Introduction to the Formation of Planetary Systems
Linas JuozėnasShare
Knowledge Ark · Universe · Chapter 05
From stardust to worlds
How do gas, dust, and ice become a family of planets? Explore the growth, collisions, and changing orbits that turn a young star’s surroundings into a planetary system.
Available solids, gas, and their distribution influence what can grow.
Material drifts, collisions reshape bodies, and the gas supply eventually fades.
A planet’s present location may be far from its original birthplace.
A planet is something the universe builds.
Long before Earth had oceans, continents, or a sky, its ingredients were scattered through the surroundings of a young Sun. Becoming a planet required those materials to gather, grow, collide, and change.
The previous chapter followed the stellar life cycle. Now we turn to the disks around young stars and the worlds that emerge within them. The starting material already includes interstellar dust; within the disk, solids can be reworked, vaporized, or condensed again.[1], [3]
Observations have revealed planetary systems very unlike our own. Explaining that diversity means asking how much material was available, how quickly it grew, and how the system evolved afterward. The Solar System gives us a detailed local example; exoplanets show how much wider the possibilities are.[11]
Ten questions about the making of worlds
Follow the building material, the growing bodies, and the evidence their histories leave behind.
Protoplanetary disks: birthplaces of planets
What does a young star have to build with?
As a star forms, some surrounding gas and dust settles into a rotating protoplanetary disk. This is a moving environment: material drifts, accumulates, feeds the star, or escapes. Temperature and chemistry vary across it.
Many gas-rich disks disperse within a few million years, although their lifetimes vary. That gives giant planets a limited window to acquire large gas envelopes. Images of rings and gaps reveal structure, but a gap alone does not prove that a planet carved it. This article explores the conditions in which planetary growth begins.[1], [2]
Explore article 01→Planetesimal accretion
How does dust become something gravity can hold together?
Small grains can stick when they collide, but growth is not an uninterrupted climb from dust to boulders. Particles may bounce, fragment, or drift toward the star before becoming much larger.
One promising route concentrates drifting solids through their interaction with gas. Dense clumps can then collapse under their own gravity into planetesimals, the building blocks of larger worlds. Collisions among these bodies and the capture of smaller, gas-dragged particles help planetary embryos grow. We will explore how several processes work together to cross the difficult gap between grains and planets.[3], [4]
Explore article 02→Formation of terrestrial worlds
When does a growing rocky body become a world?
Rocky planets assemble from solid material through accretion and collisions, including giant impacts between planetary embryos. Heat from growth and radioactive decay can melt substantial parts of a young body. Dense metal can separate toward the center, leaving a rocky mantle above it: a process called differentiation.
A planet’s later character also depends on the water and other volatile materials it acquires, the atmosphere it retains or loses, and the energy it receives from its star. The inner disk favors heat-resistant solids, but material can travel between regions. Earth and Venus illustrate how different rocky worlds can become.[4]
Explore article 03→Gas and ice giants
Why does one planet become a Jupiter and another a Neptune?
In core accretion, a growing concentration of solids attracts gas. If its envelope can cool and contract quickly enough while disk gas remains available, rapid gas accretion may follow. Growth, cooling, and disk dispersal all affect the outcome.
Beyond the water snow line, water ice adds to the solid building material. This boundary shifts as the disk evolves. A birthplace beyond it does not guarantee a giant. Uranus and Neptune contain much smaller hydrogen–helium fractions than Jupiter and Saturn. Another proposed route, gravitational fragmentation of a massive disk, requires suitable cooling conditions.[5], [4], [2]
Explore article 04→Orbital dynamics and migration
Can a planet travel far from where it formed?
A planet exchanges angular momentum with the surrounding disk. These interactions can shift its orbit inward or outward, depending on the disk and the planet. Growing planets can also settle into resonances, with repeated gravitational encounters linking their orbital periods.[6]
After the gas disperses, interactions among planets can rearrange the system further, producing collisions, tilted or elongated orbits, or ejections. Hot Jupiters demonstrate that giant planets can end up extremely close to their stars. Disk migration and later dynamical processes offer different possible routes; the observed orbit need not reveal one unique history.[2]
Explore article 05→Moons and rings
How does a planet acquire companions of its own?
Some moons grow in disks around giant planets. Others can assemble from impact debris or be captured through interactions that remove or exchange orbital energy. Simply passing near a planet does not guarantee lasting capture.
Rings are evolving populations of orbiting particles. Impacts, disrupted bodies, and exchanges with nearby moons can supply or reshape them; spreading rings can also contribute material to new moons. We will examine these connected histories, including why no single formation story explains every satellite and ring system.[7]
Explore article 06→Asteroids, comets, and dwarf planets
What can the smaller bodies tell us that planets cannot?
Asteroids, comets, and dwarf planets preserve different parts of the Solar System’s history. Their minerals, ices, shapes, and orbits help reconstruct how material was assembled and redistributed. Some retain ancient ingredients; others are fragments of larger bodies or worlds with substantial internal evolution.[8]
They are not all untouched leftovers. Heating, collisions, and reactions with water can rewrite the record. Samples from asteroid Bennu, for example, preserve evidence of extensive water-driven changes in its parent body. This article treats small bodies as witnesses with histories of their own.[9]
Explore article 07→Exoplanet diversity
How many different kinds of planetary system are possible?
Exoplanets include close-orbiting giants, compact families of small planets, and worlds between Earth and Neptune in size. The label super-Earth describes a broad mass or size category; it does not promise an enlarged Earth with familiar oceans or a habitable surface.[10]
To explain this variety, researchers compare growth, migration, atmosphere loss, and stellar environments. They also account for discovery bias: a method that readily finds large, close-in planets does not reveal every kind of system equally well. The observed catalog is evidence to interpret, rather than a complete inventory.[11]
Explore article 08→The habitable zone concept
What does the right distance from a star actually tell us?
The habitable zone describes distances where a planet could maintain liquid water on its surface under suitable atmospheric conditions. Its boundaries depend on the star and on assumptions about the planet’s climate.
Being in this zone does not establish that a world has water, an atmosphere, or life. Greenhouse warming, atmospheric loss, stellar activity, and planetary history all matter. We will use the concept as a guide to investigation, then look beyond orbital distance to the conditions that make a surface environment hospitable.[12]
Explore article 09→Future research in planetary science
Which measurements will help us tell formation histories apart?
Progress means learning more than a planet’s location. Disk observations reveal building material; measurements of mass and radius constrain planetary interiors; atmospheric spectra reveal how selected gases interact with light. Webb has already demonstrated the richness of this atmospheric evidence.[13]
ESA’s planned Plato mission is designed to connect planet discoveries with precise measurements of their host stars, including stellar ages. Together with ground-based observations, such information can help test how planetary populations change over time. This final article considers the measurements, models, and laboratory work needed to turn discoveries into explanations.[14]
Explore article 10→What happened between the dust and the world we see?
A planet’s mass, atmosphere, and orbit are the results of a history. Growth supplies material; impacts and migration rearrange it; cooling and loss change what remains.
As you read, follow three things: where the material came from, when it was assembled, and what happened afterward. Those questions connect a grain in a disk with a rocky landscape or a giant planet’s cloud tops.
How can we study a process that takes so long?
We combine snapshots of young disks with observations of mature planetary systems. Meteorites and returned samples add evidence we can examine in laboratories. Models then test whether the proposed steps can produce the properties we measure.[1], [9], [11]
Often, more than one history fits an observation. Better measurements help distinguish them. The aim is to understand which processes build different kinds of worlds—and how our own fits into that larger picture.
Sources and further reading
Research reviews and NASA and ESA explainers supporting this chapter overview. The individual articles explore the evidence and open questions in more depth.
- Ercolano & Pascucci (2017) — The dispersal of planet-forming discs: theory confronts observationsDisk lifetimes, accretion, winds, and the dispersal of planet-forming gas.
- Armitage (2024) — Planet formation theory: an overviewA theoretical overview connecting disk structure, planetary growth, and orbital evolution.
- Birnstiel, Fang & Johansen (2016) — Dust Evolution and the Formation of PlanetesimalsInherited dust, collisional barriers, particle concentration, and planetesimal formation.
- Raymond & Morbidelli (2022) — Planet Formation: Key Mechanisms and Global ModelsAccretion, giant impacts, material transport, and the assembly of planetary systems; preprint posted in 2020.
- Ikoma & Kobayashi (2025) — Formation of Giant PlanetsCore growth, envelope cooling, gas supply, and competing giant-planet formation models.
- Nelson (2018) — Planetary Migration in Protoplanetary DisksGravitational interactions between planets and disks and the resulting orbital migration.
- Blanc et al. (2025) — Understanding the formation of Saturn’s regular moons in the context of giant planet moons formation scenariosCircumplanetary disks, impacts, capture, and the coupled evolution of moons and rings.
- NASA — Asteroids, Comets, and MeteorsSmall bodies as evidence of the Solar System’s early material and evolution.
- NASA (2025) — NASA’s Bennu Samples Reveal Complex Origins, Dramatic TransformationReturned samples reveal inherited ingredients and extensive alteration by water in Bennu’s parent body.
- NASA — What Is a Super-Earth?Why a size or mass category does not uniquely specify a planet’s composition or habitability.
- Zhu & Dong (2021) — Exoplanet Statistics and Theoretical ImplicationsObserved planetary populations, discovery biases, and implications for formation and evolution.
- NASA — The Habitable ZoneSurface liquid water, stellar illumination, atmospheric conditions, and the limits of the concept.
- NASA (2022) — NASA’s Webb Reveals an Exoplanet Atmosphere as Never Seen BeforeAtmospheric observations of WASP-39 b demonstrate how spectra probe chemistry and clouds.
- ESA — Plato factsheetThe mission’s planned planet discoveries and measurements of host-star properties and ages.