Alex McColgan

It was probably captured as recently as 2017.

Although, before you warmed the idea of Earth having two satellites, it seems that this is only a temporary relationship.

Astronomers are expecting it to escape Earth's gravity again by April 2020.

This is due to its chaotic orbit.

It never really got settled into a stable orbit, and so will be ejected again in just a few months.

A very similar thing happened in 2006 with an asteroid called 2006 RH120, where Earth had a second moon for just short of a year, before again it was ejected.

And just like back then, once this one leaves Earth's orbit, we'll be back to just the one natural satellite.

Or will we?

There is another asteroid orbiting Earth at a distance of about 9 million kilometres, much further out than the Moon, called 2016 HO3.

Except, although it's circling around Earth, it's not really orbiting the Earth at all.

Instead, it's orbiting the Sun, and its orbit is very similar to Earth's, just a little more eccentric.

At some points of this orbit, it is ahead of the Earth, and 6 months later it falls behind the Earth.

This is something known as a quasi-satellite, and Earth has 5 of them that we know of.

Moons which circle the Earth, and are even influenced by its gravity, but without truly being in orbit.

And so unfortunately, they aren't and can't be classified as true moons.

We aren't the only planet to have quasi-satellites, in fact they all might, considering these objects are very difficult to spot.

But why do gas giants have so many irregular moons, and Earth only has the one, right now anyway?

Well, being big and massive is only part of the answer.

Yes, the more massive you are, the bigger your gravity, but Earth's gravity is hardly a slouch.

It's easily comparable to Saturn, Neptune and Uranus.