What type of solar system remnants primarily constitute the Oort Cloud?

Icy planetesimals

The Oort Cloud is fundamentally composed of icy planetesimals. These are defined as the building blocks of planets that failed to accrete into larger structures. Having been ejected from the warmer, denser inner and middle regions of the early solar nebula, these primordial materials represent the deep-frozen storage of the original solar nebula. They are heavily weighted toward volatile substances, often termed 'ice' in astronomical language, including water ice, frozen methane, ammonia, and carbon monoxide. Their longevity and composition stem from remaining largely untouched by significant heating or radiation for billions of years due to their extreme isolation.