Jupiter is the largest and most massive of all the planets. It has almost 2.5 times the mass of the other planets eight planets combined, and over 1,300 Earths could fit inside it. Jupiter bears the name of of the most important of all the Roman gods (known as Zeus in Greek mythology). The planet has the largest family of moons in the Solar System, its members named after Jupiter’s lovers, descendants, and attendants.
Orbit. Jupiter is the fifth planet from the Sun. It lies approximately five times as far away as Earth, but its distance from the Sun in not constant. Its orbit is elliptical and there is a difference of 76.1 million km between its aphelion and perihelion distances. Jupiter’s spin axis tilts by 3.1°, and this means that neither of the planet’s hemispheres point markedly towards or away from the Sun as its moves round the orbit. Consequently, Jupiter does not have obvious seasons. The planet spins quickly about its axis, more quickly than any other planet. Its rapid spin throws material in its equatorial region outward. The result is a bulging equator and a slightly squashed appearance.
Structure. Although it is the most massive planet (318 times the mass of the Earth), Jupiter’s great size means that its density is low. Its composition is more like the Sun’s than any other planet in the Solar System. Jupiter’s hydrogen and helium is in a gaseous form in the outer part of the planet, where the temperature is about -110℃. Closer to the centre, the pressure, density, and temperature increase. The state of the hydrogen and helium changes accordingly. But about 7,000km deep, at about 2,000℃, hydrogen acts more like a liquid than a gas. By 14,000km, at about 5,000℃, hydrogen has compacted to metallic hydrogen and acts like a molten metal. Deep inside, at a depth of about 60,000km, is solid core of rock, metal, and hydrogen compounds. The core is small compared to Jupiter’s great size but is about 10 times the mass of Earth.
Magnetic field. Jupiter has a magnetic field – it is as if the planet has a large bar magnet deep inside it. The field is generated by electric currents within the thick layer of metallic hydrogen, and the axis joining the magnetic poles is tilted at about 11° to the spin axis. The field is stronger than of any other planet. Is is about 20,000 times stronger than Earth’s magnetic field and has great influence on the volume of space surrounding Jupiter. Solar wind particles streaming from the Sun plough into the field. They are slowed down and rerouted to spiral along the field’s magnetic lines of force. Some particles enter Jupiter’s upper atmosphere around its magnetic poles. They collide with the atmospheric gases, which radiate and produce aurorae. Other charged particles (plasma) are trapped and form a disc-like sheet around Jupiter’s magnetic equator. Electric currents flow within this sheet. High energy particles are trapped and form radiation belts, similar to, but much more intensive than, The Van Allen belts round Earth. Jupiter’s magnetic field is shaped by the solar wind, forming vast region called the magnetosphere. It size varies with changes in pressure of the solar wind, but the tail is thought of about 600 million km.
Atmosphere. Jupiter’s atmosphere is dominated by hydrogen, with helium being the next most common gas. The rest is made up of simple hydrogen compounds – such as methane, ammonia, and water – and more complex ones such as ethane, acetylene, and propane. It is these compounds that condense to form the different coloured clouds of the upper atmosphere and help give Jupiter its distinctive banded appearance. The temperature of the atmosphere increases towards the planet’s interior. As gases condense at different temperatures, different types of clouds form at specific altitudes. All the while, the gas in Jupiter’s equatorial region is heated by the Sun, and this rises and moves towards the polar regions. Cooler air flows from the polar regions at a lower altitude to take its place, creating in effect a large circulation cell. This hemisphere-wide circulation transfer would be straightforward if Jupiter was stationary. It it not – it rotates, and speedily at that, and a force known as the Coriolis effect deflects the north-south flow into an east-west flow. As a result, the large circulation cell is split into many smaller calls of rising and falling air. These are seen on Jupiter’s surface as alternating bands of colour. Jupiter’s white bands of cool rising air are called zones. The red-brown bands of warmer falling air are known as belts.
Weather. Jupiter has no notable seasons, and the planet’s temperature is virtually uniform. Its polar regions have similar temperatures to its equatorial regions of internal heating. Jupiter radiates about 1.7 times more heat than it absorbs from the Sun. The excess is infrared heat left from when planet was formed. Most of Jupiter’s weather occurs in the part of this atmosphere that contains its distinct white and red-brown cloud layers and is dominated by clouds, winds, and storms. The rising warm air and descending cool air within the atmosphere produce wings, which are channelled around the planet, both to the east and west , by Jupiter’s fast spin. The wind speed changes with latitude; wings within the equatorial region are particularly strong and reach speeds in excess of 400kph. The solar and infrared heat, the wing, and Jupiter’s spin combine to produce regions of turbulent motion, including circular and oval cloud structures, which are giant storms. The smallest of these storms are like the largest hurricanes on Earth. They can be relatively short-lived and last for just days at a time, but others endure for years. Jupiter’s most prominent feature, the Great Red Spot, is an enormous high-pressure storm that may have first been sighted from Earth over 340 years ago.
Rings. Jupiter’s rings system was revealed for the first time in an image taken by Voyeur 1 in 1979. It is a thin, faint system made of dust-sized particles knocked off Jupiter’s four inner moons. The system consists of three parts. The main ring is flat and is about 7,000km wide and less than 30km thick. Outside this is the flat gossamer ring, which is 850,000km wide and stretches beyond Amalthea to Thebe’s orbit. On the inside edge of the main ring is the 20,000km thick doughnut-shaped halo. Its tiny dust grains reach down to Jupiter’s cloud tops.
Moons. Jupiter has over 60 known moons, over two-thirds of which have been discovered since January 2000. Only 38 of the moons have been given names, and several have yet to have their orbit confirmed. The recent discoveries are typically irregularly shaped rocky bodies a few kilometres across, and are thought to be captured asteroids. By contrast, Jupiter’s four largest moons are are spherical bodies that were at the same time as Jupiter. Collectively known as the Galilean Moons, they were the first moons to be discovered after Earth’s Moon. As they orbit Jupiter, passing between is and the Sun, their shadows sweep across the planetary surface; from within the shadow, the Sun appears eclipsed. A triple eclipse happens just once or twice a decade.
Average distance from the Sun: 778.3 million km
Cloud-top temperature: -110℃
Number of moons: 63
Observation Jupiter is bright and easy to spot. It has a maximum magnitude of -2.9. Even at its faintest it is brighter than Sirius, the brightest star in the sky. Jupiter is best seen at opposition, which occurs once every 13 months.
Rotation period: 9.93 hours
Orbital period (length of year): 11.86 Earth years
Mass (Earth = 1): 318
Cloud-top gravity (Earth = 1): 2.53
Volume (Earth = 1): 1,321