The asteroid that wiped out the dinosaurs may not have been the whopper scientists thought.

An analysis of the chemical remains of the asteroid that can still be found in sediments under the sea today shows the rock was about 2.5 miles wide, according to Francois Paquay, a geology graduate student at the University of Hawaii.

That’s significantly less than the up-to-12-mile-wide space boulder that past researchers have suggested was the dinosaur-killer, according to the research published Friday in the journal Science.

The dinosaurs, which ruled Earth for 160 million years, went extinct at the end of the Cretaceous period about 65 million years ago. Scientists have studied a number of scenarios to explain the event, including global climate change.

The killer asteroid theory was bolstered two decades ago, when scientists found in rocks dating from the Cretaceous period a band of iridium, a metal rare on Earth but common in meteorites. The later discovery of the 110-mile-wide Chicxulub crater off Mexico’s Yucatan peninsula seemingly clinched the debate.

Paquay said he and his research team took a different approach to the problem of determining the size of the asteroid. Instead of focusing on iridium, he looked at sediments containing a form of the super-dense metal osmium.

Paquay measured the distribution of an osmium isotope found in meteorites. Because the percentage of osmium in meteorites is known, he could then calculate the overall size of the object.