Jupiter, high up in the sky during National Astronomy Week, is the largest planet in the Solar System. It’s so large that it dominates the Solar System’s planets, as it contains over twice as much material as the rest of the planets combined. We don’t particularly feel its effects on Earth, but it has captured many asteroids into its orbit and regularly bends the tracks of comets.
Jupiter is what’s known as a gas giant. It’s not made of rocky material like Earth, but like the Sun is made of hydrogen and helium. If you send a space probe into it, it will just keep going until the increasing gas pressure crushes it, as there’s no solid surface. What we see as a surface is simply the top of a swirling mass of gas, which has divided itself into bands and zones. It’s actually about a tenth of the diameter of the Sun. If it were very much larger, it would have started to glow or even shine and become a second sun. Most of the stars we see are actually double stars, but presumably our Solar System didn’t have enough material to make two suns at the time of its formation, so we got the Sun and a much smaller Jupiter.
If you can take a look at Jupiter through binoculars you’ll see something more than just a dot of light – you’ll see up to four of its moons as well. These moons, first seen by Galileo in 1610, changed our understanding of the Universe for ever. If you look at the night after night you’ll soon see that their positions change because they are in orbit around Jupiter itself. When Galileo saw this, he realised that the doctrine that the Earth was the centre of the Universe was wrong, as here was another body which was the centre of its own little family. That got Galileo into a lot of trouble, but in the end it people accepted that the Sun was at the centre of the Solar System and, eventually, that it was just one of many other stars in the Milky Way Galaxy and that our galaxy is just one of millions of others.
If you look at Jupiter at 7 pm over the next three nights you’ll see its moons like this:
Io is the closest in, Europa the next and then Ganymede. Callisto is the most distant from Jupiter, so moves the slowest. Where is Europa on 9 February? It’s behind Jupiter. By the time it emerges, Io has gone behind Jupiter, so throughout the evening you’ll see only three moons.
Check out this page with more information about Jupiter.
Four fun facts about Jupiter
The centre of Jupiter is so dense that the hydrogen there behaves like a metal.
A red spot on Jupiter appeared suddenly in the 19th century and has been there ever since. But in recent years it has been shrinking, and no-one knows what will happen next.
Its moon Io is the most volcanic world in the Solar System. Its internal heat arises from being so close to Jupiter, whose enormous gravitational field creates tides in the interior around as Io orbits and heats it up by friction.
Europa has a layer of ice covering what is believed to be an ocean, which may even have some form of life, although there is no proof of this.
Missions to Jupiter
Hannah Bilby, Dynamics Test Engineer at RAL Space, explains how she helps make sure spacecraft survive launch, and their journeys out to distant planets, including the incredible JUICE mission which is currently on its way to Jupiter, due to arrive in July 2031.