Making a planet takes a cloud of gas, a lot of dust, and tens of millions of years. The process starts with the collapse of a massive molecular cloud and ends with violent collisions between protoplanets, each one reshaping the final world. Every planet in our solar system, from Mercury to Neptune, formed through this same general sequence, and telescopes like the James Webb Space Telescope are now watching it happen in real time around other stars.
It Starts With a Dying Star
Before a planet can form, the raw materials have to exist. Hydrogen and helium were produced in the Big Bang, but the heavier elements that make up rocky planets (iron, silicon, oxygen, carbon) were forged inside stars and scattered into space when those stars exploded as supernovae. These explosions seed the surrounding gas clouds with the building blocks of future worlds.
Supernovae also inject short-lived radioactive isotopes into nearby clouds. These isotopes, particularly aluminum-26, have half-lives shorter than 5 million years, meaning they couldn’t have been floating around the galaxy for long before being incorporated into a new solar system. Analysis of meteorites confirms that our own solar system was enriched by at least one nearby supernova shortly before it began forming. Research published in Science Advances proposes that a supernova occurring within about 1 parsec (3.26 light-years) of the early solar system bathed it in cosmic rays, producing these radioactive elements in just the right quantities. This isn’t a rare event: in typical young star clusters, at least one supernova occurs that close with high probability.
These radioactive isotopes aren’t just a footnote. They become the internal heat source that later drives a planet’s interior to separate into layers. Without them, rocky planets as we know them might not exist.
The Protoplanetary Disk
When a region of a molecular cloud collapses under its own gravity, most of the material falls inward to form a new star. But conservation of angular momentum flattens the remaining material into a spinning disk of gas and dust, called a protoplanetary disk. This is the construction zone where planets will be built.
The disk is overwhelmingly gas. The dust-to-gas ratio is roughly 1 to 100, meaning dust makes up only about 1% of the disk’s mass. That tiny fraction of solid material is all that’s available to build rocky planets and the cores of gas giants. The gas, mostly hydrogen and helium, dominates the disk’s behavior and will eventually be swept up by giant planets or blown away by the young star’s radiation.
Temperature varies dramatically across the disk. Close to the star, it’s hot enough that only rock and metal can exist as solids. Farther out, past a boundary called the frost line, temperatures drop low enough for water, methane, and ammonia to freeze into ice. This dividing line explains why our solar system has small, rocky planets close to the Sun and large, ice-rich gas giants farther out. Inside the frost line, there simply wasn’t as much solid material available to build with.
From Dust Grains to Planetesimals
The dust particles in a young disk are microscopic, similar in size to particles in cigarette smoke. Getting from that scale to a planet requires growth across more than 30 orders of magnitude in mass, and it doesn’t happen in one smooth step.
At first, dust grains collide and stick together through electrostatic forces, building fluffy aggregates up to about a centimeter in size. But there’s a problem: once particles reach roughly millimeter to centimeter sizes (often called “pebbles” by planetary scientists), they start drifting inward toward the star due to drag from the surrounding gas. Left unchecked, this drift would dump all the solid material into the star before it could grow any larger.
The leading solution to this problem is called the streaming instability. As pebbles drift through the gas, they naturally clump together in dense filaments. When these filaments become dense enough, they collapse under their own gravity, jumping straight from pebble-sized particles to solid bodies roughly 100 kilometers across. These bodies are called planetesimals, and the evidence for this jump comes from the large fraction of millimeter-sized grains (chondrules) found in meteorites, which suggests that planetesimals formed from the gravitational collapse of pebble clumps rather than from gradual, grain-by-grain growth.
Building Protoplanets
Once planetesimals exist, gravity takes over as the primary growth mechanism. Planetesimals attract one another, collide, and merge. The largest ones grow fastest because their stronger gravity sweeps up more material from a wider area. This runaway growth produces a handful of protoplanets, each one roughly Moon-sized to Mars-sized, within the first few million years.
As these bodies grow, internal heat begins to transform them. The radioactive decay of aluminum-26 heats their interiors to temperatures above 1,200 K, enough to melt metal. At around 1,600 K, silicate rock begins to partially melt as well. Once more than about 20% of the silicate has melted, the body’s viscosity drops enough for heavy molten metal to sink toward the center, forming an iron core. Lighter silicate material rises to form a mantle and eventually a crust. This process, called differentiation, is what gives rocky planets their layered internal structure. It happens remarkably early, within the first couple million years of a planetesimal’s existence, while the disk is still around.
Whether a body fully differentiates depends on when it formed. Bodies that accreted early, while aluminum-26 was still abundant, got enough heat to fully separate into core and mantle. Those that formed more than about 1.2 million years after the first solids ended up only partially differentiated, with a core and mantle buried under an unmelted outer layer.
Rocky Planets vs. Gas Giants
What happens next depends on where in the disk a protoplanet is growing.
Inside the frost line, solid material is scarce, so protoplanets stay relatively small. They continue growing through collisions with one another in a drawn-out process called the giant impact phase. About half of the collisions during this stage are “hit-and-run” events where the two bodies graze each other and separate again, sometimes losing mass in the process. The other half result in mergers. The Moon likely formed from one such collision: a graze-and-merge impact between the proto-Earth and a Mars-sized body. These giant impacts are global, energetic events that reshape a planet’s composition, density, and thermal state. Different sequences of collisions can produce similar final planets, which is why planetary scientists increasingly use probability models to study this phase.
Outside the frost line, the story is different. Ice adds significantly to the available solid material, so protoplanetary cores can grow much larger, reaching 10 to 20 Earth masses. At that threshold, something dramatic happens: the core’s gravity becomes strong enough to capture hydrogen and helium gas directly from the disk. Gas accretion accelerates rapidly, and within a relatively short time, the planet balloons into a gas giant. This is the core accretion model, and it’s the leading explanation for how Jupiter and Saturn formed.
There’s an alternative idea called disk instability, where a particularly massive disk fragments directly into planet-sized, self-gravitating clumps of gas without needing a solid core first. This could explain gas giants that orbit very far from their stars, where core accretion would take too long. Both mechanisms may operate in different systems.
How Long the Whole Process Takes
The journey from dust to protoplanet takes roughly tens of millions of years. But the final stage of assembly, when protoplanets collide and merge into finished rocky planets, can stretch considerably longer. For Earth, this last phase may have taken up to 100 million years.
Gas giants need to form faster, because they must capture their massive atmospheres before the disk’s gas disperses. Protoplanetary disks typically last only 3 to 10 million years before the star’s radiation and winds strip the gas away. Any gas giant that hasn’t captured its atmosphere by then will remain a bare, oversized rocky core.
What Makes a Planet a Planet
Size matters, but not in the way you might expect. A body becomes spherical when its own gravity is strong enough to overcome the rigidity of its material and pull it into a rounded shape, a state called hydrostatic equilibrium. For icy bodies, this happens at diameters around 400 kilometers. For rocky bodies, the threshold is higher, around 600 kilometers, because rock is stiffer than ice.
Saturn’s moon Mimas, made mostly of ice and just 400 kilometers across, is the smallest known body that gravity has shaped into a sphere. Ceres, the largest object in the asteroid belt, achieves the same thing at 940 kilometers with its rockier composition. But being round alone isn’t enough to qualify as a planet under the current definition adopted by the International Astronomical Union. A planet must also orbit a star and have cleared its orbital neighborhood of other debris, which is what separates planets from dwarf planets like Ceres and Pluto.
The full recipe, then, is straightforward in concept and staggering in execution: seed a gas cloud with heavy elements from dead stars, collapse it into a disk, let dust grains stick together and clump into planetesimals, grow those into protoplanets through collisions and gravity, and wait while giant impacts finalize the architecture. The ingredients are simple. The timescale is not.