Mighty Jupiter, largest of all the planets, dominates the night sky. Although Venus is brighter, Jupiter is the most prominent of the five classical planets. Jupiter orbits further out than Earth, so is free to wander as far from the Sun along the ecliptic as he pleases. Unlike Venus, Jupiter can shine down from directly overhead and be visible all night when at the right point in his orbit. Dominating the night sky with his icy white brilliance, it’s no wonder so many ancient cultures associated him with the king of their gods.
The Roman god Jupiter was the seventh child of Saturn, leader of the titans. In revenge for his father’s habit of eating his children, Jupiter led the other gods in revolt against the titans. In the process, he rescued his siblings (including Pluto, Neptune and Juno, whom he later married) from his father’s stomach. Having won, Jupiter assumed control of the sky and became king of the gods. As god of the sky, he controlled lightning, thunder and rain, and his messenger was the eagle. Jupiter had the power to transform himself into any animal, he could grant immortality to mortals, end their suffering by raising them into the stars, or condemn them.
He was a jolly old fellow (the word ‘Jovial’ stems from an alternative form of his name), who lived life to excess. When not ruling and judging from his throne, he indulged in countless affairs with both goddesses and mortal women. He fathered over a hundred children, including Mars, Apollo and Mercury.
His grandsons, through Mars, were Romulus and Remus, legendary founders of Rome, so he was honoured as the patron god of Rome. Oaths were sworn in his name, he was commonly invoked in matters of law and government, and his temple was the official place of state business and sacrifices.
The planet Jupiter has a fair bit in common with its mythological namesake. It is the largest of the planets, and is so massive that it affects the orbits of most other Solar System bodies. Most of the comets we know were scooped up into their current orbits by Jupiter as they fell in from the distant outer reaches of the Oort Cloud. The asteroid belt is similarly shaped and shepherded by Jupiter. Jupiter has more moons than any other planet, with 64 at the last count. The four biggest (Io, Europa, Ganymede and Callisto) are so large and bright that they are easily visible through any decent pair of binoculars. If the conditions are just right, they can sometimes even be seen with the naked eye.
Jupiter spins around its axis extremely rapidly – although its diameter (143000km) is almost twelve times that of earth, its day is under 10 hours long. This is such a rapid rotation that instead of being round, like most planets, it is distinctly oval. Jupiter is classified as a gas giant, and is composed of a small dense core surrounded by a vast ocean of an atmosphere, composed mostly of hydrogen and helium. Jupiter’s enormous gravity causes such high atmospheric pressure that it becomes liquid about a quarter of the way in from the “surface” (although strictly speaking, gas giants have no true solid surface).
The visible cloud tops, which give the impression of surface, are wild and turbulent, with powerful winds dragging the clouds into distinct stripes, or bands. In the 20th century, robotic probes like Voyager I and II returned close-up photographs which showed countless cyclones, storms and hurricanes, many of which are larger than the Earth. One of these, a massive super-anticyclone, is so large that it was even visible to Galileo when he made his first observations four hundred years ago, and is still blowing: The Great Red Spot.
Today we know a great deal about the planet: it’s enormous mass, it’s immense magnetic field, it’s vast radiation belts (similar to Earth’s van Allen belts, but so much bigger), the non-stop lightning in its atmosphere and more. Jupiter is now even known to have a ghostly ring system, similar to Saturn’s, but less than 1% as bright.
Jupiter most recently entered the public eye in 1994 when a passing comet (named after it’s co-discoverers, Shoemaker-Levy 9) passed nearby. The comet’s orbit was disturbed by Jupiter’s enormous gravity, and swung by so close that it was ripped apart by tidal forces, leaving it as a string of cometary fragments. In the process, it lost enough energy that it collided with Jupiter on a later pass, leaving a row of enormous dark blotches in the upper cloud layer which persisted for almost a year. Astronomers were overjoyed, since most had expected the impacts to leave little (if any) visible marks at all, and the constant churning cloud action should have erased all evidence within days or hours. The fact that we got such a spectacular show is evidence of how much more there is to learn about this majestic planet.