Jupiter is the biggest planet in our Solar System, being more than twice the size of all the other planets put together. It is unlike our planet Earth in that it appears to be made of gas rather than rock - such planets are known as 'gas giants'.

Jupiter would appear to be an inhospitable place to live. Being much further from the Sun than we are, it should be rather cold, although exactly how cold we are not sure. Jupiter is a strong source of radio waves, which suggests that there are things going on inside the gas that are generating energy. Is this enough to raise the temperature and make it habitable?

This Entry looks at the practicalities of living on or around Jupiter, as seen by modern science and also as portrayed in fiction.

John Carter goes to 'Eurobus'

Edgar Rice Burroughs wrote a whole series of stories about the planet Mars. In one of them, Skeleton Men of Jupiter, published in 1941, the hero John Carter is kidnapped from Mars and brought to Jupiter, known to its inhabitants as Eurobus. Burroughs imagined the planet to be like a bigger version of Earth, with a solid surface, mountains, plains and oceans. The air was clear at ground level but a high-level dense cloud layer prevented the inhabitants from seeing the Sun and us from seeing the surface. Like Earth, Burroughs's Jupiter had a molten core. Giant volcanoes all over the planet provided both light and heat so that the temperature was conducive to life as we know it.

Jupiter is a massive planet - that is, it has a lot of mass. The force of gravity at its surface should be more than 2.5 times that on Earth, so a person would weigh more than 2.5 times as much on Jupiter as they would on Earth. Carter reported that gravity on the surface appeared to be slightly less than that on Mars, which would put it at about one quarter of Earth's surface gravity. His explanation for this was that the enormous speed of rotation of the planet throws anything on the surface outwards and this centrifugal force counteracts the huge gravitational pull. Burroughs may have made a mistake in his calculations, or he may have been bending the truth to make a good story. It is certainly true that the planet appears to turn at a remarkable rate, doing one full rotation every 9.9 hours. But for the centrifugal force to be large enough to cancel the gravity as Carter said, the planet would need to spin more than three times as fast, at a rate of once every 3.1 hours.

Our figure of 9.9 hours is based on the clouds - we can't see through them to the surface if there is one. It is conceivable that the planet could spin at three times the speed of the clouds above it, but it seems unlikely. If this were the case, the inhabitants would experience violent winds all the time.

A World of Gas

Current thinking is that Jupiter is not a solid world like Earth. It is almost entirely gaseous, being composed of hydrogen and helium (they make up 99% of its mass). Taking the 'surface' as the level where we can see the planet's clouds, the pressure of the gas increases with depth below the surface. This is because at any point, the gas must support the weight of all the gas above it. At a certain depth, the pressure is so high that the hydrogen starts to liquefy, although there is no distinct boundary between the gaseous and liquid hydrogen. It is thought that there is a solid centre to the planet, but nobody knows for sure. If there is, it is likely to be small relative to the planet's size, although it still could be large compared with the Earth. Some have even speculated that the solid core of Jupiter might be made of carbon rather than hydrogen, and that the intense pressure will have caused it to crystallise, making a diamond the size of the Earth!

Living in what is effectively an entirely gaseous world would be quite a different thing from the solid planet envisaged by Burroughs. Once you take to the air, you find that there is much more living space on Jupiter than anywhere else in the Solar System. It has a surface area more than 100 times that of Earth, and the atmosphere is so much thicker that you would probably be able to use at least one hundred times as much of it vertically as on Earth, making the total 'floating space' about 10,000 times that available on Earth.

Arthur C Clarke's A Meeting with Medusa

In 1971, Arthur C Clarke wrote a novella about mankind's first visit to Jupiter. The mission was piloted by a lone man, Howard Falcon, who had been involved in an accident and whose body had been rebuilt with certain improvements which made him suitable for the task. Clarke's story had a long description of descending through the various cloud layers and what Falcon saw as he did so. He was surprised to find life on Jupiter, but of a kind very different to life on Earth. Two creatures in particular were worthy of note - a giant floating jellyfish-like creature a mile across, which is the Medusa of the title, and a creature like a manta ray which preyed upon the medusas.

In this story, Clarke is ambiguous on whether there is actually any solid surface on Jupiter.

The Algebraist

Iain M Banks' book The Algebraist is not actually about Jupiter but describes a similar fictional planet, Nasqueron, in another part of the galaxy. The issues are the same - there is no solid ground but vast amounts of atmosphere. Anything that lives must float. The planet is inhabited by a species, the Dwellers, who occupy not just the described planet but also most of the gas giants of our galaxy. They have been civilised so long that they are considered a 'post-civilisation culture' - they have fallen from civilisation and have many 'barbarous' practices (such as hunting their own young for sport), but still retain a lot of the technology of a civilised species.

It's interesting that the juvenile form of the species looks like a manta ray - it is possible that Banks was inspired by Clarke's earlier story.

Living in the Gas

Despite the accounts in these stories of living 'on' Jupiter by floating in the clouds, there are in fact certain problems. Jupiter's atmosphere is subject to very violent storms - these can last a long time. One, known as the Great Red Spot, has been raging for at least 350 years since it was first observed in the 17th Century. Another problem is that Jupiter has a very strong magnetic field. Hydrogen under extreme pressure behaves as a metal and conducts electricity. Eddy currents in the metallic hydrogen layer of the planet cause the magnetic field to be the strongest of any planet in the Solar System. And strong magnetic fields are not good for life.

The Moons

If the planet is off limits to humanity, then perhaps somewhere can be found to live on some of the many moons. Jupiter has more than 60 moons, but four of them are of particular note - the Galilean moons, discovered by Galileo Galilei, are comparable in size to our own Moon, being between 90% and 150% as big. After the probes Voyager 1 and Voyager 2 flew past Jupiter in 1979, giving us highly detailed images of the moons, people started entertaining the idea of life on one of Jupiter's moons.

Io

Io is the closest of the Galilean moons to Jupiter. It is close enough to the planet to be affected by tidal forces. All the moons are pulled by Jupiter's gravitational field, but Io is so close that one side of it is significantly closer to the planet than the other, resulting in a different gravitational pull on the two sides. This causes an effective stretching force on the moon. As the moon revolves around the planet in an elliptical orbit, the strength of this stretching force changes, causing the moon to expand and contract. This in turn causes the core of the moon to heat up, so Io has a molten core rather than a solid one, and there are volcanoes all over its surface which constantly deposit new layers of magma on the surface. There's no point in making a map of Io as it changes continually. This is not a hospitable place for life.

Ganymede

Ganymede is the biggest moon in the Solar System, even bigger than the planet Mercury. Initially it was thought that it was a rocky sphere like our own Moon. This would present challenges similar to living on the surface of the Moon, although with much more spectacular views - the King of the Planets would dominate the night sky.

To live on a rocky Ganymede, giant domes would have to be built and filled with air. This is discussed in great detail in Gregory Benford's 1983 novel set on Ganymede, Against Infinity. Another novel about living on Ganymede is Robert Heinlein's 1950 Farmer in the Sky, in which a full-scale terraforming project is undertaken, grinding up the surface to make soil for growing plants.

More recently it has been determined that the surface of Ganymede is not rock but water ice. This had led to speculation that if the planet is warm inside like Io, then the ice might have melted below the surface, meaning that there would be a layer of water. While exciting, this is more likely to be the case on Europa, so it will be discussed for that moon.

Callisto

Callisto is smaller than Ganymede and further from the planet. It appears to be also covered in a layer of ice, but its extra distance from Jupiter means that the warming by tidal forces is minimal and there is not likely to be any liquid water. Not the kind of place to raise your kids.

Europa

Perhaps the most interesting moon is Europa. It's not close enough to its parent planet to be really hot like Io but there is still a fair amount of internal heat caused by tidal forces. Like Ganymede and Callisto, it is covered in a layer of ice, and it is very likely that this is warm enough below the surface to be liquid water. Humans can live below the surface of our own ocean for long periods (for example in nuclear-powered submarines) so living in such an ocean should also be possible.

A more interesting possibility, though, is that life may have already evolved in this environment. There should be enough organic compounds on Europa to form life, and a nice warmish ocean to nurture it. Could we already have another form of life in our Solar System?

Arthur C Clarke's 2010: Odyssey Two

Stanley Kubrick's film 2001: A Space Odyssey is rightly considered a modern masterpiece. It is often stated as being based on the book of the same name by Arthur C Clarke, but according to Clarke it was more of a collaborative project. He wrote the book and Kubrick wrote the screenplay together, each one feeding off the ideas of the other. In places they went in different directions - Kubrick's ending to the film was mystical while Clarke's book ending was matter-of-fact. One major difference, though, was that Clarke set the action around the planet Saturn, while Kubrick decided that Jupiter would be more appropriate.

In 1982 Clarke published a sequel, 2010: Odyssey Two. It was a sequel, however, to Kubrick's version of 2001 rather than his own, since it was based on Jupiter. One of the reasons for this was probably because in the meantime, the Voyager probes had visited Jupiter and sent back lots of pictures of the moons, showing them to be really interesting places to set a story. 2010: Odyssey Two is one of the best descriptions in fiction of the moons of Jupiter. In addition, it was made into a very fine film, 2010, directed by Peter Hyams and starring Roy Scheider, John Lithgow and Helen Mirren. This is essential viewing if you want to get a good feel for what it's like to be hovering over Io with Jupiter above you.

A fascinating part of the book and movie involved rescuing an abandoned spaceship which was floating near Io. The subject of life on Europa, on the other hand, was the central point of the plot. Mysterious and intelligent aliens had noticed, or possibly created, life on Europa and were now helping it along, including imposing a ban on humans approaching the moon.

We won't spoil the dramatic ending by revealing the far-reaching method by which the aliens ensured the survival of life on Europa. Let's just say that it was, in the words of David Bowman, 'something wonderful'.

So Why Go to Jupiter?

Various reasons have been proposed why we should want to live in or around Jupiter.

• As the population on Earth keeps growing, we are running out of both space and resources. By moving out into the Solar System, we give ourselves room to keep growing, and the resources to do so, although the type of life would be very different from that on Earth.

• In the spirit of exploration, we want to try new things and explore new worlds. We might even meet a few aliens along the way who could teach us a thing or two. What better place to start than the worlds of our own Solar System?

• Thinking really long-term, our Sun is going to be around in its present form for only another six billion years. At the end of that period, it will change into a red giant, getting much bigger and hotter. The Earth will be destroyed in this process. If we haven't already died out or found somewhere else to live, that will be the end of us.

Of course going to Jupiter at the moment would be ferociously expensive. But if we could make the project self-sustaining, then we could have a community of humans living in the clouds of Jupiter or in the oceans on the Galilean moons. Just think of the adventure!

Image of Jupiter courtesy of NASA