The formation of large planets is influenced by various factors, including the composition of the protoplanetary disk and the distance from their parent star. The process of planet formation begins with the accumulation of solid material, such as dust and ice, in a protoplanetary disk surrounding a young star. Over time, these small particles collide and stick together, gradually forming larger and larger objects known as planetesimals.
In the outer regions of a protoplanetary disk, where temperatures are cooler, volatile substances like water and methane can condense and freeze, leading to the formation of solid cores made primarily of rock, metal, and ice. These cores serve as the building blocks for planets. However, beyond a certain distance from the star, called the frost line, the temperatures are low enough for volatile gases to remain in a gaseous state rather than condensing into solids.
When a solid core reaches a certain mass (typically several times the mass of Earth), its gravitational pull becomes strong enough to attract and capture significant amounts of surrounding gas, primarily hydrogen and helium. This process is called gas accretion. In the outer regions of the protoplanetary disk, where there is an abundant supply of gas, the growing core can accumulate large amounts of gas, eventually becoming a gas giant.
On the other hand, closer to the star, within the frost line, the temperatures are higher, and volatile gases remain primarily in a gaseous state. As a result, the solid cores that form in these regions have less access to gas, and their growth is limited. They tend to become smaller, rocky planets like Earth, Mars, or Venus.
Therefore, the formation of gas giants or rocky planets depends on the availability of gas and volatile materials in the protoplanetary disk and the location within the disk where the planet forms. Outer regions with abundant gas favor the formation of gas giants, while inner regions with less gas content are more conducive to rocky planet formation.