We have now some information on the primordial disc. These objects are now being observed by Earth based and spaceborne telescopes. In this chapter we would like to study how from this disk the terrestrial planets (and so the Earth) may form. The key to understand this process is the gravitational instability of a flat object. We should be familiar with the Jeans instabilityJeans instability which refers to a limit condition when on a spherical object the pull of gravity overcome the pressure force while we are much less familiar when the instability refer to a flat object. The pieces resulting from this instability may give origin to the embryo of the planet that we call planetesimalsPlanetesimals. The subsequent interactions of such planetesimals give raises to rocky planets. To illustrate this path we will follow the illuminating work of Philip ArmitageArmitage, Philip while this mechanism was first introduced by Viktor SafronovSafronov, Viktor  of the Moscow University in a precious little book of 1969 and by Peter GoldreichGoldreich, Peter and Robert WardWard, Robert of Caltech in a paper five years later.