Planetary moons form through giant-impact accretion (Moon-forming impact), in situ formation in circumplanetary disks (Galilean moons), or capture of small bodies (outer moons). Orbital mechanics (Kepler's laws, orbital resonances, tidal evolution) determines satellite stability, migration, and long-term dynamical evolution.
The solar system contains over 200 known moons, and they did not all form the same way. Building on your understanding of planetary formation and gravitational physics, satellite formation can be grouped into three distinct mechanisms, each leaving characteristic signatures in a moon's orbit, composition, and relationship to its host planet.
The first mechanism is giant-impact accretion, best exemplified by Earth's Moon. In the leading model, a Mars-sized body struck the proto-Earth roughly 4.5 billion years ago, ejecting a disk of molten and vaporized rock into orbit. This debris rapidly coalesced into the Moon. The impact origin explains several otherwise puzzling facts: the Moon's bulk composition is strikingly similar to Earth's mantle (because the debris came mainly from the outer layers of both bodies), the Moon is depleted in volatile elements (blasted away by the energy of the collision), and the Moon orbits close to Earth's equatorial plane. This is a violent, one-off event — not a gentle assembly process.
The second mechanism is co-formation in a circumplanetary disk, which produced the large regular satellites of the giant planets. Just as the Sun formed with a disk of gas and dust that spawned the planets, Jupiter and Saturn each had their own miniature accretion disks. The four Galilean moons of Jupiter — Io, Europa, Ganymede, and Callisto — formed within Jupiter's circumplanetary disk, which is why they orbit in nearly circular, prograde, equatorial orbits with a systematic density gradient (denser closer in, icier farther out, mirroring the temperature gradient in the disk). This formation pathway produces ordered satellite systems that resemble miniature solar systems.
The third mechanism is gravitational capture, which accounts for the many small, irregular moons of the outer planets. These are bodies — typically former asteroids or Kuiper Belt objects — that wandered too close to a giant planet and were captured into orbit, often aided by gas drag in the planet's early envelope or three-body interactions. Captured moons are easily identified by their orbits: they tend to be distant, eccentric, inclined, and frequently retrograde (orbiting opposite to the planet's rotation). Once in orbit, Kepler's laws and tidal forces govern a satellite's long-term evolution. Orbital resonances — where two moons' orbital periods form simple integer ratios — can stabilize or destabilize orbits. The famous Laplace resonance among Io, Europa, and Ganymede (1:2:4 period ratios) continuously pumps Io's orbital eccentricity, driving the intense tidal heating that makes Io the most volcanically active body in the solar system. Tidal evolution also drives orbital migration: Earth's Moon spirals slowly outward as tidal friction transfers angular momentum from Earth's rotation to the Moon's orbit, a measurable process that has been tracked by lunar laser ranging since the Apollo missions.