cloud region was previously thought to be a source of long-period comets that pass close to Earth.
Their orbits are thought to change when they are nudged by the gravity of a nearby star. These events are thought to trigger an increase in the frequency of comets entering the inner Solar System - a phenomenon called "comet showers".
But simulations carried out by Nathan Kaib and co-author Thomas Quinn at the University of Washington in Seattle suggest the inner Oort Cloud could instead be a major source of the long-period comets crossing Earth's path.
This suggests many observable comets come from a different source than previously thought. But the findings also have implications for investigating the causes of mass extinctions.
The exact number of objects in the inner Oort Cloud is unknown. But by assuming the maximum possible number, the researchers showed that only two or three objects from this region should have struck our planet over the last 500 million years.
The gas giants Jupiter and Saturn act as a barrier, preventing most of them from hitting us. That point was reinforced recently when a huge scar appeared on Jupiter's surface, likely evidence of an impact.
A minor extinction event on Earth about 40 million years ago has been attributed to one of these comet showers. The University of Washington research suggests this was probably the most intense shower on record.
"That tells you that the most powerful comet showers caused minor extinctions and other showers should have been less severe, so comet showers are probably not likely causes of mass extinction events," said Mr Kaib.