Luminosity is the amount of light given off by an object. Along with temperature, it is one of the most important factors that define a Habitable Zone.
Plants and luminosity
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 simulate the light on Mars, allow a plant to receive only 44% of light. They will grow without major challenges.
- To simulate the environment on Jovian moons, reduce light to 3.7%. 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. Plants that grow on the floor of a dense forest are suited for such an environment.
- Beyond Neptune most plants cannot survive, with the exceptions of 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 (Earth-like) 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
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 would increase the number of possible terraforming planets.
Agriculture in the future
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, and aren't supplemented with artificial light, they will be less productive. On Mars, you would need over twice the amount of area as on Earth to produce the same amount of food. On a Jovian moon, you would need 25 times as much.
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 of whether settlers would prefer genetically modified plants or whether they would prefer to add some light to their crops. For Barnard's star, a red dwarf, settlers might need to use blue lamps to compensate for the lack of blue light emitted by the star.
Main article: Population Limit
As shown above, population on distant worlds, like the satellites of Saturn, will be far smaller than Earth's population, 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 shield to reflect part of the light, so we assume plants here will have similar productivity to those 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 these things. Keeping in mind that the costs to transport food, oxygen and water (and to send back human excrements and carbon dioxide) are very high over the huge distances of space, it is unlikely that settlers will receive food from home. This might work for asteroids and inner rocky planets, where food can be transported using solar sails.