How Does a Planet Stay in Orbit Without Losing Inertia?

A planet in motion remains in a stable orbit around the sun mainly because of two things: the nature of the gravitational force and because virtually no other forces act on the planet. The momentum of a planet changes during its orbit but not, in general, its inertia -- which relates solely to its mass. These facts also hold true for planets orbiting other stars, moons orbiting planets, stars orbiting each other and any other objects subject subject to gravity by virtue of having mass.

Inertia and Momentum

Inertia is often confused with momentum. In both common language and physics, the word “inertia” refers to resistance to change. In physics, however, inertia comes from mass of an object. Momentum, on the other hand, is a property of a moving object that involves both its mass and velocity. The inertia of a planet only changes when its mass changes, and this will change the orbit only if the change in mass is accompanied by a force on the planet, such as in an asteroid impact.

Inertia and Energy

Inertia can also be confused with energy. The total energy of an orbiting system combines the kinetic energy in the motion of the objects and the gravitational potential energy between them. As one object orbits another, the total energy of the system does not change. Energy will only be lost or gained by some external influence.

Central Forces and Elliptical Orbits

There are two conditions for a stable orbit. The force pulling two objects together must point directly on a line between them, and force must have a particular relationship to the distance between them. The force of gravity, which is proportional to the product of the masses of the two objects and inversely proportional to the square of the distance between them, is one such force. As long as these two conditions are met, one object can travel in a stable elliptical orbit around the other, with the other object being at one of the foci of the ellipse.

Periods, Mass and Distance

Gravitational orbits have another special property. Any object that orbits another with a given average distance will have the same orbital period (for instance, year) regardless of its mass (inertia). So if you suddenly replace the Earth with a bowling ball traveling at the same speed and direction, it would continue in the same orbit as the Earth, even though its momentum would be smaller. Because an object speeds up and slows down during an elliptical orbit, its velocity, and therefore its momentum, is constantly changing. Its total energy, however, does not -- at least as long as nothing drastic happens.

Empty Space and External Forces

Space is largely empty, but some forces do perturb the orbits of planets. These forces include the solar wind, gravitational forces from other planets and collisions with asteroids. Fortunately, because the gravitational force between the Earth and sun is large compared to these other forces, the effects are very small. Still, the gravitational forces among planets do cause their orbits to wobble ever so slightly. In fact, the planet Neptune was discovered by making calculations from wobbles in the orbit of the planet Uranus.

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