In what ways did the formation of the Galilean satellites mimic the formation of the planets? In what ways were the two formation processes different?

A circumplanetary disk is a dense, orbiting ring of material that forms around a young planet. Imagining the early stages of our solar system, we often picture a vast, turbulent cloud of dust and gas known as a 'solar nebula'. When a planet, such as Jupiter, begins to form within this nebula, it exerts a strong gravitational pull that attracts and collects the surrounding material.

This accumulation of dust, ice, and gas coalesces into a flat, rotating disk encircling the planet, quite similar to how Saturn's rings look today. However, the key difference is that in a circumplanetary disk, the materials are not just static; they collide, combine and gradually form larger bodies within the disk.

For the Galilean satellites of Jupiter, this process was crucial. These moons originated from the dense circumjovian disk, exemplifying planetary system formation on a smaller scale. The coherence of this disk provided the structure necessary for satellite formation, just like the larger protoplanetary disk did for the planets around the Sun.

The concept of planetesimal accretion opens a window into the dynamic and violent early phase of planetary formation. A planetesimal is essentially a solid object arising amid a disk of matter around a star, formed when granular matter in the disk begins to stick together and clump, typically ranging in size from a kilometer to hundreds of kilometers across.

Through a process known as 'accretion', these planetesimals collide and stick together, gradually building up mass. Imagine a snowball effect where dust and rock bind due to electrostatic forces and gravitational interactions, growing into larger and larger celestial bodies.

In the early solar system, myriads of planetesimals swarmed, colliding and merging to eventually form protoplanets, and with enough mass, clearing their orbits and becoming the planets we are familiar with today. This bottom-up formation scenario is fundamental to understanding how the robust, spherical bodies in our solar system came to be.

A protoplanetary disk is the broad, flat region of gas and dust encircling a young star, often considered the birthplace of planets. This is the larger-scale equivalent of the circumplanetary disk but around a star. When a star forms from the collapse of a molecular cloud, its gravity attracts surrounding material, spinning it into a flattened disk.

Within this disk, dust particles stick together, forming larger structures that aggregate into planetesimals, similar to the earlier stages of planetesimal accretion. As these planetesimals continue to collide and merge, they eventually give rise to full-fledged planets. This process is not a serene, steady progression but a chaotic and tumultuous series of events that can lead to the formation of various planet types based on their distance from the star, composition of the disk, and the dynamics involved.

Reflecting back on the exercise, while planets form within the vast protoplanetary disk around a star, the Galilean satellites instead formed within the circumplanetary disk of Jupiter, showing a parallel yet distinct miniaturized version of planetary genesis.