Planets and moons acquire their orbital velocity primarily from the process of their formation within a rotating protoplanetary disk. Here's a step-by-step explanation of how it happens:
Protoplanetary Disk: Planets and moons are formed from a rotating disk of gas and dust known as a protoplanetary disk, which surrounds a young star. This disk is a remnant of the star's formation process.
Conservation of Angular Momentum: The protoplanetary disk possesses a certain amount of angular momentum, which is a property related to rotation. According to the conservation of angular momentum, as the protoplanetary disk contracts due to gravity, its rotation rate increases. This principle can be observed in everyday life when an ice skater spins faster as they pull their arms inward.
Accretion: Within the protoplanetary disk, small dust particles and gas gradually come together through collisions and stick together to form larger objects, known as planetesimals. These planetesimals further collide and merge, growing in size through a process called accretion.
Orbital Motion: As planetesimals and protoplanets continue to accrete, their combined mass and gravitational attraction become significant. The gravitational force pulls these objects inward, and as they fall toward the star, they gain orbital motion. This motion is tangential to their trajectory, perpendicular to the gravitational force.
Equilibrium: Over time, as more material accumulates and collisions occur, an equilibrium is reached between the inward pull of gravity and the tangential motion of the objects. This results in stable orbits around the star.
Conservation of Energy: The orbital velocity of planets and moons is determined by the balance between the gravitational force pulling them inward and the centrifugal force generated by their orbital motion. These forces allow the objects to remain in a stable orbit without spiraling into the star or drifting away.
In summary, planets and moons acquire their orbital velocity from the rotational motion of the protoplanetary disk during their formation. As they accrete mass, their gravitational attraction and centrifugal forces balance out, resulting in stable orbits around the central star.