The formation of the solar system by gravitational instability:   Prediction of a new planet or another Kuiper-type belt
  The early gas-dust solar nebula is considered: the gasdynamic theory is used to study the gravitational Jeans-type instability in its protoplanetary disk. The implications for the origin of the solar system are discussed. It is shown that a collective process, forming the basis of the gravitational instability hypothesis, solves with surprising simplicity the two main problems of the dynamical characteristics of the system, which are associated with its observed spacing and orbital momentum distribution. The analysis is found to imply the existence of a new planet (or another Kuiper-type belt) at a mean distance from the Sun of 87 AU. 
Evgeny Griv and Michael Gedalin