Glossary · Thiên văn học
Accretion
The process by which a body grows by gravitationally attracting and gathering surrounding material. In the model of Solar System formation, planetesimals large enough to attract their neighbours grew this way into protoplanets and then planets.
In the solar nebula, condensed grains quickly joined into larger and larger chunks, until most of the solid material was in planetesimals a few to a few tens of kilometres across. Going from planetesimal to planet takes a substantial step up in size: some planetesimals were large enough to attract their neighbours gravitationally and grow — this is accretion. The intermediate steps are not well understood, but ultimately several dozen centres of accretion seem to have grown in the inner Solar System, each attracting surrounding planetesimals until it reached a mass similar to that of Mercury or Mars: the protoplanets.
The process was violent. Each incoming planetesimal was accelerated by the protoplanet's gravity and struck with enough energy to melt both itself and part of the impact area. Soon the whole protoplanet was heated above the melting temperature of rock. The result was differentiation: heavier metals sank toward the core and lighter silicates rose toward the surface.