Moons

Moons of the Solar System

We’re all familiar with the Moon. It’s the second brightest astronomical object we can see from Earth – by far the brightest thing in the night sky. It’s one of the most familiar astronomical objects, second only to the Sun itself.

But familiarity doesn’t always bring understanding. That silver disk that brightens the night gets called the Moon, but really it’s a moon. It might be the only one that Earth has but there are more out there, orbiting the other planets and minor planets.

In fact there are many more – hundreds of them – and some of them are among the most interesting places in the whole solar system.

planets and satellite moons

The solar system is full of objects, and almost all of them orbit the Sun. The solar system formed from a giant, spinning disk of matter, and as gas, rock and ice collected into the objects we now see, most of it kept spinning in the same direction around the disk’s central point – where the Sun now burns.

There were exceptions though. Sometimes one of these objects ended up in a spot where a nearby planet’s gravity was stronger than the Sun’s, and when that happened it would fall into a new orbit around the planet.

What is a moon?

A natural satellite is a celestial body that orbits another one of greater mass. Earth is a natural satellite of the sun. Moons are a sub-class of natural satellites – the ones whose orbit isn’t around the Sun.

A moon can be almost any size. Jupiter has 67 of them and they range in size from S/2003 J 12, which is less than a kilometre in diameter, to Ganymede, the ninth largest object in the solar system – larger than Mercury.

The definition of a moon is quite loose. In most cases it’s quite simple – if it orbits a body that isn’t the sun it’s a moon. Very small ones, especially if they’re too small for their own gravity to have pulled them into a sphere, are sometimes called moonlets.

Where it can be difficult is when objects are closer in size, but astronomers have a rule of thumb for this. The centre of an orbit isn’t actually the centre of the larger object; both large and small are orbiting around a point called the barycentre.

barycenter earth moon

The barycentre of Earth’s orbit around the Sun is 449km from the Sun’s centre, well inside the star, so anyone watching from outside would see Earth orbiting and the Sun wobbling slightly.

Because the Moon is much closer to Earth’s size than Earth is to the sun the barycentre of its orbit is further out, roughly 4,670km from the centre of Earth – but still about 1,700km below the planet’s surface.

If the barycentre is inside the large object astronomers count it as a small object orbiting a large one – but if it lies outside the large object it’s classed as a double body. There’s some flexibility though. Pluto and its largest moon, Charon, orbit a barycentre that’s about 600km above Pluto’s surface.

Where can we see moons?

Moons are scattered throughout the solar system. Of the eight planets, six have moons. The exceptions are Mercury and Venus, which are so close to the sun that it would be very difficult for their gravity to capture a satellite.

Our Moon is the most central one in the system. Moving out, Mars has two. At least 118 of the larger objects in and around in the asteroid belt have moons.

Jupiter has 67 known moons, and Saturn at least 62. Uranus and Neptune are far enough out that small moons are hard to detect, but they both have multiple known ones. Even tiny Pluto has at least five.

So far we know of 173 moons orbiting planets and almost 300 around other objects. Unfortunately most of them are too small and distant to be seen easily, but good binoculars or an inexpensive amateur telescope can pick up some. The easiest to see are the four Galilean moons of Jupiter.

moons of Jupiter

Types of moon

There are many different types of moon. Anything that’s captured by gravity and pulled into an orbit can become one, so there are few real limits on what they can be like. The smallest are just irregular lumps of rock.

Larger moons can be similar to small planets, pulled into a round shape by their own gravity and with layered structures inside. Our Moon is rock, very similar to Earth’s crust (which it used to be part of) with a small, partly molten core.

Jupiter’s moon Io has more active volcanoes than Earth, while its neighbour Europa has a rocky core with a 100km deep ocean over it, under a shell of ice up to 30km thick. Europa is one of the places most likely to have extra-terrestrial life.

Moons haven’t had as much attention as planets, which is a pity. They’re fascinating, and astronomers are now paying them a lot more attention. We can expect a lot more interesting moon facts to be discovered in the next few years.

Click on the links below to learn more about the various moons and satellites in our amazing solar system.