Luminosity and temperature are the most important factors that define a Habitable Zone.
Plants and luminosity Edit
Plants need light to survive. A terraformed planet cannot exist without life. Unless scientists will use other more exotic types of life, plants are needed. A plant needs both red and blue light in order to survive. If you give them only red or only blue light, they will die.
I made some experiments on my own. They are simple to conduct. Put plants into boxes and allow them to have only a limited hole for light. Keep plants inside for a specified amount of time. The results are as follow:
- To stimulate the light on Mars, allow a plant to receive only 35% of light. They will grow without major challenges.
- To simulate the environment on Jovian moons, reduce light to 1%. I tested this with grass and grain. Results were positive. Plants are able to survive (even if not easy) and feed the settlers.
- To simulate conditions around Neptune, allow only 0.1% of light. It might look very dim, but some plants can survive. Don't expect trees to grow in such an environment. Those plants that grow on the floor of a dense forest, they are the best candidates.
- It looks like beyond Neptune plants cannot survive, except maybe for some algae. Settlers will have to use additional light sources to make their crops grow.
It is very important to know that not all stars generate the same kind of light. Some are more red, some are more blue, with excess of infrared or ultraviolet. For more details, go to Habitable Zone and follow the links. Our sun, Sol, has the most perfect light for life. Other stars are lacking of something, so that their habitable zones are shorter. Planets that are outside the borders will not be easy to terraform.
Genetically modified plants Edit
geneticists will probably try to make plants able to use only red or only blue light. In some extreme environments, plants could be modified to work with infrared or ultraviolet light. This can increase the number of possible terraforming planets.
Agriculture in future Edit
It is possible that settlers will want to have similar cuisine to what we have today. Others will have less options and will eat only algae. If plants don't get much light from their hosting star, the plants will produce less food. On Mars, you would need to harvest maybe 3 times more crops, while on a Jovian moon, 100 times, unless you don't give plants extra light or you don't use better hybrids.
In a closed environment, like a dome on an asteroid or on a Kuiper belt object, it might be better to use artificial light.
It is a question if settlers would prefer genetically modified plants or they would like to add some light to their crops. For Barnard's star, a red dwarf, settlers might need to use blue lamps for compensation.
Maximum population Edit
As shown above, population on distant worlds, like the satellites of Saturn, will be far smaller then population of Earth, because they would not have the power to create food as needed. On the other hand, for Venus and Mercury, even if Sol's radiation is stronger, there would be some sort of shields to reflect part of the light, so we assume plants will give similar productions to what we see on earth.
Food can be transported, but this will come with massive problems. Humans don't need just food, they need also air, water and minerals. Plants also need all these. Keeping in mind that the costs to transport food, oxygen and water (and to send back human excrements and carbon dioxide) are very high at big distances, it is unlikely that settlers will receive food from home. This might work for asteroids and inner rocky planets, where food can be sent with solar sails.