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. 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.