Planetesimal

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Planetesimal is a term used in astronomy and planetary science to describe small celestial bodies that are formed from dust and small particles in a protoplanetary disk around a young star. These objects are the building blocks of planets and range in size from a few meters to several kilometers in diameter. The process of their formation is a critical step in the theory of planet formation and is closely related to the concepts of accretion and the solar nebula hypothesis.

Formation

Planetesimals form in the protoplanetary disk that surrounds a new star. This disk is composed of gas and dust, remnants from the formation of the star. Through a process known as accretion, dust particles in the disk gradually collide and stick together, forming larger and larger bodies. Two main theories explain the formation of planetesimals: the gravitational instability model and the core accretion model.

Gravitational Instability Model

This model suggests that planetesimals form when a region of the protoplanetary disk becomes gravitationally unstable and collapses under its own gravity. This can lead to the rapid formation of planetesimals over a wide range of sizes.

Core Accretion Model

The core accretion model proposes that dust particles in the disk slowly come together through electrostatic forces and form larger solid cores. Over time, these cores grow large enough to attract additional material through their gravitational pull, eventually forming planetesimals.

Characteristics

Planetesimals are characterized by their irregular shapes, lack of atmosphere, and composition, which can vary depending on their location in the protoplanetary disk. Those formed closer to the star tend to be rocky due to the high temperatures that vaporize volatile materials, while those formed further away can contain ices and organic compounds.

Role in Planet Formation

Planetesimals are crucial in the process of planet formation. As they continue to collide and merge, they can form larger bodies known as protoplanets. This process can eventually lead to the formation of terrestrial planets or the cores of gas giants, depending on the initial mass and composition of the planetesimal and its environment in the protoplanetary disk.

Research and Exploration

The study of planetesimals and their role in the solar system's formation is ongoing. Observations from telescopes and space missions to asteroids and comets (considered to be remnants of the early solar system's planetesimals) provide valuable insights into their composition, structure, and the early conditions of the solar system.