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One moon among many (for youth)

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The Moon is fascinating, truly a world unto itself... and it is far from being the only moon! In our solar system alone, there are over a hundred: they are extremely different and interesting worlds! There are without a doubt also moons outside the solar system, orbiting planets that revolve around stars other than the Sun.

Here are the answers to a few questions to help you discover the moons in the solar system and elsewhere in space.

What is the difference between a moon and a planet?

The difference between a moon and a planet is that a planet orbits a star, and a moon orbits a planet. There are also moons orbiting dwarf planets (like Pluto) and even asteroids!

The Moon is Earth's natural satellite. Earth also has artificial satellites, which are objects sent into space by humans, and they help us communicate, navigate more easily, monitor our planet's resources, and more.

There is often some confusion surrounding the different celestial bodies in the solar system. From left to right: our star, the Sun; the planet Mars; the dwarf planet Pluto; and the moon Pan, which orbits Saturn. (Credit: NASA)

Contrary to what you may think, a moon can be bigger than a planet! A moon is not defined by its size: it simply has to be in orbit around a planet or another celestial body that is not a star. In other planetary systems (or solar systems), there may very well be moons as big or even bigger than Earth.

According to astronomers, it is possible, but rare, for a moon to have its own moon. This type of celestial body is called a subsatellite or submoon!

Which planet has the most moons?

The planet Saturn. (Credit: NASA/JPL-Caltech/Space Science Institute)

Saturn has the most moons. For now.

Up to recently, Jupiter had the most moons (79 in total), but that changed in when 20 new moons were discovered around Saturn, which now has 82 moons in total. And… it could change again because our instruments are increasingly powerful and allow us to continue discovering smaller moons we didn't know about before!

Mercury and Venus are the only planets in our solar system with no moon.

Ganymede, Io, Europa and Callisto, Jupiter's four largest moons, were discovered over 400 years ago by Galileo using a refracting telescope! That's why they are called Galilean moons. With this discovery, humanity understood that Earth was not the centre of the universe at all.

Why don't the moons look alike?

The solar system contains moons with extremely different appearances. From left to right: Phobos, Titan, Callisto, Enceladus, Ganymede and Miranda. (Credit: NASA)

Moons come in many shapes and sizes! There are small, potato-shaped moons that are barely a few kilometres long, like Deimos (Mars); and moons that are very large and sphere-shaped like a planet, like Ganymede (Jupiter) and Titan (Saturn), which are bigger than Mercury and Pluto.

Some are covered in craters, like Callisto (Jupiter); others are covered in volcanoes, like Io (Jupiter); geysers, like Enceladus (Saturn); or giant canyons, like Miranda (Uranus). Some are very smooth, like Europa (Jupiter).

Moreover, moons do not all follow the same type of orbit around their planet. Some are very close to their planet, like Phobos (Mars), which takes less than 8 hours to complete a revolution. Others are very far from their planet, like Psamathe: it takes this moon 25 years to complete one revolution around Neptune!

Why so many differences? One of the major reasons is that moons are not all formed the same way: some are formed around their planet, others were captured by the gravitational force, and others still were created as the result of giant collisions!

While Titan is bigger than our Moon, Enceladus, one of Saturn's other known moons, is approximately 10 times smaller than our Moon. The gravity there is much less powerful than on Earth, so much so that you could easily jump 40 metres high (higher than a 10-storey building)!

Is there extraterrestrial life on the moons?

We don't know yet. Some moons could be home to simple and small life forms, kind of like bacteria on Earth, or even slightly more complex life forms living beneath the surface.

Artist's rendition of the surface of Europa, one of the large moons orbiting Jupiter, seen on the horizon with the Sun in the distance. (Credit: NASA/JPL-Caltech)

Scientists are studying how life appeared on Earth and how certain life forms evolve in locations that seem very inhospitable on our planet. By comparing this information with the conditions found on different moons, these are some of the places where we think it would be promising to search for extraterrestrial life.

  • Titan, one of Saturn's moons, is the only moon with a dense atmosphere. This allows for the presence of lakes and rivers on its surface. Only one "small" difference with Earth: these lakes and rivers do not contain water, but instead contain liquid methane and ethane! Even though swimming in a lake of methane, the main component in natural gas, is not recommended for humans, we know that many terrestrial bacteria would be perfectly happy there! 
  • On Europa, one of Jupiter's moons, plumes of liquid can be seen very high in the air, coming from fissures in the ice! We believe it has liquid oceans beneath a thick layer of ice, where there could be life forms similar to the ones found in Earth's subterranean lakes. 
  • Enceladus, another one of Saturn's moons, is much smaller than Titan and Europa, but much like Europa, it probably has an ocean beneath its surface. This is another place where life could exist.

Do planets outside the solar system (exoplanets) also have moons?

Artist's rendition of an exomoon orbiting a giant gaseous exoplanet. (Credit: NASA/JPL-Caltech)

Since the 1990s, we have known that there are planets outside the solar system, called exoplanets. Thousands have been discovered, and we suspect there are exoplanets orbiting almost every star.

In our galaxy, the Milky Way, there are hundreds of billions of stars. Can you imagine how many possible planets that represents? According to astronomers, these planets, much like the planets in our solar system, probably have moons, which are called exomoons. However, exomoons are very difficult to detect. We will have to wait a little longer before the discovery of an exomoon is confirmed.

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