Alex McColgan

So what's the difference?

Well, simply put, there's another force of gravity in the solar system called the Sun.

Moons in orbit around planets are constantly being tugged and pulled at by the Sun.

However, they are within something called the planet's Hill Sphere.

Within the Hill Sphere of a planet, the planet's gravity has more of an influence on the object than the Sun's gravity.

And this influence will be different depending on how close you are to the Sun, because the closer you are, the stronger you are under the influence of the Sun's gravity.

This means that the Hill Sphere around Earth isn't very big, only 1.5 million kilometres.

If Earth was closer to the Sun, its Hill Sphere would be smaller.

If it was further away, it would be bigger.

And now you can see why these gas giants have so many captured moons, their hill spheres are simply much bigger all the way out there.

In fact Neptune, while not being anywhere near as massive as Jupiter, has a much bigger hill sphere, meaning it can capture stray asteroids which are tens of millions of km away.

In order for Earth to permanently capture an object, it would have to happen in just the right way, likely needing some helpful gravitational tugs from our Moon along the way to tame its orbit.

And as far as we know, that hasn't happened yet, meaning anything that does come too close only sticks around for a few orbits before it leaves again.

So, enjoy 2020 CD3 while you still can.

Can a moon have a moon?

Well, yes, in theory it can, and yet we don't see any examples of this in our solar system.

So why is that?

The first important thing to know is that all celestial objects have gravitational pull.

That includes tiny little asteroids, all the way to giant stars or even black holes.

The mass of an object dictates how strong its gravitational pull is.