Kuiper belt/Scattered disc

[FeynmanMH42]

Why has this not coalesced yet to form a planet?

[tony873004]

It never will. Eccentricities are too high, so any collisions would not be low-velocity. Collisions will grind things down instead of build them up. They also move very slowly, and the volume of space out there is huge, so collisions are not even that likely.