Even if we manage to prove a planet is within all the planetary habitability conditions that might support lives, it will be too far in distance that we hardly can put it in real useful investment for any human being to travel to such planet for a gamble of his future, unless major breakthrough in interstellar journey makes such journey so short as if today's intercontinental travel.
Put our vision back to Solar system, our choice seems to be Mars: the red planet in our star system. Though Mars is a small planet that following conditions rule out Mars being a suitable planet for human next mass habitat of colonies.
1. Atmosphere being too thin to protect colonies from direct impact of mini asteroid, which would otherwise burn out in Earth environment.
2. Mars is too far from the Sun means energy is too weak to support planetary scale of warming. Though engineering solution can create heat shelter to insulate colonies from heat lost.
3. Gravity of Mars is only 38% of Earth, it has long term damage to human lives living too long without artificial gravity compensation. Any potential lives grown in such gravity environment will be a different specie from planet Earth.
How about Venus. It has hot gas atmosphere seems to be quite impossible to overcome. But in my opinion, if we cast our human history of a million years of natural evolution, we have to say thank you to the tribe that took their migration out of African jungle. Our ancestors did what the choice that matters to future generations of civilization. In real term our modern civilization started 250 years ago when engines were developed.
Geo-engineering of environment so far is quite limited by human society but it is nonetheless happening, especially in the view of long-term impact by what we have been doing. Planet Venus could be a potential of second home to humanity if we plan for a time frame of 10,000 years, which is only a micro second of universe evolution, or 1% of a million years. How to cool down Venus atmospere using outer space light weight material shelter fully or partially surrounding the planet? There have been some sepulative publication on such visions. In the top of Venus atmosphere, about 50 km in altitude, the temperature and pressure is just like planet Earth at sea leve, which makes it perfect environment for buoyancy floating colonies, which can be a factory of phototyythesis, or tourist hotels. In these floating cities, we can create factory pumping Venus air into floating colonies. Algea in special built photosythesis tanks can absort CO2 to grown more algea while oxygen will be used by human beings living in the floating colonies. We can then fabricating carbon fiber net using dry algea material to make light weight net material. It would take time to cover Venus outter space with such net to block Solar light, which will cool down Venus very quickly. Other solutions is to put such net in Lagrangian point L1, which is between Sun and Venus for better heat insulation to block more input of solar energy while heat radiation from Venus happening in the same time. Second solution give potential space engineering control to solar energy level that matches Earth envrionement. By doing so we are making the Venus the Second Earth in Solar system, well, almost.
In order to have continously algae based photosynthesis process on the upper Venus atmosphere, water is the essential to support. Water can be found through comets by towing these comets from asteroid bell. So the algae photosynthessi in upper Venus will create carbon fiber net for shelter while O2 will be stored until Venus air temperature drop. In order to fast track the cooling, actively pumping Venus air into space using solar energy. Due to this is simple graveitational process, we can expect energy netrual. excessive Venus atomostpere gaes can be liquified and trasfered into deep space of outter Solar system. Then the O2 will be release back into Venus atmospere plus comets water. Then introducing algae into Venus suface for further terraforming.
(To be continue)
* Fogg, Martin J. , Terraforming: Engineering Planetary Environments, SAE Press, 1995. ISBN 1560916095, ISBN 978-1560916093
* Jump up ^ Landis, Geoffrey A., "Terraforming Venus: A Challenging Project for Future Colonization," paper AIAA-2011-7215, AIAA Space 2011 Conference & Exposition, Long Beach CA, Sept. 26-29, 2011.
^Wikipedia: Discoveries of extrasolar planets