For most of human history, the existence of planets outside our solar system was a matter of speculation. Today, we know of thousands of exoplanets, and increasingly powerful observing instruments continue to expand the list of distant worlds. Behind every planetary system—whether it be a few planets orbiting a Sun-like star or a cluster of mini-Neptunes around a red dwarf—lies a fundamental process of disk formation and planetesimal accretion.
This topic—Formation of planetary systems—examines how protoplanetary disks evolve into formed planetary structures. From the condensation of initial dust particles and ice grains to the growth of massive gas envelopes for Jupiter-like giants, we will review the key stages leading to the formation of rocky planets, gas giants, and a variety of exoplanetary configurations. Below is a brief overview of the key concepts discussed:
Protoplanetary disks
Young stars form from collapsing molecular clouds and are often surrounded by disks of gas and dust—these circumstellar disks are where planet formation begins.
Planetesimal accretion
Small solid particles collide and stick together, eventually becoming larger planetesimals. As they grow and transform into protoplanets, the future planetary structure of the system takes shape.
Formation of rocky worlds
The inner, hotter regions are dominated by rocky materials, which is where Earth-like planets form. Their accumulation, differentiation, and preservation of atmospheres determine whether Earth- or Venus-like worlds will form.
Gas and ice giants
Further away from the star, beyond the ice line, ice is abundant, allowing solid cores to grow rapidly and fuse together to form huge layers of hydrogen and helium, forming planets like Jupiter or Neptune.
Orbital dynamics and migration
Newly formed planets interact gravitationally with the disk and with each other, often migrating inward or outward. Phenomena such as "hot Jupiters" demonstrate how unexpected orbits can be in these early transformations.
Satellites and rings
Planetary moons may form with the planet in small circumplanetary disks, or they may be captured if a separate body is caught in the planet's gravitational pull. Rings may form from broken moons or from remnant debris disks.
Asteroids, comets and dwarf planets
Not all of the material accreted into large planets. The asteroid belt and Kuiper belt objects represent the remnants of planetesimals, or "missing" protoplanets, preserving the conditions of the early Solar System.
Diversity of exoplanets
Observations of distant worlds have revealed a stunning diversity—super-Earths, hot Jupiters, mini-Neptunes, lava worlds, and more—a result determined by the properties of the protoplanetary disks, the stellar environment, and the migration history.
The concept of the vital zone
Predicting where liquid water might exist on a planet's surface in orbit is important in the search for potentially habitable worlds. However, factors such as the activity of the star and the composition of the planet's atmosphere must be considered to determine true habitability.
Future research in planetary science
New space missions, giant telescopes, improved theoretical models, and detailed surveys of exoplanets will further refine our understanding of planetary formation, distribution, and potential habitability.
All of these thematic sections show how stellar disks, accreted from interstellar dust and gas, become complex families of planets, moons, and smaller bodies.By understanding this chain of processes—from protoplanetary disks to giant planet formation and orbital rearrangements—we can better understand not only the origins of our own solar system, but also the myriad exoplanetary systems that dot the cosmos.