core accretion
How do you build a planet from a thin haze of gas and dust? Imagine a dusty whirlpool circling a newborn star. The dust grains are no bigger than smoke particles, but they bump into one another and stick, like dust bunnies gathering under a bed. Pebbles glue into boulders, boulders into mountains, mountains into a small rocky world. Once that world grows heavy enough, its gravity starts pulling in everything nearby — first more rock and ice, then, if it gets big enough fast enough, the surrounding gas. Core accretion is this patient, build-it-up-from-grains recipe for making planets.
In the standard picture, a solid core grows step by step inside the disk around a young star. Near the star it is too warm for ice, so cores stay small and rocky, making worlds like Earth. Farther out, beyond the so-called snow line, water freezes into ice and adds plenty of extra solid material, letting a core grow to roughly 10 times Earth's mass. At that point its gravity is strong enough to grab huge amounts of hydrogen and helium gas from the disk in a runaway, ballooning into a giant like Jupiter. The whole sequence has to finish in only a few million years, before the star's light and winds blow the gas away — which is the model's tightest constraint.
Core accretion is the leading explanation for how most planets, including the ones in our Solar System, came to be, and it naturally explains why the rocky planets sit close to the Sun and the gas giants sit farther out. Its main difficulty is speed: building a 10-Earth-mass core fast enough to catch the gas, especially in the cold outer disk, is hard, which is why a rival idea (disk instability) is invoked for some distant giants. The model keeps being refined as we discover planetary systems wildly different from our own.
Jupiter is thought to have built a rocky-icy core of perhaps 10 Earth masses just beyond the snow line, then grabbed gas so fast it ended up over 300 times Earth's mass — yet its dense core is still buried under all that hydrogen, a frozen record of how it began as a solid seed.
A giant planet starts as a small solid core, then balloons by swallowing gas.
Core accretion is the favored model but not a finished story: making a big enough core fast enough remains its hardest test, and some systems may need other mechanisms.