The planets in our solar system are not arranged in a perfectly flat plane, but rather in a roughly flat or disk-like shape called the "ecliptic plane." The ecliptic plane is defined by the orbital paths of the planets around the Sun. It is a result of the initial conditions and processes that led to the formation of the solar system.
The formation of the solar system is thought to have started from a rotating cloud of gas and dust called the solar nebula. As the cloud collapsed under its gravity, it began to flatten into a spinning disk due to conservation of angular momentum. The majority of the material gathered near the center to form the Sun, while the remaining material coalesced into smaller clumps called protoplanetary disks.
Within these protoplanetary disks, particles and gas began to accrete together due to gravity, forming planetesimals and eventually protoplanets. These protoplanets continued to grow by accreting more material and undergoing collisions and mergers. The disk's flat shape influenced the alignment of the protoplanets' orbits, leading to the planets' relatively coplanar arrangement.
This process is known as the nebular hypothesis or the protoplanetary disk model, which provides a plausible explanation for why the planets in our solar system lie roughly on a flat plane. However, it's important to note that there is still ongoing research and exploration to better understand the exact details and variations in planetary formation within different systems.