Scientists still believe that a massive slab of rock crashed into Earth and caused a cataclysmic explosion that wiped out the dinosaurs. A new study from Harvard, however, changes the theory of the origins of that space rock.

Harvard researchers say the effect was not from a mammoth asteroid that came from relatively nearby - somewhere between Mars and Jupiter - but rather from a smaller comet that originated farther out from a region known as the Oort Cloud at the edge of the solar system, the Harvard Gazette reports.

Using statistical analysis and gravitational simulation, Avi Loeb, Frank B. Baird Jr. Professor of Science at Harvard, and Amir Siraj, an astrophysicist, say a large part of a type of comet emanating from the Oort cloud, a sphere of debris at the edge of the solar system, was tipped off course by the gravitational field of Jupiter and sent close to the sun, the tidal force of which shattered parts of the rock.

One of those fragments, about the size of Boston, struck off the coast of Mexico and caused what had come to be known as the Chicxulub crater. That raises the incidence of comets like Chicxulub because these fragments cross the Earth's orbit and strike the planet once every 250 million to 730 million years.

"Basically, Jupiter acts as a kind of pinball machine," said Siraj, who is also joint president of Harvard Students for the Exploration and Development of Space. "Jupiter kicks these incoming long-period comets into orbits that bring them very close to the sun."

It's because of this that long-period comets, which take more than 200 years to orbit the sun, are called sungrazers, he added.

Further investigation found that comets within a range of 10 kilometers to 60 kilometers (between 6 miles and 37 miles) would be broken apart by sufficiently powerful forces into smaller fragments - similar to what happened to the comet Shoemaker-Levy 9 when it struck Jupiter in 1994.

The most widely accepted theory for what caused the mass extinction of dinosaurs is known as the "Alvarez hypothesis," after the late scientist Luis Alvarez and his geologist son, Walter.

In 1980, they suggested that the extinction event may have been caused by a massive asteroid or comet entering Earth. This finding was based on their study of sedimentary deposits at the Cretaceous-Paleogene boundary discovered all over the world - which included extremely high amounts of iridium - a metal more commonly observed in asteroids than on Earth.