Posted with permission from Newsweek

There are about seven times more large comets at the outer edge of our solar system than once thought, according to NASA. While these comets spend most of their time billions of miles away from the sun, when they do edge into our region of the solar system, they have the potential to collide with planets—including Earth.

Scientists used the Wide-field Infrared Survey Explorer (WISE) spacecraft to work out the size and numbers of short- and long-period comet populations. This refers to the orbital period—long refers to comets that take more than 200 years to orbit the sun, while short-period comets take less than 200 years.

Their findings, published in the Astronomical Journal, show there are vastly more long-period comets than previous estimates had indicated. “The number of comets speaks to the amount of material left over from the solar system’s formation,” lead author James Bauer said in a statement. “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 a shell of icy objects that surrounds the solar system, far beyond Neptune.

In the study, scientists looked at long-period comets and Jupiter-family comets—comets that have an orbital period of less than 20 years and that are controlled by Jupiter. Comets are formed from material that was left over from the formation of the planet, so studying them helps us understand the evolutionary history of the solar system.

In total, in the area of the sky surveyed, scientists found 56 long-period comets and 108 short-period comets. "Over the course of the eight months of the survey, our results indicate that the number of long-period comets passing within 1.5 au [about 140 million miles] are a factor of several higher than previous estimates," they wrote. 

By getting a better understanding of how many long-period comets there are and how they differ from Jupiter-family comets, researchers are able to gain a key insight into how and why the solar system appears to us as it does.

Researchers found there are far more long-period comets than there are Jupiter-family comets, and the that long-period comets are on average twice as big, measuring at least 0.6 miles across. “Our results mean there’s an evolutionary difference between Jupiter-family and long-period comets,” Bauer said.

They also discovered long-period comets passed the sun far more often than had been thought. This finding is important, as it has implications for the risk of comets hitting Earth and indicates more of them have probably affected planets, such as delivering icy materials to them. “Comets travel much faster than asteroids, and some of them are very big,” Mainzer said. “Studies like this will help us define what kind of hazard long-period comets may pose.”