The third phase of the colonization of a new planet involves seeding it with the alien species, plus whatever else is necessary to establish a clone of the home planet, at least for alien civilizations of categories A1 and A3. In the best of the sweet spot worlds, the planet that has been discovered and selected as a destination has life matching the DNA coding scheme of the alien planet, or if there are more than one on the alien planet, one or more of them. There is sufficient oxygen in the atmosphere, generated by some form of photosynthesis, which might be chlorophyll or some other photosynthetic chemical that evolution on Earth has not discovered, and which Earth technology has not invented.
All of the planetary parameters and stellar parameters are within tolerable bounds. There are many of them: the star must be of an acceptable class, not a large, hot star with only a short life expectance, and not a red dwarf with little light and phase-locked planets in close. The planet must have gravity near the gravity of the home planet, and the metallicity of the solar system must be high enough so there are elemental resources in sufficient quantities available near the surface of the clone world. Climate, specifically maximum wind speeds and ice coverage must be below some limits, and there would need to be oceans to moderate temperature swings. Eccentricity cannot be too much or seasons will be too extreme, and axial tilt has to be in some middle range. There needs to be a magnetosphere to protect the atmosphere itself and any land-living lifeforms from solar flux. The planet has to have settled down tectonically, so that there are not basalt floods erupting any more, or at least only in the long-term future; nor can there be too high a level of earthquake activity.
There cannot be an intelligent alien civilization already there in sufficient numbers to upset the goals of the arriving aliens to populate the planet with their own citizens. How many would be a nuisance is not clear, but numbers in the hundreds of millions would probably be a barrier. Perhaps even hundreds of thousands would be, as the arrivers will come in very small numbers at first.
The new alien civilization will not be coming in ships full of thousands of aliens, as the costs of doing this are very high and are not needed. They have a choice in designing the seed ships: bring a small number of alien adults, probably in hibernation, or a robotic ship able to start the first generation from the ship’s own resources or gathered from the more easily available resources on the planet’s surface. The overall choice of strategy, as well as the innumerable details of design, would be determined by a cost benefit analysis. The goal of the design is to establish a colony of a small number of alien citizens, able to grow into a large population. There would be a high reliability requirement, perhaps 99+% as well as it can be estimated, as sending a second ship would double the expense, and the delay, while increasing reliability is not likely to double the cost of the ship, meaning the size of the payload.
Let’s compare the two alternatives. In the robotic seed scenario, they will have high technology ships, able to create a starting number of alien citizens, and host them on the new planet. A planet with all the favorable attributes listed above should be easy to populate, but there are some simple necessities that will have to be generated from supplies carried on the starship. These include the production of the first generation of aliens, training and educating them, providing them with the necessities of life such as food, water, hygiene, protection, shelter, and certainly more for the youth period of the first generation. What is not carried on the ship must be found on the planet and collected, processed, and prepared for use. If the period of youth for the alien civilization is ten, twenty or thirty years, this means that the ship’s lander will have to provide supplies for that period, and then some more until the new adults can begin to take the load of provisioning the colony on their own. This is a large amount of supplies to have to carry across interstellar distances.
In the hibernation scenario, there would still have to be high technology ships, but after landing, there would be a shorter time in which the ship’s robotics would have to provide sustenance, and everything else needed, to the arriving alien adults. The landing site would have to be in an area where they could obtain the necessary resources to preserve the population. There would have to be easily obtainable materials to build basic infrastructure, such as shelter. Versatile manufacturing equipment on the lander would have to be used for years, and therefore must be highly reliable, but even so, input materials will have to be found on the planetary surface.
The landing site may have to be selected from the arriving seed or hibernation ship, if it was not done by the probe. There is a clear tradeoff between what instruments the probe had to carry, and what instruments not related to colony formation the initial colonization ship would have to bring along.
As an initial evaluation, it would seem that a seed ship would require much more payload than a hibernation ship. The weight needed to carry a small number of adult passengers and keep them alive for the voyage time, and then awaken them is certainly more than it would take for an incubation system for producing new citizens from basic organic materials. However, the supplies needed to raise the young citizens for a period of years would probably outweigh by many times the hibernation equipment.
A third alternative exists. Instead of having hibernating adult citizens, dormant robotics could be the main weight of the initial colonization ship, and then the robots on board could build some infrastructure that would reduce the amount of supplies the ship and its lander would have to carry for the purpose of sustaining the new young citizens during the period of their youth.
In order to better understand the tradeoffs between seed ships, hibernation ships, and robotic ships, some more details of colony building would have to be investigated, and perhaps some very rough estimates of weights and times done.
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