Let us consider the gravitational force acting on each mass M by adjacent particles be F. and the gravitational force acting on each mass M diagonally be F1
The net force, Fnet=RMv2 Along the centre of circle, 2F+F1=RMv22((2R)2GMM)+((2R)2GMM)=RMv2 ⇒ R2GM(21+41)=Rv2 ⇒ v=21RGM(22+1) Hence, the speed of each particle is 21RGM(22+1).