Comets are the icy remnants left over from the formation of the Solar System. They circle the Sun in highly elliptical orbits that can range in length from several years to several million years, depending on their origin, and while they are usually quiet and dark when they get close enough to the Sun they are briefly heated enough to melt—technically sublimate—some of their frozen material, forming a cloud of gas and dust and a long tail sometimes big and bright enough to be visible from Earth.

But for the majority of their travels most comets are dark and difficult to spot, especially those originating from the Oort Cloud, an enormous spherical zone of icy debris surrounding our Solar System 186 billion miles away. Now, using infrared data from NASA’s WISE spacecraft, researchers have concluded that there are many more so-called “long period” comets visiting from the Oort Cloud than previously suspected—at least seven times more—and that they’re larger than we thought, too…many over half a mile across.

The number of comets speaks to the amount of material left over from the solar system’s formation,” said James Bauer, lead author of the study and now a research professor at the University of Maryland, College Park. “We now know that there are more relatively large chunks of ancient material coming from the Oort Cloud than we thought.”

While comets are thought to have delivered much of the water and possibly even the organic material necessary for life to the early Earth, they also pose an unlikely but definitely-not-zero risk of impact should their orbits intersect Earth’s…a risk with increased consequences due to comets’ size and velocities.

While there are currently no comets known to be on a collision course with Earth in the foreseeable future, it’s important to be able to identify as many as we can to be able to accurately plot their orbits.

“Comets travel much faster than asteroids, and some of them are very big,” said Amy Mainzer, NEOWISE PI and study co-author based at NASA’s Jet Propulsion Laboratory. “Studies like this will help us define what kind of hazard long-period comets may pose.”

Read the full story from NASA’s Jet Propulsion Laboratory here.