Planetary engineering is the application of technology for the purpose of influencing the global environments of a planet. Its objectives usually involve increasing the habitability of other worlds or mitigating decreases in habitability to Earth.
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Perhaps the best-known type of planetary engineering is terraforming, by which a planet's surface conditions are altered to be more like those of Earth. Planetary engineering is largely the realm of science fiction at present, although recent climate change on Earth shows that human technology can cause change on a global scale.
Terraforming
Terraforming is the hypothetical process of deliberately modifying the atmosphere, temperature, or environment of a planet, moon, or other body to be similar to those of Earth in order to make it habitable by humans.
Seeding
Seeding is a term used for the process of introducing microbial or algae species on a planet or moon already offering habitable regions. Jupiter's moon Europa is a good example as it has a well protected ocean under a thick ice crust. Living on the surface of the ice crust is not possible, but some microorganisms from Earth could possibly survive in the oxygen rich environment of Europa's subsurface ocean. In the case of Europa it might be possible to incrementally seed with the species in their trophic (see foodchain) context and to build up an aquatic ecosystem.
Geoengineering
Geoengineering is the application of planetary engineering techniques to Earth. Recent geoengineering proposals have principally been methods to tackle human-induced climate change by either removing carbon dioxide from the atmosphere (e.g. using ocean iron fertilization) or by managing solar radiation (e.g. by using mirrors in space) in order to negate the net warming effect of climate change. Future geoengineering projects may preserve the habitability of Earth through the sun's life cycle by moving the Earth to keep it constantly within the habitable zone.
Habitability Maintenance
Planetary engineering can also be applied to keep habitability of Earth in the far future. The rate of weathering of silicate minerals will increase as rising temperatures speed up chemical processes. This in turn will decrease the level of carbon dioxide in the atmosphere, as these weathering processes convert carbon dioxide gas into solid carbonates. Within the next 600 million years from the present, the concentration of CO2 will fall below the critical threshold needed to sustain C3 photosynthesis: about 50 parts per million. At this point, trees and forests in their current forms will no longer be able to survive. Unlike other animal species, humankind has the potential ability to prolong the habitability of the planet earth by extracting and reinjecting inert carbon trapped underground back into the biosphere.