Why are there two types of planets?
As we look out into our solar system as well as others, we notice a pattern among the planets: there are small, rocky worlds, and enormous, gaseous “Jovian” ones. But why is this? Why is there a such a distinct line between the two, so that Jovian worlds are not intermingled with terrestrial ones and vice versa? The answer lies in the birth of our solar system and something called the frost line.
When our star was just beginning its life, it was surrounded by a protoplanetary disk or solar nebula that consisted of 98% hydrogen and helium gas, 1.4% hydrogen compounds (stuff like water, methane, and ammonia) and the rest (0.6%) was rocks, minerals, and metals (visual aid).
Naturally, the vast majority of the solar nebula that was hydrogen and helium could not condense. Throughout the entire disk, all rocks, minerals, and metals were able to condense, which is where we get the terrestrial planets in the inner solar system from as well as the rocky cores of comets and planets in the outer solar system.
But, specifically in our solar system, there is a line just beyond the asteroid belt that marks the frost line. Within the frost line, rocks and metals condense, but hydrogen compounds stay gaseous because it is so warm. Beyond the frost line, rock and metals condense and hydrogen compounds freeze.
The reason that the Jovian planets are so enormous compared to our small rocky world is because there was far more of the hydrogen compounds (three times as much) compared to rocky material in the early solar system, and there was more material outside of the frost line than within it. This is also why comets only come from the Kuiper Belt and Oort Cloud. As the solar system grew older, planetesimals collided, eventually “growing” larger and larger so much that they had a significant enough gravitational pull to draw in more and more hydrogen compounds. The planets ballooned, resulting in the gas giants we see in our solar system today.
Pretty cool, huh?
(x) (text source: my astronomy textbook)