An outer planet is far away from its hosting star. Temperatures are low (-100 C and below). With the help of greenhouse gasses, terraforming is (at least in theory) possible. It must be not too far, so that the light is enough for Earth-like plants to survive. There are two theoretical models. If the planet (or moon) has huge mounts of ice, it will become an ocean world. however, if it contains just the right amount of water, it can become a new earth.

Water planet Edit

After the ice melts, the planet (or moon) will be covered with a global ocean. There will be no dry land. This is how Europa, Ganymede, Callisto or Titan will most probably look like. The ocean will be many kilometers deep. Humans might live on floating continents or even floating cities.

Because of the high distance to its hosting star, the planet will receive only a tiny amount of heat. For Jovian moons, this is 1% of the radiation received on Earth. On the other hand, greenhouse gasses will prevent heat from escaping. The ocean itself accumulates heat and oceanic currents help that heat circle around the planet. Also, there will be very small variations of heat between seasons and even between hemispheres.

Jovian moons have their axis tilted only by 3 degrees, so they don't have seasons. In case of Saturnian moons, they are up to 50 degrees tilt. Since Saturn needs 29 years to complete its tour around Sol, at the poles there will be a cycle of 15 years endless light and 15 years endless night. Even so, temperature changes will not be significant, because of the ocean. Most probably, day-night changes will be unnoticeable, while differences between polar day, plar night and equator will be less then a degree.

Even if the axis is 90 degrees tilted and polar night will last for 40 years (like the moons around Uranus), there will be not major differences of temperature. And because of that, winds and oceanic currents will be slow. Waves will be small. In such an environment, water gets separated into layers. Differences of salt composition will also occur. The top layer receives more oxygen, while deeper layers will get full of toxic gasses and will lack of vertebrates. Only some algae and bacteria will be able to survive there. We can see that stratification on Earth, in Black Sea. However, if the planet has an active core and generates underwater volcanoes, this can help water to mix.

The reduced Luminosity will also affect oceanic life. Plants will not survive too deep inside the ocean. They will prefer to stay closer to the surface, where they will be hunted by fish and animal plankton.

The air will have a humidity of nearly 100%, fog and clouds will always be present, but it will rain less, because temperature does not decrease to allow water to condensate. Water and air temperatures will be almost the same.

By creating a Ground Insulation layer, we can prevent the entire ice to melt and we can more easily create continents. However, the planet or moon will not last like this forever.

Rocky planet Edit

It is possible that a rocky planet was pulled away from its former orbit and moved here. Also, moons with not much water (like Io) can exist close enough to their planet and at some point could be ejected into a heliocentric orbit. Without much water, they can become new Earths.

To understand how will such a planet look like, let's take Earth away from its orbit and move it somewhere between Jupiter and Saturn. Then, add greenhouse gasses, to keep water liquid.

During day, the sun will bring only 1% of its energy to the planet (compared to Earth). In response, during night, greenhouse gasses will allow the planet to cool only by 1% of how much it would around Earth orbit. As a direct result, day-night temperature will vary by only 1%. Even in case of a planet that rotates in 30 days (assuming a day of 15 Earth days), temperature variations will be of only 1 C. However, if we have a terraformed planet at the orbit of Ceres and a low rotation period, temperatures will change more visibly. As the planet cools in night time, rains will start to fall. During the day, temperatures will slowly increase and the atmosphere will consist of less moisture.

Seasons will also be less visible even if at the orbit of Saturn the year will last 29 Earth years. On Earth, these are the temperature records [1]: 56.7 C and -89.2 C, with an average global temperature of 15 C. Supposing that our planet also has an average temperature of 15 C and solar energy is only 1% of what is around Earth, temperatures will vary 100 times less. The extremes will be below 5 degrees C.

Because temperatures are not so different, there will be almost no winds. The air will accumulate more water. Since temperature does not decrease, rains will be rare and of low intensity. Rivers will be very small, but because of the high humidity, they will never dry. Deserts are unlikely to exist.

However, at some point it is possible that a hole in greenhouse gas layer might form. Temperature will decrease fast and air pressure will change, creating winds and rains. Once the winds are formed, the gap will close and greenhouse gas layer will make again its uniform cover. After the incident, temperature will remain low (maybe even below freezing point) and the sky will be blue for a number of days. Gradually, temperature will increase and humidity will also rise to initial levels. It is possible that settlers will want to create these changes in their environment. They might even celebrate days with clear blue sky.

Ice planet Edit

This is a cooler version of the ice planet. Settlers might want to preserve some ground for further buildings. There are a few ways to do that. First of all, by keeping temperatures just below freezing, they will keep a thin layer of ice above. Another way, more risky, is to use salts that are able to change melting point, but they might be toxic to the environment. It is highly possible that, with low currents, water stratification will occur, so that water with almost no salt will come to the surface.

In this scenario, if global temperature is kept at 0 C, there will be some parts of the planet covered with ice and some parts of the ocean will be exposed.

Atmosphere Edit

Greenhouse gasses have a higher molecular mass, so they will stay closer to ground surface. Since winds will be so slow, it is possible that gasses will tend to separate. Higher mountains will be exposed to a deep freeze.

Without winds, there will be no storms, just very long rains bringing low amounts of water. Without storms, there will be no lightning to bring nitrous compounds on ground. This will affect plant life. On the other hand, gaps in the greenhouse gas might be just enough to create storms.

Some moons experience strong tidal forces. On Titan, this generates winds. Without tidal winds, settlers might look for other alternatives to mix the gasses in their atmosphere.

The number of outer, cold planets, is far greater then the number of inner, Earth-like planets, since the volume of space outside classic Habitable Zone is far larger then the habitable zone itself, combined with the space between the zone and the star.