system
Given the following post, respond as if you're commenting on Reddit:
human
Why is the Kuiper belt a disc but the Oort Cloud is a sphere?
gpt
The Nice Model explains this. The Kuiper Belt (at that time a disk of planetesimals without planets) was already here after the solar system formed, back when the planets were much closer to the sun (ye the gas giants moved outwards after). Due to gravitational interactions between planets (mostly orbital resonance between Jupiter and Saturn) and also between planets and planetesimals, the last 3 planets moved outwards into the planetesimal disk, flinging many of them towards the inner solar system. Many of these are intercepted by Jupiter, which due to its large mass and their trajectories, flung them outwards into deep space in all directions. The stuff that got flung out never fully escaped the Sun's gravity and started orbiting it. This is what's known as the Oort Cloud today. The disk of stuff which is now the Kuiper Belt still remains.