A gas giant is a giant planet made almost completely of gasses. The Solar System hosts four gas giants. Around other stars, other similar planets exist. For a classification of gas giants, see Gas giants (theoretical models).
In theory, there are many ways a gas giant can become useful for settlers. Some methods don't actually try to make these planets habitable for any living organism, while other methods propose ways to transform them into at least partially habitable worlds.
Non-Terraforming Methods
The following methods try to transform the planets into something useful for settlers that will move to their satellites.
Artificial Sun
Main article: Artificial sun.
The idea is that nuclear fusion reactions can be started inside the core of a gas giant. Giant planets have enough hydrogen, but their mass is lower then what is needed to start a thermonuclear reaction. If they had enough mass, they would already become M - type stars or at least Brown Dwarfs. Still, there is the idea, at least in science-fiction movies, that large enough giants, like Jupiter, can be artificially transformed into new stars.
If somehow hydrogen or at least deuterium fusion starts inside the core of Jupiter, the planet will expand, internal pressure will decrease until it no longer matches the conditions required for nuclear reactions to continue. The artificial star will be short lived. Still, the time needed for the new failed star to cool down is longer then a human lifetime.
Another way to heat-up a gas giant is to create a False Star. All we need is just to heat-up the planet until it reaches 3000 degrees, which is enough to create the illusion of a star. Then, it will take centuries to cool down. This can be done with huge explosions. However, theoretical models suggest that many gas giants have hot layers of gas below their upper atmosphere. If somehow we manage to mix-up their gas layers, we can bring heat to the upper layer. This process needs to be maintained over centuries, in order to keep temperatures high enough.
The Impact Theory also works in case of gas giants. If we manage to collide two large objects (for example a Jupiter-class planet with a Neptune-sized one), the impact can generate just the heat we need, also mixing-up their atmospheres. However, the energy needed to disrupt the orbits of two giants is huge and only a highly-advanced civilization will be able to do so. Also, there will be a high risk of collision between the diverted planet and the moons. Increasing mass of the main planet will also affect orbit of its moons. The method is too risky.
Another method to heat-up a gas giant is the Moon Impact Theory. Colliding for example Jupiter with one if its large moons will not create the needed heat. Still, there is a possibility that some gas giants might be binary. Imagine a Jupiter with a Neptune-size moon orbiting close enough, like Pluto and Charon. Then, at higher distance, there are other moons, some of them large enough for human colonization and orbiting the binary. Colliding the planet and its giant moon will not affect too much the motions of outer planets, but will heat-up the planet up to 2000 degrees.
Artificial Luminosity
In one sci-fi novel, a self-replicating lantern was thrown into Neptune. After many years, it managed to create many copies of itself, floating in the upper part of the atmosphere and creating enough light to sustain plant life on Triton, which was terraformed with the help of greenhouse gasses.
The idea might sound impossible with current technology, but it might be possible to create a fleet of nuclear-powered stations, floating like a hot-air balloon, on the upper atmosphere of a gas giant, providing the needed light for a human colony on its moons. The process can also generate limited amount of heat.
Industrial Usage
Main article: Industrial colonization.
Gas giants are large and partially recycle their atmosphere. Extracting raw materials from them is almost impossible and anyway mining asteroids is much more easy. Still, to avoid filling the space with debris and covering planets and asteroids with garbage, many would agree that dumping things into a gas giant is a much better alternative.
Possible Colonization Methods
Creating a colony on a gas giant looks like something impossible. If you try to land on a giant planet, you will only get into hotter and denser gasses, until the pressure will make your ship implode. They don't have a solid surface. Also, their winds are a real problem, reaching over 1000 km/h. They have huge storms, hurricanes that can be larger then the diameter of Earth and not only. Lightning has been noted to exist on Jupiter and Saturn. The changing in temperature between gas layers also means that in different layers of the atmosphere it might be raining. We might have also snow or hail, made of various materials.
So, is it possible to create a colony on a gas giant?
Balloon Colony
The most easy way is to create a floating station. It will be a giant hot-air balloon, standing inside the upper layers of the atmosphere, where pressure is acceptable for humans, weather is not quite a nightmare and temperature can be within tolerable limits. The colony will be on a continuous movement around the planet.
A large colony is not possible in this way. Strong winds will tear it apart soon. We can imagine a set of several balloons connected with a cable, to form a row. Another version is to have a large balloon high enough and a counter-weight at many km below it. The colony will be somewhere between. Since lower air is hotter, temperature differences between the balloon and the counter-weight can be used to generate electricity.
The balloon colony has two major problems. First of all, gas giants have a large amount of hydrogen and helium in their upper atmosphere, that means we cannot use a lighter gas to keep the balloon up. We will need to heat it. The second problem is that creating a launch pad for spacecrafts on a floating town is very difficult. Ship platforms will always be moving and will not support too much weight. Escaping the gravity of a gas giant requires much more energy (and larger ships) then escaping the gravity of a rocky planet.
Large Floating Colony
One theoretical model suggests that winds on a gas giant are not felt at their real speed. This is because any floating object will be moving with the same speed the gas is moving. If this is true, then a city can float above a gas giant. Also, it is possible that some giant planets are not as aggressive as others. We know that Uranus has less powerful winds then Neptune. In theory, as a gas giant gets older, it loses its internal heat, which powers-up all its winds and hurricanes. So, a planet old enough (for example a gas giant orbiting Barnard's Star) could be just suitable for the creation of a floating city.
Construction of a floating colony will be very difficult, because materials have to be imported from elsewhere. We cannot build with gasses. Very important will be the equilibrium of forces. Hot air balloons (or maybe hydrogen balloons if the atmosphere is heavier) will counter the weight of the whole city. If anything bad happens, the city will sink to the abyss or will rotate with one side up.
Energy can be generated using wind turbines. There should be speed differences between various layers of gas.
Microbe Terraforming
It has been theorized that small organisms (like algae and bacteria) can survive on a gas giant. These planets might have all ingredients needed for life. Microorganisms can survive in the currents, as long as the pressure does not increase too much. Some scientists argued that Alien life might already exist inside the clouds of some giants.
It might be possible for us to insert life forms from Earth to create a breathable atmosphere of a gas giant. Maybe, that will be possible with the use of genetically-modified algae. All their life, these algae will produce oxygen (which will slowly burn the methane or hydrogen). The microorganisms are heavier then the surrounding gas, so eventually they will fall to the lower atmosphere, together with the carbon they gathered, but still some will remain, lifted by upper currents, to renew the population. On a long timescale, the process can transform a gas giant into a planet with breathable atmosphere. This way, a floating colony becomes more possible.
Complete Terraforming Models
Is it possible to completely terraform a giant planet? In theory, it is. There are two methods that can do this:
Atmosphere Removal
To remove the atmosphere of a gas giant is very difficult, just impossible with our current technology. Also, sending such a huge amount of gas into outer space, will have catastrophic consequences. The gas will interact with the solar wind, creating huge radiation belts and solar storms. As the gas is pushed away, it can affect colonies on planets and asteroids it will encounter. For colonies on the local moons, it will be a nightmare.
After removing excess atmosphere, the mass of the giant planet will decrease with more then half. A significant part of its moons will escape into solar orbits. The freshly exposed core is very hot, probably much over 10 000 degrees. So, we will have to wait a long time for it to cool down, unless we have the technology to do so.
Encased Planet
Another theoretical model suggests the construction of a large artificial surface above the atmosphere, like a giant shell. The surface will need to be extremely strong, to support the forces beneath it. No known material is suitable for this. Only an extremely advanced civilization might be able to create an encased planet.
The Universe is full of giant planets. They exist in the Solar System as well as around many nearby and distant stars. Giant planets have been discovered also to be free-floating around the galaxy. Even if we might never be able to terraform them, understanding the importance they have in each solar system and the complex processes that happen inside them and in the surrounding space is vital if we want to colonize their moons.