Huge comets are seven times more likely to hit earth than previously thought.
Space boffins used data from Nasa’s WISE spacecraft to determine the nucleus sizes of comets, and estimate the number out there.
Some comets in the distant parts of the solar system, between 186 billion miles (300 billionkms) away from the sun, known as the Oort Cloud, can be up to 60 miles across.
However, they may only pass the sun every 200 years or so, making them harder to study.
Comets are made up of materials, including rock and ice, left over from the formation of the universe, and it was previously thought there were less than there actually are.
If anything of a few miles or more across hit the Earth, it could destroy the entire planet, wiping out all life.
The findings published in the Astronomical Journal, also found more comets than thought are leaving the Oort Cloud and passing the sun.
A Nasa spokesman said: “NASA’s WISE spacecraft, scanning the entire sky at infrared wavelengths, has delivered new insights about these distant wanderers.
“Scientists found that there are about seven times more long-period comets measuring at least 0.6 miles (1km) across than had been predicted previously.
“Researchers also observed that in eight months, three to five times as many long-period comets passed by the Sun than had been predicted.”
James Bauer, lead author of the study and now a research professor at the University of Maryland, College Park, said: “The number of comets speaks to the amount of material left over from the solar system’s formation.
“We now know that there are more relatively large chunks of ancient material coming from the Oort Cloud than we thought.”
The Oort Cloud is too distant to be seen by current telescopes, but is thought to be a spherical distribution of small icy bodies at the outermost edge of the solar system.
The density of comets within it is low, so the odds of comets colliding within it are rare.
Long-period comets that WISE observed probably got kicked out of the Oort Cloud millions of years ago.
Amy Mainzer, study co-author based at NASA’s Jet Propulsion Laboratory, Pasadena, California, said: “Our study is a rare look at objects perturbed out of the Oort Cloud.