How do Planetesimals form?

• A. Dust and ice particles sticking together to form Planetesimals after the inner disk is cleared
• B. Gas clumping directly into planetesimals
• C. Direct condensation of rocks into large bodies
• D. Collision of comets forming planetoids

Answer: (A)

Planetesimals form when solid particles in a young star’s protoplanetary disk collide and stick together, growing from microscopic dust and ice grains into larger bodies. This coagulation happens as grains settle toward the disk’s midplane and encounter each other at gentle enough speeds to stick, with icy grains especially sticky beyond the snow line. As sizes increase, gravity starts to play a bigger role, and dense clumps of solids can collapse or be amplified by processes like streaming instability, producing kilometer-sized planetesimals. This solid-growth path is the cornerstone of planetesimal formation, rather than gas clumping into solids, direct rock condensation into large rocks, or formation from comet–planetesimal collisions.