Final answer:
The Kuiper belt is flat because it consists of materials influenced by Neptune's gravity and aligned with the planetary ecliptic plane, while the Oort cloud is spherical due to objects being scattered in all directions by giant planets. The study of Kuiper belt objects enhances our understanding of the solar system's formation.
Step-by-step explanation:
The reason why the Kuiper belt is flat but the Oort cloud is spherical comes down to the history and dynamics of our solar system's formation. The Kuiper belt consists of ice-and-rock planetesimals that are remnants of the building blocks of the planets and are significantly influenced by the gravity of Neptune. These interactions during the early evolution of the solar system resulted in a flatter, disc-like structure as the orbits of the Kuiper belt objects were aligned with the ecliptic plane in which the planets orbit. In contrast, the Oort cloud is a distant spherical shell of icy objects that were scattered in all directions by gravitational interactions with the giant planets, creating a more isotropic distribution around the Sun. Astronomers study the Kuiper belt using new, powerful telescopes and by measuring the composition of comets that originate from it. Comparing these Kuiper belt comets with those from the Oort cloud can provide insights into the different compositions and histories of these celestial bodies. As such, the Kuiper belt helps us understand the early evolution of our planetary system and the material within the solar nebula disk.