Final answer:
The orbits of planets align in a plane due to the formation of the solar system from a spinning cloud of gas and dust known as the solar nebula.
Step-by-step explanation:
The phenomenon of planets aligning in a plane can be explained by the formation of the solar system from a spinning cloud of gas and dust known as the solar nebula. As the solar nebula collapsed and flattened into a rotating disk, the planets formed within this disk. Due to the conservation of angular momentum, the planets inherited the same plane of rotation as the original disk. This is why all the major planets in our solar system orbit the Sun in roughly the same plane.