Planetary accretion is the process by which a planet grows through the gradual accumulation of small bodies, typically rocky or icy, into a larger body. The process is thought to occur over the lifetime of a star system, with planets forming from the dusty disks that surround young stars. The word “accretion” comes from the Latin verb “ad-crescere”, meaning “to grow”.
The most widely accepted model for planetary accretion is known as the core accretion model. In this model, a newborn planet begins its life as a small rocky or icy core surrounded by a disk of gas and dust. Over time, the core grows in size as it draws in more material from the disk. Eventually, it becomes massive enough to begin capturing gas directly from the disk, at which point it rapidly begins to grow into a full-fledged planet.
There are several different mechanisms by which small bodies can be added to a growing planet’s core. One is direct impact: if two small bodies happen to collide head-on while orbiting a young star, they can stick together and form a larger body. Another is gravitational capture: if one body happens to stray too close to another body that is significantly more massive than itself, it can be pulled in by gravity and become gravitationally bound to that body. Finally, there is accretion by sublimation: if an icy body approaches a hot young star too closely, it will begin to vaporize; some of this vapor will then condense onto the nearby surfaces of other bodies (including the young planet’s core), adding mass to those surfaces in turn.
Over time, as more and more small bodies are added to its mass, a growing planet will begin to experience increased levels of gravity and tidal forces. These forces will cause any remaining gas and dust in its vicinity – including that within its own atmosphere – to be drawn inward toward theplanet’s center of mass. This infall of material onto the surface oftheplanet ultimately leadstoits growthintoa full-fledgedgas giantplanet likeJupiteror Saturnin our own Solar System.
The final stage in planetary accretion involves two additional processes: differentiation and volcanism. Differentiation occurs when heavier elements (such as iron) sink down toward the center ofa newly formed planet due toboth their greater densityand also convective motionswithin themolten mantlelayer surroundingthecore . This sinking segregates these heavy elements away from lighter materials (such as silicates), resulting inthe formationof adistinct crust ontheplanet’s surface . Volcanismis thenthought toprovide amechanismforthe escapeofthesetracesofheavy elements backintotheatmosphereor outerspace , wheretheycan onceagain take partin future episodesofplanetaryformation .