The core accretion model (Lissauer 1993; Pollack et al. 1996; Safronov 1972) is a multi-step formation process which starts with the formation of a heavy element core which forms when small solid dust grains and ices ($\leq \mu m$) sediment and coagulate through collisions into larger particles. These particles (~cm in size) proceed to stick together until eventually objects ~1-100 km in size form, known as planitesimals. Beyond this size, gravity becomes important in the development of pairwise planetesimal accretion forming planetary embryos. The embryos eventually form into a planetary cores or protoplanets. When the thermal speed of the surrounding gas drops below the escape velocity of the newly formed core, gas starts to accrete around the core. Thermal pressure dictates the ever increasing growth rate which defines the runaway gas accretion phase (D’Angelo et al. 2011). This continues until the reservoir of gas within the gravitational reach of the planet is exhausted.

The core accretion model has in recent years emerged as the dominant formation mechanism with a large body of observations supporting it. One such observation is the steep increase in the giant planet occurrence rate as a function of host star metallicity above solar metallicity (Fischer & Valenti 2005; Santos et al. 2004). The terrestrial planets of the solar system together with the supersolar compositions and likely high mass cores of Jupiter and Saturn (Aatreya et al. 2003; Guillot 2005; Militzer et al. 2008) are further examples in favour of the core accretion model. Uranus and Neptune also have envelopes enriched in heavy elements, however pose more of a challenge with their predicted formation timescales exceeding classical calculations of the lifetime of the solar nebula (Safronov 1969). More recent studies have found that this timescale can be lowered significantly under certain assumptions such as a modest enhancement of the initial surface density and if the growth takes place preferentially during the runaway planetesimal accretion phase (Pollack et al. 1996). These results are echoed by Helled & Bodenheimer (2014) who highlight the fact that different conditions in the protoplanetary disk, as well as the birth environments of the planetary embryos, can lead to the formation of planets with vastly different masses and compositions. This would provide a natural explanation for the large diversity of intermediate mass exoplanets currently observed.