Final answer:
Young planets in protoplanetary disks primarily display spiral motion due to gravitational interactions with gas and dust remnants, causing them to spiral inward, although some can halt and remain in the system.
Step-by-step explanation:
The tracks of young planets in their protoplanetary disks exhibit spiral motion. This phenomenon occurs because the planets interact with the remnant material in the disk. As a young planet travels at a faster velocity than the surrounding gas and dust, it experiences a form of frictional 'headwind', which causes the planet to lose energy and spiral inward towards the star. While some protoplanets might plunge into the star, others, like hot Jupiters, manage to halt their spiral and remain in the planetary system. The spiral motion reflects the gravitational influences and dynamics within the early solar system, including the conservation of angular momentum.