Question

What made it possible for the planet to become as big as they are

Answer 1

Answer:

Planets can gain mass by being hit by asteroids, comets, and other planets. Most of this happens in the early history of a star system; later on, there aren't enough massive objects to hit the planets anymore to make them grow appreciably. When our Solar System was forming, gravity gathered billions of pieces of gas and dust into clumps which grew larger and larger to become the planets. The force of the collision of these pieces caused the newly forming planets to become hot and molten.