Final answer:
Jovian planets formed through a two-step process where planetesimals first collide to form a solid core, and then this core accretes gaseous material to become a gas giant. Jupiter and Saturn acquired large amounts of gas while Uranus and Neptune got less, affecting their composition. Migration within the disk is also possible, explaining closer hot Jupiters.
Step-by-step explanation:
The formation of the jovian planets, Jupiter, Saturn, Uranus, and Neptune, involves a two-step process within the solar nebula theory. The first step is the accumulation of a solid core through the collisions and amalgamation of planetesimals. These planetesimals collide, stick together, and gradually form a larger core. The second step occurs when this core becomes massive enough to sweep up gaseous material from the surrounding protoplanetary disk. Jupiter and Saturn, for instance, were able to accrue significant amounts of hydrogen and helium, giving them a composition similar to the Sun's. Uranus and Neptune, however, collected less gas, leading to their current icy and rocky compositions. Observations of exoplanets and the behavior of planet-forming disks indicate that jovian planets likely formed several astronomical units away from their stars but were capable of migrating closer over time due to gravitational interactions.