How Planets Form: From Dust to Worlds in Protoplanetary Disks

Before a single exoplanet can be detected orbiting a distant star, it must be born—assembled from nothing more than dust and gas swirling in a protoplanetary disk. Understanding how planets form is the crucial foundation for interpreting the diverse populations of worlds we now observe. The journey from microscopic dust grains to fully-fledged planets involves competing physical processes, mysterious timescales, and dramatic migrations that reshape entire solar systems. By studying planet formation theory alongside observational data, we can decode why exoplanet systems look the way they do.

The Protoplanetary Disk: A Stellar Nursery

When a young star first ignites nuclear fusion, it is surrounded by a rotating disk of gas and dust—the leftover material from the star's own formation. This protoplanetary disk is not a permanent feature; it typically survives for only a few million years before planets consume it, stellar winds blow it away, or radiation evaporates it into space. Yet in those brief few million years, planets must assemble themselves from the atomic building blocks scattered throughout the disk.

The disk is not uniform. Near the star, it is hot; far from the star, it is cold. This temperature gradient creates distinct chemical zones where different materials can exist in solid form. Close to the star, only metals and rocky minerals survive the heat. Farther out, beyond the "snow line" (where water freezes into ice), more volatile compounds remain solid. This chemistry profoundly influences what kinds of planets can form where, seeding the different planetary architectures we observe with telescopes today.

Protoplanetary disks are visible to modern telescopes like the James Webb Space Telescope, which can image the gaps and rings carved by emerging planets. These observations have revolutionized our understanding by showing that planet formation is often well underway before any planet is fully grown.

Core Accretion: Building Worlds Bottom-Up

The dominant pathway for planet formation follows a remarkably simple but slow process: core accretion. In this model, planets are built piece by piece, starting from the smallest scales.

From Dust to Planetesimals

Dust grains in the disk are microscopic—roughly a micrometer across. Yet they do not remain isolated. As the disk swirls, dust particles collide gently with one another. Electrostatic forces and van der Waals interactions cause them to stick, forming larger aggregates. Through countless collisions over thousands of years, grains grow into millimeter-sized pebbles, then centimeter-sized rocks, then meter-sized boulders.

The growth eventually stalls at a critical threshold: objects roughly a meter across become too large to stick efficiently to their neighbors, yet too small to gravitationally attract material toward themselves. This seemingly insurmountable barrier is called the meter barrier. However, turbulence in the disk can concentrate pebbles into dense filaments and clumps, allowing runaway growth that leaps across the meter barrier in localized regions. Once an object reaches kilometer scales, gravity takes over, and accretion accelerates dramatically.

Objects at kilometer scales are called planetesimals, and they become the building blocks of planets. A planetary core grows by colliding with and absorbing countless planetesimals, a process that can take hundreds of thousands to millions of years.

From Cores to Planets

A rocky core roughly 0.1 Earth masses can grow rapidly once planetesimals accumulate around it. As the core grows, its gravity increases, pulling in material from larger volumes. The timescale matters critically: if the core reaches approximately 5–10 Earth masses before the disk's gas dissipates, it becomes massive enough to gravitationally bind the surrounding hydrogen and helium, growing into a gas giant. If accretion is slower, the disk gas drifts away before the core becomes large enough to trap it, leaving behind a smaller rocky or icy world.

Gravitational Instability: The Fast Track to Gas Giants

Core accretion explains how most rocky planets form, but it may not tell the whole story for the largest, most massive planets. In certain conditions, the disk itself can become unstable to gravitational collapse, allowing giant planets to form directly and rapidly without assembling a substantial solid core first.

When a protoplanetary disk is very massive and sufficiently cool, regions can fragment spontaneously under their own gravity, collapsing into planet-sized objects within 1,000 years or less. This gravitational instability mechanism is much faster than core accretion and can produce planets of extreme mass in brief timescales. Observing very young systems with giant planets very close to their stars suggests that this process may occur, though it is likely rare compared to core accretion.

The two formation pathways—core accretion and gravitational instability—may coexist in the same system, with some planets forming via each mechanism.

Migration: Reshaping Planetary Systems

Here lies one of exoplanet science's greatest puzzles: hot Jupiters, massive gas giants orbiting closer to their stars than Mercury orbits the Sun. These planets should not exist in their current locations. A gas giant cannot form so close to its parent star; the heat would have prevented the accumulation of planetesimals. Yet hot Jupiters are common.

The explanation is orbital migration. As a forming planet orbits within the disk, its gravity sculpts the gas and dust around it, creating density waves. These waves exert a torque on the planet, causing it to drift inward or outward. For most gas giants, the net torque drives inward migration, carrying them from their birthplace in the cooler outer disk to the inner solar system.

Migration can be dramatic and chaotic. Giant planets may scatter one another through gravitational interactions, causing some to be ejected from the system entirely while others are flung inward. A single surviving giant may end up in a completely different orbit than where it formed. Late-stage migration, after the gas disk has dispersed, is driven purely by gravitational interactions between planets and can cause catastrophic rearrangements of planetary orbits.

The precise migration history of a system leaves fingerprints on its final architecture. Systems with planets in neat, circular, co-planar orbits suggest calm migration and/or system stability. Systems with highly eccentric or misaligned orbits bear the scars of violent gravitational encounters.

Connecting Formation History to Transit Detection

At the S.O.L.A.R.I.S. project, citizen scientists search for planets using the transit method on NASA TESS data. Every detected exoplanet represents a successful outcome of a specific formation and migration history. The characteristics we observe—orbital period, planet radius, atmospheric composition—are clues to the origin story.

Consider orbital period: planets in very short-period orbits (a few days) were likely massive enough to migrate inward, or their systems experienced gravitational scattering. Planets in longer-period orbits may have formed in place or migrated less dramatically. Planet radius encodes information about composition, which depends on where the planet formed relative to the snow line. A massive, puffy gas giant formed near the snow line may have subsequently spiraled inward. A small, dense super-Earth likely formed in the inner disk where only rocky materials could accumulate.

When we catalog exoplanet populations using systems like S.O.L.A.R.I.S., we are implicitly sampling the outcomes of billions of different formation and migration histories across the galaxy. The relative frequencies of hot Jupiters, super-Earths, and other planet types tell us how often each formation pathway produces planets, which regions of protoplanetary disks are most hospitable to planet birth, and how violent the typical planetary system's youth must be.

Decoding Exoplanet Populations Through Formation Theory

Modern exoplanet science proceeds by iterating between theory and observation. Formation models predict which combinations of planet masses, orbital spacings, and compositions should be common. Observations of exoplanet populations (including contributions from citizen scientists using TESS data) test those predictions. Mismatches drive refinement of theory.

For example, the discovery of numerous mini-Neptunes in close orbits was unexpected. Core accretion alone struggles to explain why so many planets of intermediate size exist so close to their stars. New theoretical work proposes that photo-evaporation (stripping of an atmosphere by the star's radiation), collision-driven loss of atmospheric layers, and compositional separation during migration can all produce the observed population.

Similarly, the detection of planets in multi-planet systems with tight orbital spacings challenges our understanding of migration damping and resonance trapping. Planets can lock into orbital resonances during migration, maintaining specific period ratios even after the disk dissipates. Observing these resonant systems reveals the dynamical memory of formation.

The future belongs to integrating formation models with large observational datasets. Each new exoplanet discovered and characterized contributes to a statistical portrait of planetary system origins. Through this process, we transform the abstract physics of protoplanetary disks into a concrete, empirical understanding of how the universe populates itself with worlds.