Core accretion is the leading scientific model for how planets form. It describes a bottom-up process: microscopic dust grains in the disk of material around a young star collide and stick together, gradually building larger and larger bodies until they become full planets. The process unfolds over millions of years and explains the formation of everything from rocky worlds like Earth to gas giants like Jupiter.
How Core Accretion Works
Every planet starts as dust. A young star is surrounded by a spinning disk of gas and fine solid particles, called a protoplanetary disk. Within this disk, tiny grains of rock, ice, and metal bump into each other and stick together, forming progressively larger clumps. These clumps eventually grow into kilometer-sized bodies called planetesimals, which are big enough for their own gravity to start pulling in more material.
Planetesimals then collide and merge to form protoplanets, rocky bodies roughly the size of the Moon to Mars. This is the core-building phase, and it’s driven almost entirely by the accumulation of solid material. What happens next depends on how large the core grows and how quickly it does so.
Rocky Planets vs. Gas Giants
The core accretion model actually describes two possible outcomes, and the fork in the road comes down to timing and mass.
For rocky planets like Earth, the process is relatively straightforward. Protoplanets continue to collide and merge in a series of giant impacts over tens of millions of years, well after the gas in the disk has dissipated. Without a thick envelope of gas to capture, the result is a dense, solid world. Earth’s final major collision, the impact with a Mars-sized body called Theia that formed the Moon, is thought to have occurred during this late stage.
Gas giants follow a different path. If a protoplanet can grow to roughly 10 Earth masses while the disk’s gas is still present, its gravity becomes strong enough to pull in and hold enormous quantities of hydrogen and helium from the surrounding disk. This triggers a phase called runaway gas accretion, where the planet’s atmosphere balloons rapidly. The result is a planet like Jupiter or Saturn, with a relatively small solid core buried under a massive gaseous envelope.
The key constraint is time. Observations of young stars show that the gas in protoplanetary disks dissipates within roughly 1 to 10 million years. A core that hasn’t reached that critical mass threshold before the gas disappears will never become a gas giant. It will remain a rocky or icy world instead.
The Pebble Accretion Shortcut
One persistent challenge with the traditional version of core accretion is speed. Building a core large enough to capture gas by colliding kilometer-sized planetesimals can take a very long time, sometimes longer than the gas disk actually lasts. This is especially problematic for planets far from their star, where material is more spread out and collisions are less frequent. In 1969, the physicist Viktor Safronov showed that forming cores at the distances of Uranus and Neptune via planetesimal collisions would take longer than the disk’s entire lifetime.
A newer refinement called pebble accretion helps solve this problem. Instead of relying solely on collisions between large planetesimals, this version of the model accounts for millimeter-to-centimeter-sized particles (pebbles) that drift inward through the disk. A growing protoplanet can sweep up these pebbles very efficiently because the surrounding gas slows the pebbles down, making them easier to capture. Pebble accretion can build planetary cores 100 to 1,000 times faster than planetesimal collisions alone, bringing the formation timeline comfortably within the disk’s lifespan.
The Competing Model: Disk Instability
Core accretion isn’t the only proposed explanation for planet formation. The main alternative is called disk instability, and it works in a fundamentally different way. Instead of building a planet from the bottom up over millions of years, disk instability proposes that a massive protoplanetary disk can fragment under its own gravity, collapsing directly into planet-sized clumps in as little as tens of thousands of years.
The two models make different predictions about what a forming planetary system looks like. In core accretion, a growing planet creates a detectable disturbance in the disk that tracks the planet’s position. In disk instability, the entire disk becomes turbulent and corrugated, and the exact location of the forming planet is hard to pin down. Distinguishing between these signatures requires extremely high-resolution imaging, on the scale of milliarcseconds.
Most astronomers consider core accretion the dominant pathway for planet formation, particularly for planets in our solar system and for the thousands of exoplanets discovered so far. Disk instability may still play a role in forming very massive planets far from their stars, where core accretion struggles with its timescale problem, but the observational evidence increasingly favors core accretion for the majority of known planets.
What Telescope Observations Show
Recent data from the James Webb Space Telescope has added new support for the core accretion model. By measuring the chemical makeup of exoplanet atmospheres, astronomers can test whether a planet’s composition matches what core accretion predicts.
One example is WASP-80 b, a warm gas giant orbiting a small star. JWST captured its complete emission spectrum across a wide range of infrared wavelengths. The planet’s atmosphere showed higher-than-solar levels of heavy elements, which is exactly what you’d expect if it formed by core accretion beyond the water ice line (the distance from its star where water freezes into solid ice) and then migrated inward through the disk, picking up metal-rich ices along the way. Its composition matched the broader population of hot gas giants thought to have formed through the same process.
These atmospheric measurements are becoming a powerful tool. Planets formed by core accretion should carry a chemical fingerprint of the solid material they accumulated during their early growth. As JWST continues surveying exoplanet atmospheres, these chemical signatures are helping scientists map out where and how individual planets formed, turning core accretion from a theoretical framework into something testable on a planet-by-planet basis.
Why Uranus and Neptune Are Still a Puzzle
Uranus and Neptune sit in an awkward spot for the core accretion model. They’re clearly not rocky planets, with thick atmospheres of hydrogen, helium, and volatile ices. But they’re also not gas giants in the Jupiter sense. They have massive solid cores (likely 10 to 15 Earth masses) surrounded by relatively modest gaseous envelopes, suggesting they started down the gas giant path but never completed runaway accretion.
The problem is building those cores fast enough at 20 to 30 times Earth’s distance from the Sun. Even with pebble accretion speeding things up, forming both planets concurrently at their current orbits remains difficult to model. Recent work has explored whether they might have formed closer to the Sun and migrated outward, or whether a combination of pebble accretion, gas accretion, and planetesimal accretion working simultaneously can account for their unusual size. The ice giants remain one of the most active areas of planet formation research precisely because they test the limits of core accretion in ways that Jupiter and Saturn do not.