Here's why:

* Lump of ice, rock, frozen gas, and dust: Comets are composed primarily of ice (water, carbon dioxide, methane, ammonia) mixed with dust and rock particles.

* Orbits the Sun: Comets travel in highly elliptical orbits around the Sun, spending most of their time far away from it in the outer Solar System.

Let me know if you'd like to learn more about comets!