The most MOST likely origin of the "dirty snowballs" that become comets when they are deflected into orbits that bring them closer to the Sun is the:

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Comets are believed to originate from the Kuiper belt and the much more distant Oort cloud, which are both located outside of the orbit of Neptune. These regions are thought to be populated by icy planetesimals that occasionally get perturbed by gravitational interactions with nearby planets or passing stars, causing them to be deflected onto orbits that bring them closer to the Sun. Once they approach the Sun, the heat causes the ices on their surfaces to sublimate and release gas and dust, forming the characteristic tail of a comet.

The Most MOST Likely Origin of the "Dirty Snowballs" That