They are the origins of planets.
They formed from gas and dust.
They are believed to have developed in debris disks.
According to the planetesimal theory, planets were formed from planetesimals. It all started with gas and dust-- remnants of the big bang in the form of debris disks. These disks coalesced and formed bigger and more solid forms of matter through the process of accretion.
These bigger forms of matter ten started to attract each other as they grew bigger due to gravity. They slammed into each other, either breaking smaller pieces apart or they stuck together to form even bigger pieces. These became planetesimals. As the collisions continued, the others became bigger and bigger and formed planets.
