Conservation of Angular Momentum in Planetary Formation

In any spinning system there are two forces present: centrifugal acceleration and centripetal acceleration. In the case of our earth, gravity provides the centripetal component of acceleration which pulls us towards the center of mass. Centrifugal acceleration, on the other hand, wants to pull us off from the surface of the earth. The force we feel on the earth's surface is a combination of centrifugal acceleration and gravity. Lucky for us, at our current rate of rotation and distance from the center of rotation, gravity wins out and our feet remain firmly planted.

During planetary formation, or any accretion disk for that matter — such as solar system formation, conservation of angular momentum dictates that as the mass pulls inward from gravitational attraction, it's angular velocity must increase proportionately in order to conserve its angular momentum. The increase in angular velocity also increases the centrifugal acceleration felt by the mass. At some point these two opposing forces (gravity and centrifugal acceleration) neutralize each other. At this neutral point you find an accretion of matter from which the planetary shell is formed. A similar phenomenon is noted in our solar system's formation as most of the solar system's planetary mass is located in and around the orbits of Jupiter and Saturn. This optimal distance for matter accretion was the equivalent of our solar system's gravitational and centrifugal neutral zone.


In order for centrifugal acceleration and gravity to cancel each other out at the earth's current surface, earth's rotational period would have to be roughly 84 minutes. At this rate of rotation we would feel weightless. Any faster and we would fly upwards out into space. Scientists have hypothesized that the earth rotated much more rapidly during its formation. A higher angular velocity during Earth's accretion stage indicates that the centrifugal acceleration was much more pronounced and goes a long way to explain the final radius of the earth's shell during planetary formation.