Final answer:
Planets and the Sun travel through the ecliptic plane due to the conservation of angular momentum from when the solar system formed, with all bodies originating from a rotating disk and flattening out into a disc-like pattern.
Step-by-step explanation:
The reason all of the planets and the Sun travel through the ecliptic plane in the sky is due to the conservation of angular momentum. This is because the planets, the Sun, and the Moon all formed from the same rotating disk of dust and gas around the early Sun. Over time, the material in this disk flattened out and coalesced into the celestial bodies we observe today. They all revolve around the Sun in the same direction and approximately in the plane of the Sun's own rotation, which is why we see them move across the sky in a relatively flat, disc-like pattern centered on the ecliptic. Changes in the inclination of the ecliptic are also responsible for the Sun's apparent movement north and south in the sky, leading to the changing seasons.