Let’s talk about the time scales of colonization. If the aliens who are looking around for a planet to start a clone of their own world, like category zero and category 1a do, with some important differences, they are seeking an almost habitable planet, a sweet spot planet in this blog’s parlance. It is like an empty house in good condition. You just move in. There is some difficulty about there being air to breathe, and we assume that aliens who move to the stellar traveler level breathe oxygen. Alternatives to that assumption will be discussed separately. Everybody knows that Earth wasn’t born with an oxygen atmosphere, the chemotrophs evolved a version of chlorophyll that produced it, and then it took about a hundred million years to change the atmosphere.
That’s too long to wait. One solution for the aliens is to look for a planet which has made the chlorophyll oxygen transition, and take it over, provided other conditions are right. Category zero sees this as a gift to them, and they proceed immediately. Category 1a sees it as a gift only if no intelligent life form is already there, and they proceed immediately on those which have none. Category zero would like gravity to be close to that of the home world, but category 1a would not mind modifying their stockiness to cope with a somewhat wider range.
The other solution is to add oxygen. Obviously this is not something that can be brought along in the hold of the starship, but it is an interesting question as to how fast an advanced alien civilization could transform a no-oxygen world to an oxygen world. Suppose they seeded the oceans with a life-form designed for fast growth and to be as efficient at producing oxygen as possible, given that this planet would have no ozone layer protecting the near surface waters from harsh UV, if the sun made a lot of it. Assume the atmosphere has abundant nitrogen, as early Earth had, and the rocks have enough available minerals to support a fast growth.
The atmosphere has about 4 * 10^9 million tons of oxygen. The mass of life on earth is roughly 4 * 10^6 million tons. If each organism produced its own weight of oxygen in a year, this divides out to a thousand years, give or take an order of magnitude. If these numbers relate to early Earth, the obvious question is why did it take so long on Earth to change the atmosphere? Was there a shortage of magnesium in the oceans, limiting the number of molecules of chlorophyll that could form?
If the target world was short of dissolved magnesium could the aliens find enough and increase it sufficiently to meet their thousand year target? There may be a different explanation, as magnesium is a very common element, and is the third most common element in our ocean water. Perhaps that is the result of all the sea life that used it?
The aliens might set a target as 100 thousand million metric tons of biomass, and if chlorophyll molecules occupy 1% of the total, one thousand million tons of just that molecule is needed. The molecular weight of magnesium is about 24 and chlorophyll about 930, so we have about 25 million tons of magnesium. About a million tons of magnesium is mined each year by us on Earth, where we don’t use much of it, so 25 times that is not a big deal. Thus, aliens could probably, over a thousand years, dump enough magnesium into the ocean to replace a shortage.
The conclusion from this hodge-podge of numbers is that aliens transforming a planet’s atmosphere appears feasible, given a millennium time scale. If they had been a civilization for many millennia, or even for millions of years, this is a reasonable project.
Gravity is obviously not something that can be engineered, but what about climate? Average temperature is controlled by the orbital radius of the planet and the output of the star, but it is also affected, mostly in the upwards direction, by greenhouse gases. If the original world was too cold, could the aliens add enough of a greenhouse gas to raise the temperature to something more to their liking? As discussed earlier, the answer is yes, they could banish an ice age. This is in accord with their ability to change the oxygen content of the atmosphere itself. The amount of greenhouse gas needed for a significant shift of temperature is not likely to be as large as the oxygen content, so if there exist biological methods of producing the greenhouse gas, it is likely that the engineering of the temperature could also be done.
What about wind speed? If there are prevailing high winds on the planet, would there be any way in which these could be reduced to a level commensurate with living there? Winds are generated and controlled by the differences in heating of the atmosphere that goes on in different parts of the planet. The differences in heating come from axial tilt, which is not something that might be affected, like gravity is not. However, if the difference in heating came from the difference in albedo, with the polar region being covered with snow and ice, making a climate change with greenhouse gas might mitigate the winds. Melting the ice might also allow large oceans, if they existed on the planet, to begin to flow warm tropical water into the polar regions, again reducing the heating difference. So, winds might be incidentally reduced.
Another aspect of a planet’s favorability is the length of day. Aliens would probably like a day around as long as their own was, but changing the spin rate of a planet is difficult to conceive of. The spin rate of Earth has been changed significantly by its interaction with the moon, but this option again seems beyond the pale for alien engineering.
So, the sweet spot world search includes gravity within a range, temperature tolerable or on the cold side for heating work, wind speed low or likely to be affected by ending an ice age, and day length within a range. Other conditions might exist, and should be thought through.
Does this mean that once we can observe some details of exo-planets, that the handiwork of an alien civilization would be visible? If we see an oxygen atmosphere, there would appear to be no way to tell if it was made by native or seeded organisms. However, if the greenhouse gases were detectable, and they are not of a kind that would occur naturally, with some genetic engineering triumphs, then we might be able to pick out a planet where aliens had colonized it. This may be the only detectable signature of planetary engineering available to us.
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