Meaning of accretion
Use of the term accretion disk for the protoplanetary disk leads to confusion over the planetary accretion process. The protoplanetary disk is sometimes referred to as an accretion disk, because while the young T Tauri-like protostar is still contracting, gaseous material may still be falling onto it, accreting on its surface from the disk's inner edge.
However, that meaning should not be confused with the process of accretion forming the planets. In this context, accretion refers to the process of cooled, solidified grains of dust and ice orbiting the protostar in the protoplanetary disk, colliding and sticking together and gradually growing, up to and including the high-energy collisions between sizable planetesimals.
In addition, the giant planets probably had accretion disks of their own, in the first meaning of the word. The clouds of captured hydrogen and helium gas contracted, spun up, flattened, and deposited gas onto the surface of each giant protoplanet, while solid bodies within that disk accreted into the giant planet's regular moons.
Read more about this topic: Nebular Hypothesis
Famous quotes containing the words meaning of and/or meaning:
“You will never be happy if you continue to search for what happiness consists of. You will never live if you are looking for the meaning of life.”
—Albert Camus (1913–1960)
“Do you know what Agelisas said, when he was asked why the great city of Lacedomonie was not girded with walls? Because, pointing out the inhabitants and citizens of the city, so expert in military discipline and so strong and well armed: “Here,” he said, “are the walls of the city,” meaning that there is no wall but of bones, and that towns and cities can have no more secure nor stronger wall than the virtue of their citizens and inhabitants.”
—François Rabelais (1494–1553)