Jupiter – king of the planets

Jupiter is the big daddy of the Solar System – the largest of the planets with a diameter at the equator of 142,984 km (88,846 miles), and the first of the so-called gas giants.

A true-colour image of Jupiter from the Cassini spacecraft, on its way to Saturn, shows Europa casting a shadow on the planet’s cloud tops. Credit: NASA

Oddly Jupiter’s diameter from pole to pole is a lot less – 133,708 km – so that it rather resembles a pumpkin. This is due to Jupiter’s rapid rotation. It has a day just 9.8 Earth hours long, and so bulges in a shape known as an oblate spheroid.

The planet, which is 318 times the mass of the Earth, is mainly made up of hydrogen and helium. It gives off twice as much heat as it receives from the Sun, and it might have become a star itself if it had been 80 times more massive.

Jupiter is notable for its belts or bands of various hues. They show a lot more detail than outer neighbour Saturn, evidence of violent weather patterns and produced by swirling methane, ammonia and water.

The bright bands are called zones and are where Jupiter’s weather systems move from west to east. Darker bands, where the prevailing winds blow from east to west, are called belts.

Jupiter’s most famous feature is its Great Red Spot, a raging anti-cyclone bigger than the Earth that was first recorded by astronomers in 1831. Other smaller storms blow up regularly in the belts and zones, driven by jets of hot ammonia.

The belts and bands of Jupiter. Image credit: Skymania

Temperatures of a chilly -110° have been recorded in the cloud tops, but they soar to around 5,000° deep within the atmosphere, under the enormous pressure which makes the gases more like a liquid. Jupiter’s core is thought to be solid and rocky, and about 20,000 km (12,500 miles) wide.

Jupiter has its own powerful magnetic field. It also emits radio waves. Jupiter has its own powerful magnetic field. It also emits radio waves.

A surprising discovery by NASA’s Voyager 1 probe, when it flew past Jupiter in February 1979, was that the planet is surrounded by rings of fine dust. These obviously cannot compare to the brilliant ring system surrounding Saturn.

Several space probes have studied Jupiter, either in flybys or from orbit. NASA’s Galileo probe was a huge success, circling Jupiter and studying the planet and its moons from 1995 to 2003.

It was followed by another orbiter, Juno, which arrived in 2016 to fly in polar orbits and observe Jupiter up close. Its instruments had to be shielded against the planet’s powerful radiation.

The moons of Jupiter

Jupiter has countless moons orbiting it, including many captured asteroids. The official tally had reached 79 by mid-2018. Four of these natural satellites are significantly bigger than the rest. They are known as Galilean moons after the Italian astronomer Galileo who sketched them in the early 17th century with one of the first telescopes.

Io, the innermost of the Galilean moons, is 3,630 km (2,256 miles) wide, making it slightly bigger than our own Moon. It orbits Jupiter every 1.77 days. It is very active volcanically, and the regular eruptions give the surface a pizza-like appearance.

Europa, the smallest of the Galilean moons, with a diameter of 3,140 km (1,951 miles), is a smooth, icy body, with no mountains and few craters. It orbits Jupiter every 3.55 days. NASA’s Galileo probe discovered that beneath its crust lies a salty sea more than 100 km (60 miles) deep. The water is kept liquid by Jupiter’s powerful pull, raising the possibility that Europa might be home to simple life.

Ganymede is the third Galilean moon from Jupiter, and the largest in the Solar System, being 5,262 km (3,270 miles) across. It is even bigger than Mercury. Ganymede’s surface is a mix of light and dark regions, separated by ridges and grooves. The icy surface is peppered with impact craters, and another ocean of water is believed to lie beneath the crust. Ganymede orbits once every 7.16 days.

Callisto is the outermost of the big four moons. It is the third largest moon in the Solar system, after Ganymede and Saturn’s Titan. Its ancient surface is dark and heavily cratered. Callisto measures 4,800 km (2,983 miles) in diameter and takes 16.69 days to make one orbit of Jupiter.

How to observe Jupiter

Jupiter is a fine sight through amateur telescopes. You can see fascinating detail in its gaseous surface, including the light and dark zones and belts, and that Great Red Spot that has been raging for so many decades.

Even with the smallest telescope, you will see Jupiter’s four Galilean moons. Io, Europa, Ganymede and Callisto perform a fascinating dance as they orbit their parent planet like a mini solar system.

Even if you only have binoculars and can hold them steady on, say, a tripod, you can see these moons stretched in a line around Jupiter like a string of pearls. It can be fascinating to watch their changing pattern over the hours as they orbit their home world. You can read more about how to observe and photograph the moons here.

Many amateur astronomers sketch the detail they see on Jupiter – they have to be quick because of its rapid rotation. Increasingly, amateurs are using adapted webcams to produce images of Jupiter that can show a surprising amount of detail.