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1) explain planetesimals, and state the unsolved problem

2) talk one possible way: GI of small size particles, talk under what circumstances it is possible and the implications:

1. one way: particle sticking → meter size → streaming instability + gravitational collapse → km size // two prob: further growth from particle-particle sticking stops at ~ cm due to fragmentation and bouncing.. meter size radial orbit decay

2. alternative way: the answer relies on how dense these small size particles could reach.