The accretion phase refers to the period in planetary formation when dust and gas in a protoplanetary disk come together to form larger bodies, eventually leading to the creation of planets. This phase is crucial because it involves the accumulation of matter that contributes to the building blocks of planetary bodies, impacting their final size, composition, and structure.
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The accretion phase typically occurs over millions of years as small particles collide and stick together, forming larger clumps called planetesimals.
During this phase, heat generated from collisions and radioactive decay contributes to the early thermal evolution of forming planets.
The size and composition of the initial building blocks can significantly influence the final characteristics of the planets formed.
Accretion can be influenced by various forces such as turbulence in the protoplanetary disk and the presence of other forming bodies nearby.
The end of the accretion phase often marks the transition to a more stable stage where planets undergo differentiation as they cool and solidify.
Review Questions
How does the accretion phase contribute to the overall formation of a planet?
The accretion phase is critical in planetary formation as it involves the gradual buildup of material from dust and gas in a protoplanetary disk. During this time, small particles collide and merge to form larger bodies known as planetesimals. This process not only determines the final mass and size of a planet but also influences its composition and internal structure, setting the stage for future differentiation as these bodies continue to accumulate material.
Discuss how gravitational instability can affect the accretion phase in a protoplanetary disk.
Gravitational instability plays a significant role during the accretion phase by causing certain regions within the protoplanetary disk to collapse under their own gravity. This leads to the formation of dense clumps that can grow rapidly through accretion, enhancing the formation rate of planetesimals. As these clumps interact with surrounding material, they can either grow larger or be disrupted, impacting the efficiency and outcome of planetary formation.
Evaluate the implications of varying compositions of planetesimals formed during the accretion phase on later stages of planetary differentiation.
The initial compositions of planetesimals formed during the accretion phase have profound implications for later stages of planetary differentiation. If a planet's building blocks are rich in metals or volatiles, this can lead to significant differences in how layers develop within the planet once it cools. For example, a metal-rich core may form if metallic planetesimals dominate, while a more volatile-rich composition could lead to extensive outgassing and atmosphere development. Understanding these initial conditions helps explain why planets have such diverse structures and compositions today.
Related terms
Protoplanetary Disk: A rotating disk of dense gas and dust surrounding a young star, from which planets and other celestial bodies form.
Planetary Differentiation: The process by which a planet develops distinct layers, often due to variations in density and temperature, leading to a core, mantle, and crust.
Gravitational Instability: A condition in a cloud of gas and dust where gravity causes regions to collapse, leading to the formation of structures like stars and planets.