Where orbital motion comes from

This optional article looks at how planets and moons acquire their orbital motion. Continue to Why heavy and light objects fall together for the next main lesson.

The previous lesson showed why sideways motion allows an object to orbit. To understand where that motion came from, we need to look at how planetary systems form.

Passing a planet does not guarantee capture

An object approaching a planet from far away speeds up as it falls in and slows as it travels out again. If we treat both as point masses and nothing else interferes, a visitor arriving on an escape trajectory also leaves on one. The encounter bends its path without capturing it.

Capture needs a way to transfer some of the visitor’s orbital energy elsewhere. Another body can exchange energy with it, gas can slow it down, or tidal stretching can convert orbital energy into heat and deformation.

A rotating cloud becomes a disc

Gas clouds contain a mixture of motions. If a cloud has some overall rotation, that rotation becomes faster as gravity draws it into a smaller region, much as a skater spins faster by pulling their arms in.

This is conservation of angular momentum, a measure of rotational motion. As gas particles collide, some of their motion becomes heat, which can escape as radiation. The cloud can settle into a disc while keeping much of its overall rotation.

Planets forming in that disc inherit its orbital motion. They do not need a separate sideways push once they have formed.

Where the rotation begins

The early universe contained small differences in density. As those differences grew, neighbouring concentrations of matter pulled on one another unevenly. These gravitational influences could exert a turning effect, or torque, on a forming structure.

A cloud can gain rotation through those interactions with its surroundings. When we account for the whole isolated system, its total angular momentum is conserved.

Moons have several histories

Some moons form in discs around young planets. Others are captured or form from impact debris. The leading explanation for our Moon involves material placed into orbit by a collision with the young Earth.