planetogenesi

Planetogenesis, or planet formation, is the process by which planets emerge from the gas and dust of a protoplanetary disk surrounding a young star. The basic framework posits that a rotating disk forms as a molecular cloud collapses, with solid particles growing from micron-sized grains into kilometer-sized planetesimals and, through a series of mergers and accretions, into protoplanets and eventually mature planets.

Two leading mechanisms are debated: core accretion and disk instability. In the core accretion model, dust grains coagulate into larger bodies, forming planetesimals that collide to build planetary cores. If a core reaches a critical mass, it can rapidly accrete gas, producing gas giants, or remain a terrestrial planet. This process is slow in the outer disk, where material is sparse, and is aided by mechanisms such as pebble accretion. The disk instability model proposes that parts of the gas disk become gravitationally unstable and collapse directly into self-gravitating clumps that contract into giant planets, bypassing the slow core-building stage.

Planet growth proceeds through stages: coagulation of small grains, formation of planetesimals, runaway and oligarchic growth of protoplanets, and, for sufficiently massive cores, envelope accretion. Migration within the disk, planet–disk interactions, and gravitational perturbations can alter orbits, explaining close-in giants and resonant configurations observed around other stars.

Observational evidence includes protoplanetary disks with rings and gaps, direct imaging of young planets, and the rich diversity of exoplanet systems. Isotopic dating of meteorites and planetary samples in the Solar System provides constraints on formation timescales, with Earth and the inner planets forming within the first tens of millions of years.