Answer:
This answer is not in detail but it gives you a bit of an idea of how it works.
With the "core accretion" theory, planets start as small rock-ice cores that grow as they gravitationally acquire additional mass.
The most commonly accepted mechanism for the formation of Jupiter-like planets is the core accretion model. In this model a rocky core forms through the coagulation of planetesimals until it is sufficiently massive to accrete a gaseous envelope. Initially this envelope is in hydrostatic equilibrium, with most of the luminosity provided by the accreting planetesimals. Once the core reaches a critical mass, however, hydrostatic equilibrium is no longer possible, and a phase of rapid gas accretion occurs.
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