How It Works

Why do planets have moons?

Planets have moons because early in their formation they were introduced to other space- faring rocks that either crashed into the planet and threw off debris, or were trapped in the gravitational pull of the planet. Over billions of years the orbits of these rocks (or debris), now under the influence of the planet’s gravity, were squashed into a spherical shape that kept them encircling their host planet.

To visualise how a moon becomes ensnared, imagine the earth is a ball placed on a floating sheet of frictionless paper (to represent gravity). The ball depresses the paper and if you roll a coin (to represent a moon) around this depressed area at a fast enough speed, it will circle the ball indefinitely.

So, why don’t moons have their own moons? The answer is that the planet they are orbiting has a much stronger gravitational pull, so over time it would selfishly take other objects in as another moon for itself.

  • ahsaine youssef

    i love everything about planets , so i can discover how it moves around the sun

  • So what about our moon, why doesn’t the sun selfishly pull it into it’s orbit?

    • Ah, John, the Moon is somewhat in orbit around the Sun. It’s just that the Earth exerts a greater influence upon it, so it stays in a circular orbit around our planet. Let’s not forget that by orbiting Earth, which orbits the Sun, the Moon must have some of the same rotational motion around the Sun.