Apart from Mercury and Venus, all our major (and some minor) planets have moons, sometimes multiple and even ring systems! Given their abundance, it seems like moons are a common occurrence, so how much do we know about them?

Moons Come in a Variety of Types

The term “Moon” isn’t really proper and usually these bodies are called natural Satellites. That being said, there are SO MANY types of “moon” that I simply couldn’t go through all of them here, most notably are trojan moons, quasi-satellites, irregular moons, subsatellites and asteroid moons.

We know of over 300 celestial bodies with moons. Just our main planets alone are home to 205 natural satellites! Most moons are 10’000 x less massive than their host planet, with the biggest exception to that general statement being us. Aren’t we such special snowflakes?

Moons themselves can be a range of sizes since our classification system is quite lacking. Large moons, such as Pluto’s Charon, interfere with their planet’s motion quite a fair amount due to gravity. These cases could easily be labelled as a binary system or double planet, however there isn’t a clear divide between binaries and just big moons.

I would have thought that a big moon could become a double planet once the centre of gravity of the system – the barycentre – is outside the big planet. Maybe that’s something I’ll argue for when I become a scientist!

There has also been some interest in calling particularly small moons or asteroid moons moonlets, although there’s no clear definition for them either. The name “minimoon” has been thrown around as well.

Moon Formation can Vary in Method

Moons can be formed from 3 main methods: Capture, impact, or in situ.

Planets can “capture” a celestial body, essentially pulling it into orbit using its gravitational attraction. It’s generally thought that Mars’ moons, Phobos and Deimos, were asteroids that were captured.

Earth’s moon, however, was most likely created by a violent clash between the Planet and an asteroid. This idea is supported by evidence such as the extremely small core within the Moon compared to Earth, and the fact that the Moon is similar in composition to Earth.

The last method, formation in situ, is a bit more complex. Similarly to how a star can have a protoplanetary disk, an early planet can have a circumplanetary disk of which dust and gas can coalesce into a moon. There’s not much evidence of this within our solar system, however astronomers have detected these disks around exoplanets!

Take a look at PDS 70c below. The centre of the image is the star, but to the right is a little planet and a slight haze around it. That’s the circumplanetary disk! How cool is that!?

Some Moons don’t Stay Forever

Temporary moons are what they say they are: temporary moons! These (tiny) satellites get captured by the planet for a while, but eventually crash into it or simply leave orbit. 2020 CD3 was a minimoon which briefly entered Earth orbit around 2016 and left in 2020.

These years are approximates since it was difficult to observe, being only a meter in diameter!

This image below was taken by the Gemini North observatory in Hawai’i, and they have some absolutely stunning images!

That’s all for moons today! I will try to get back into posting more content as I’m still learning to balance my studies with other activities!