Gravity is one of the most important factor that defines a celestial body and has a strong influence on habitability. Each planet, moon or asteroid has its own gravity.
Above habitable Edit
If gravity is too strong, human life is impossible. This is the case of White dwarfs, Neutron stars or Black holes, but also the case of many stars. of course, life is impossible on such bodies because extreme temperatures. But, they are not the only celestial bodies with extreme gravity.
A black dwarf is a cooled white dwarf. In theory, without an external source of heat, the temperature can be close to absolute zero. If there is a nearby star, temperatures will be higher, even around 300 K (27 C). In this case, we might find conditions for life. Gravity is about one billion times higher then on Earth. In these conditions, any object will simply disintegrate. No human will be able to survive. They are surrounded by a highly compressed atmosphere, where pressure is so high, that nothing will survive.
Gas giants sometimes have a high gravity, but even in the case of Jupiter, it is not too high, so that a person would survive. But, some exoplanets are found to have a greater mass then Jupiter, at the same diameter, resulting in higher gravity. Terraforming such giants is very hard, if not impossible.
High gravity planets could exist in theory. If a planet has a huge nucleus made of heavy metals (like lanthanides), surrounded by a small crust, gravity will be high. Assuming a light atmosphere, the planet can be habitable, but at a radius over 3 times that of Earth, its gravity will be over 7 times that of Earth. Terraforming would still be possible, but humans will never adapt to such conditions.
Within terraforming limits Edit
The upper terraforming limit is a gravity where humans can survive. After a few generations, it is possible for humans to live at 4G or 5G. The lower limit is where atmosphere can be held for a limited time frame. See Atmosphere around small bodies for details.
Humans will have to adapt to this. See Future races for details.
High gravity planets Edit
In case of a Super-Earth, gravity is expected to be up to 3 times higher then on Earth. Settlers will find hard to live in these conditions, but their children, born in these conditions, will be adapted. They will grow shorter then us and will have much stronger muscles and bones. Their heart and blood vessels will need to be stronger. However, since humans are not made to live in this environment, elders will often suffer from arthritis, heart diseases, arteritis, bone diseases and working accidents.
Living in this environment is a dangerous task. Because of gravity acceleration, it will be risky to jump. For many of us, jumping down in an one-meter deep hole is not a problem. But on a world with high gravity, the acceleration will make us break our bones. On Earth, if you drop a hammer from your hand, if it falls on your feet, it will make you feel pain, but you will not have broken bones. On a Super-Earth, you will certainly break your bones. This is because gravity acceleration is multiplying the force every second. Even things that we don't give attention, will harm you. Try to avoid staying in the rain, because even a drop of water will have enough energy in it, to hurt you.
Nature will have to adapt. Trees will grow smaller and wider. If the atmosphere is not much denser, birds will find impossible to fly. Animals will also face the same problems. Life will be unaffected in shallow waters, because the Archimedes law. Everybody will love to stay in water. In deep water, things will be different, because pressure increases faster then in Earth oceans. So, if gravity is 3 times higher, at 20 meters deep, it will be like at 60 meters deep in Earth's oceans.
The landscape will be strongly affected. Rivers will flow much faster, with a great force of erosion. After terraforming, you will see deep canyons forming. If the planet had rivers before and geology is not too active, there will be no mountains around, but large plains and highly eroded hills. Tides will also be far smaller.
Visitors from other planets will probably lie down or stay in water, in order to survive.
Engineers will face many problems. A lot of work should be made in creating cranes and other lifting devices. Buildings will need to be stronger. Probably you will never see sky-scrappers there. Building a long bridge will be a hard task. And if the planet has an active nucleolus, earthquakes will be a big challenge. Transportation will also be a major problem. Vehicles will climb much harder and will need stronger brakes. Even the friction with the ground will be higher, meaning that each vehicle will need more energy to gain the same speed it gets on Earth. One major advantage is that hydroelectric power plants will generate more energy. Technological progress will be slow, because the many problems created by massive gravity.
Traveling to space will be a much harder task then it is on Earth. The escape velocity required for a spaceship is far greater, resulting in huge fuel consumption.
On a Super-Earth, Demography will be interesting. First, not many people will go to such a world, so there will be only a small number of settlers. Then, generation after generation, their number will increase, mainly by natural birth and only limited by immigration. Not many people will interfere with them, so that the planet will have a large internal autonomy, but a low technological level. Population will slowly spread across land. Because large buildings are hard to made, there will be no large cities, but villages and suburbs. Since transportation systems will face many problems, there will not be strong connections between people. so, residents will be encouraged to spread in new villages instead of moving to cities. Then, since surface of a Super-Earth is larger then Earth itself, the planet can sustain a much greater population then an Earth-like planet.
Earth - like planets Edit
In case of an Earth - like planet, life will be similar to what we see on Earth. Building standards found on Earth should be also applied here.
Small Earth planets Edit
Majority of terraforming candidates in Solar System are planets and moons smaller then Earth. They can hold a breathable atmosphere around them long enough for a human colony to exist. Gravity is between 20 and 60% of what we have on Earth. People living in these worlds will adapt fast. Their children will grow a bit higher then us and will have less muscular tissue and weaker bones, but still, they will look similar to us. If one day they decide to visit Earth, they will feel like us visiting a Super-Earth. Because lower gravity means less effort to the heart and less stress to blood vessels, they will have less diseases. These low gravity worlds will also host hospitals and resorts for sick people from worlds with higher gravity.
Things that look impossible on Earth, will be possible there. On Europa or Luna, walking on water will not be a dream. How is that possible? In low gravity, inertia increases, so that if you jump up, you will fall down in far more time. Also, your weight is far smaller. So, walking fast and with small steps above water will be possible. People will be able to jump over an obstacle of two meters high or to jump down a few meters. Transporting a big luggage will not be as hard as on Earth. Swimmers will be able even to jump from water like dolphins. However, all these advantages will not be the same for children born here, since their muscles will be less developed. Walking will be a different experience. On Moon, jumping was the best way to travel, while on Mercury, it looks like running is the best way. If gravity is small enough and if atmosphere is dense, some people will even fly with artificial wings.
For nature, it will not be hard to adapt. In fact, nature will find this world better then Earth. Plants will grow higher and thinner, while animals will find more easy to climb in trees or to jump over obstacles. In oceans, life will benefit of lower pressures. What we see on Earth at 20 meters deep, we should see on Lunar oceans at 120 meters deep. So, divers will reach incredible depths without much effort. In the air, flying will also be easy. Domestic hens will fly like pigeons.
Rivers will flow much slower, generating less erosion. Also, landslips will be more rare. As a direct result, landscapes will be longer preserved. Watching a waterfall will be interesting, to see all water drops falling slower. Fish will find more easy to swim upstream. But, since rivers will flow much slower, in case of a flood, water will also move away slower then on earth.
Technology will triumph on low gravity worlds. Since majority of these planets are geologically inactive, earthquakes are also very rare. So, we will see constructions that on Earth simply cannot exist. Sky-scrappers will reach incredible heights. Because buildings don't need to be so resistant, constructors will invest more in other domains. Transportation routes will also be more easy. Air vehicles will fly with less energy consumption. Ground vehicles will lose less energy by ground friction and will climb more easily. Bridges will require less investment.
Access to outer space is more easy from a planet with low gravity, because the escape velocity is smaller.
The demography of such a planet will show a strange pattern. From the beginning, settlers will love it. After the first pioneers, many settlers will come. Immigration will be stronger then birth rate, resulting in a fast population increase. Cities will rise and grow. It will all look very good in the first days. However, because the planet is small, its surface is even smaller. Soon, population density will reach a dangerous level, depending on the environment. At that point, local government will take actions to limit immigration and population density will plateau. It is also plausible that, trying to avoid overpopulation, the government will invest in terraforming another planet.
Very small terraformed bodies Edit
Terraforming these bodies is very hard, because they are below the point where atmosphere is safe (see Atmosphere around small bodies for details). If somehow they are terraformed, their atmosphere will be so puffy, that they will double planet's diameter. Solar winds, low escape velocities and gravity perturbations will blow the atmosphere away fast. A strong and continuous work is needed to keep the fragile equilibrium.
People will belong to the Cosmic race. they will be higher, with even less muscular tissue and with visibly thinner bones.
Everything will be different there. Walking will be hard or useless, instead, jumping will be the way to foot travel. Eating will be a different experience, since food can easily be thrown from your plate. Drinking will required closed bottles, because liquids will be attracted by tenuous forces and will 'climb' away from a glass. People will be able to do incredible things. The Romeo & Juliet balcony scene will never exist, because Romeo will simply jump up to Juliet.
For nature, things will be even more interesting. Plants will grow without problems, you could in theory see trees climbing over 1 km (in practice, their height is limited by the instructions they have in DNA code, so they will stop at some point). Also, not only birds, bats and insects will fly. In this low gravity, even dogs can swim in the air. Many animals will probably not be interested in flying, but in jumping. There will be no problem for a dog to jump after a cat in a tree.
Scientists will find building very easy, far more easy then on a small Earth. On the other hand, huge maintenance costs will be a continuous thread for settlers. As long as the economy is strong enough, they will afford the price, but in case of a long recession, things will get very dangerous. For the entire population, if maintenance is not made, there will be only two choices: death or emigration.
Demography of such a world is interesting. Population will increase fast, reaching a high density in short time. Then, we will see a delicate balance between increase and decrease of population. As long as people has where to work and are well paid, they will stay. People searching for a better life will try to settle here. If population reaches a too high level, laws will appear, trying to limit immigration. But if something goes bad, people will move away fast. Decrease in population density will result in less tax payers and less money for maintenance services. Increasing taxes will bring poverty and will make even more people to emigrate. If the process is not corrected, in the end, maintenance will not be possible anymore and a disaster will strike.
Below terraforming limits Edit
If a celestial body has a too low gravity, terraforming is too expensive or even not possible. Paraterraforming is still an option. Many paraterraforming candidates exist in Solar System. In case of asteroids, this is the only way to create an Earth-like climate. For more details, see Global paraterraforming. There still are ways to inhabit a celestial body without terraforming or paraterraforming. For this, see Industrial colonization.
Another, very unusual way of inhabiting a celestial body, is by inhabiting existing or artificial caverns. This method has no official name. I suggest naming it Internal paraterraforming.
With no gravity Edit
There are many asteroids that are too small, so that their gravity can be considered as equal to zero. Also, this is the environment that you will find inside a space station without artificial gravity.
In case of a very small moon or a tiny asteroid, there will still be some gravity, too small to be sensed by humans, but still strong enough to allow floating objects to fall on the ground in a long time scale. However, humans can build inside an asteroid where the overall gravity is equal to zero.
On a tiny body, like Rosetta's comet, if you try to walk on the surface, the force we use on Earth to walk, will be enough to send you into orbit. Try to jump and you will travel forever around the Sun. However, humans will stay much of the time inside domes and underground habitats.
Without gravity, everything will be different. The Super - Cosmic Race will be formed. First generation of settlers will look like us, but their children will not be quite as we are. without gravity before birth and in the first years, they will understand only in theory what 'up' and 'down' means.
Every aspect of life will be different. Women will not be happy to wear skirts, because the lack of gravity will move them up. Eating, drinking, will be from tubes and bags, because food fragments and water drops will float in the room for a long time. Eating a slice of bread or any other classic food will generate floating garbage, that can float in the air and even be breathed. If you sneeze, tiny drops of saliva will float in the air until they reach a surface. There is no gravity to take them to the floor. In fact, there is no floor.
Smoking is different in a world without gravity. On Earth, we are used to see smoke going up, because heated air is lighter. Without gravity, the smoke will slowly move in all directions, it will mix slower with air, but not that slow to stop the cigarette from burning. Some air currents are created by temperature differences.
NASA made experiments with plants and animals in zero-G. Fish will swim in all directions, birds and insects will also fly in any direction. It will be harder for animals to breed. Laying eggs (for birds or reptiles) requires some gravity, but without it, the eggs will leave the nest. Even building a nest is more complicated.
The zero-G environment is good for many industrial purposes. Many corporations will like to have factories in outer space, for this reason.
Artificial gravity Edit
The most common way to create some sort of gravity is by using centrifugal force. Many scientists have proposed ring-shaped or disk-shaped bases. In the center, gravity will be zero, but the further you move, the stronger gravity will be. Using of centrifugal force also increases orbit stability.
It is possible that in future other technologies will be developed, to create artificial gravity on orbiting stations and spaceships.