Why is it thought that a disk appeared in the solar nebula?

The Nebular Hypothesis offers a compelling explanation for the origin of the solar system. It suggests that about 4.6 billion years ago, our solar system started as a vast cloud of gas and dust called the 'solar nebula'.

Over time, this nebula underwent a process of gravitational collapse, initiating a sequence of events that led to the formation of the Sun and the surrounding planets. The hypothesis outlines that as the nebula condensed, it triggered an increase in the rotation speed, a concept closely related to the conservation of angular momentum, which we will delve into in the subsequent section.

The hypothesis is foundational because it demonstrates how complex systems, like our solar system, can evolve from simple beginnings solely under the influence of physical laws.

Angular momentum is a physical quantity that represents the rotational inertia of a spinning object. When it comes to the formation of the solar system, the conservation of angular momentum plays a pivotal role. This principle asserts that if no external torque or force is acting on a system, its angular momentum will remain constant.

In the case of the solar nebula, as the cloud collapsed, the conservation of angular momentum meant that as the particles came closer together, they sped up. This increase in rotational speed forced the nebula to flatten into a disk, because the fastest way for the nebula to rotate without losing angular momentum was to spread out along the plane perpendicular to its rotational axis. This formation of a disk is what we observe in young stellar objects across the universe and provides an understanding of disk-shaped formations in other astrophysical contexts.

The protoplanetary disk, often referred to as an accretion disk, forms as a natural consequence of the collapsing solar nebula and the conservation of angular momentum discussed earlier.

This disk is primarily composed of gas and dust that orbits the central protostar—in our solar system's case, the proto-Sun. It is within this dynamic, spinning environment that the building blocks of planets, or 'planetesimals', begin to emerge. Over time, the disk material coalesces due to gravitational and electrostatic forces, leading to the next key phase: planetesimal formation. Such a disk not only provides the raw materials needed for planet formation but also represents the early 'nursery' of the nascent solar system.

Planetesimal formation represents a fundamental stage in the development of a robust solar system. These solid bodies, anywhere from a few kilometers to several hundred kilometers in diameter, are formed from the coagulation of dust and small particles within the protoplanetary disk.

Over time, as these planetesimals orbit the young Sun, they accrete more mass through collisions and sticking to one another, a process described by the nebular hypothesis. As they grow, their gravitational influence also increases, leading to the formation of protoplanets and, eventually, the full-fledged planets of our solar system.

The understanding of planetesimal formation is essential in grasping how a diffuse cloud of gas and dust can transform into a system of well-organized, orbiting bodies—a key focus for both astronomy and planetary science researchers globally.