Final answer:
Planets composed of heavy elements beyond hydrogen and helium were formed from materials enriched by previous generations of stars through a. supernova explosions and nucleosynthesis, leading to the progressive enrichment of the interstellar medium.
Step-by-step explanation:
The elements heavier than hydrogen and helium, which make up the planets, most likely originated from a. supernova explosions that occurred before the solar system formed.
In stars more massive than about 8 solar masses, nuclear reactions can produce elements as heavy as iron through a process called nucleosynthesis.
Over time, as stars die, they release these heavy elements into the interstellar medium, where they can become part of the raw material for new stars and planets.
This progressive enrichment led to the formation of planets like Earth, composed of silicon, iron, and other heavy elements, only possible after several generations of stars had the opportunity to create and spread these materials.