Jupiter

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When the ancient astronomers named the planet Jupiter after the mighty king of the gods, they were spot on. Though they couldn't have known it, before telescopes were invented, Jupiter is by far the biggest planet in the whole Solar System. It has more than twice as much material as all the other planets put together. And the amazing thing is that most of it is hydrogen and helium, the lightest of all the elements. Actually, these are the most common elements in the whole Universe, and stars are mostly made of hydrogen and helium, but even Jupiter is not big enough to have the nuclear reactions in its centre that would turn it into a star.

Even so, Jupiter has an influence on much of the Solar System. Comets in particular are easily attracted by its gravitational pull, and in 1994 a large comet was seen to plunge into the planet, leaving black marks that took weeks to disappear. Others have been seen since then, giving Jupiter the reputation of being the 'vacuum cleaner of the Solar System'!

Jupiter also had an effect on our understanding of the Universe. The story goes back to 1610, when the Italian scientist Galileo Galilei used one of the first telescopes to look at Jupiter. What he saw amazed him, and you can see the same thing using ordinary binoculars if you can find Jupiter in the sky. On either side of the planet are up to four little stars, which are actually the four largest moons of Jupiter. They orbit the planet, as moons do, so each night you look they are in different positions. In fact, in some cases even a couple of hours is enough to show a difference in position.

Galileo saw this, and realised that here was a Solar System in miniature, with objects going around a central body. The only problem was that at that time the general belief was that the Earth was the centre of the Universe, and everything had to rotate around Earth. A few people, such as Copernicus in Poland, had dared to suggest that maybe the Sun was at the centre and not the Earth, but they were regarded as dangerous revolutionaries by the Church authorities, whose verdict on anything to do with the Universe was final.

But here, in front of Galileo's eyes, was evidence that the Church was wrong. When he said so, he was branded as a heretic by the authorities and had to spend his life under house arrest. At least he wasn't burned at the stake, like poor old Giordano Bruno, a few years previously.

Fortunately, things have moved on and you can now view Jupiter without any fear of being arrested.

Photo: Black scars on Jupiter photographed by the Hubble Space Telescope  in 1994 after a comet strike.
Credit: STSci