system
Answer the questions
human
Rewrite this passage in simpler language by removing technical terms.

Relevant data:
'The theoretical model proposes a two-step process for the formation of planetary systems: first the accretion of planetesimals, followed by protoplanet formation via gravitational instability.'
gpt
The idea says that planets are made in two parts: small rocks clump together first, and then bigger bits of rock form because of gravity.