If an alien civilization wants to do colonization, there is really no substitute for an alpha-habitable world. This has all the things already completed that are necessary for the alien civilization to simply move in and get started resurrecting its culture from scratch. Alpha-habitable means life has started there, and that presumes that the planet is not in the throes of basaltic flooding, or suffering from intense planetesimal bombardment, or has intense stellar radiation cleansing the surface of life. It means the atmosphere is okay. It means gravity is in the right range, and planetary parameters are just fine. The star type is right. Everything is there, except for the aliens. It is like a planet with a red carpet laid out for respected guests to arrive.
But, like any esteemed establishment, you have to get there at the right time. There is a window of opportunity. Getting an idea of how broad that is can be done by examining the progress of life on Earth, our only example.
The planet formed about four gigayears ago, and the first bacterial fossils that can be confirmed come from about three gigayears ago. Some time before that life originated, perhaps first chemical life and then biological life. Part of this gigayear was devoted to the evolution of the first biological cells, and an earlier part was devoted to the reformation of the planet by impact. Still, coming this early is not a good idea, as there is no oxygen before about two and a half gigayears ago. It took this long for photosynthesis to originate and evolve, and then become so prevalent and prolific that it could change the entire atmosphere. It would be possible to migrate after oxygen is established, but the whole land surface is just rocks with maybe some sand. Still not a hospitable planet to move to.
Cells keep evolving, and we get eukaryotes and multicellular creatures, but all in the oceans. There are animals in the oceans by 600 megayears back, and the first entry onto land starts about 450 megayears back. This was likely some fungus, and it needed to evolve a lot before living on land was common. Eventually it happened, and plants began spreading over the surface. There were the first animals, the predecessors of the dinosaurs, at about 300 megayears back. Insects were there by then as well.
Dinosaurs themselves arrive around 200 megayears ago, meaning there would be meat to eat, if the colonists wanted to do some hunting, and forests and jungles as well. Mammals would be present by 200 megayears ago, but only small ones. Sometime later birds arrive.
Earth may be a special case, as there was a major extinction event, one of many in its history, about 66 megayears ago, which is when the dinosaurs bid Earth goodbye. Since then, mammals have dominated the surface of the planet. On another planet, there might not be such an extinction event, such as might be caused by a major impact or a basaltic flood, meaning that mammals might have to out-evolve the dinosaurs, or perhaps simply continue to co-exist with them. Climate change was undoubtedly happening, as the Earth went through multiple ice ages separated by warm periods when there was no ice at all, even at the poles. This may have had an effect on the competition between mammals and dinosaurs.
We can now see how short a colonization window there is. From the dawn of the Earth, however you define it, maybe 4 gigayears ago, a colonist hopeful would have to wait at least until about 200 megayears ago, which is only five percent of its lifetime. The window closes about now, as within a few hundred more years, humans will possibly have developed their technology enough to have the planet pretty much sewn up. Whether or not an alien civilization would simply displace an existing intelligent species is something to be discussed further.
When we were comparing how many alpha-habitable worlds there are, little account was taken of the length of time in preparation there would be. Out of a thousand worlds that are destined to become alpha-habitable, most would still be in the earlier stages of change, so that they wouldn't be alpha-habitable at all at any snapshot in time. The numbers that are actually alpha-habitable, are less than five percent of the total.
On the other side, if the planet is alpha-habitable, how long does it stay that way? Perhaps life just goes on and on, waiting for some extraterrestial colonist ship to come its way, and it could last this way for a gigayear or more. On the other hand, each year that passes might mean that an animal species starts to use tools and communicate, the two hallmarks of beginning intelligence. It could very well be that instead of talking about the probability that an alpha-habitable planet develops intelligence, the mean time to develop intelligence after the emergence of mammals might be a better way to quantitize this. Perhaps half develop intelligence 200 megayears after mammals emerge, and three-quarters after 300 megayears, and seven-eighths by 400 megayears. This assumes there is no condition on most planets that prevents intelligence from appearing, and although this subject has not been thoroughly explored, none has appeared.
With this way of looking at things, a 200 megayear window is about all that an alien civilization could expect, and they could be unpleasantly surprised on some planets to find it happens after only 100 megayears. With a window this small, the number of alpha-habitable planets, without life, is much smaller, that previously estimated. Instead of a thousand, there might only be fifty or a hundred when the first alien civilization in the galaxy is about to start hunting for a colonization site.
During the million year reign of the alien civilization on its first colonized solar system, there might be one of the hundred or so alpha-habitables that begins the ascent to intelligence and a second alien civilization. Before it uses up the hundred, there might be a fifty percent chance this would happen. The second alien civilization would find almost no worlds for them to colonize, thanks to the first alien civilization's diligent efforts to vacuum the galaxy of potential sites.
While the numbers tossed around here can certainly be altered, it appears that the galaxy is home to former homes of great alien civilization, now inhabited by no one at all, or by a post-golden age civilization that relies on sustainable energy and materials. Few pristine planets for colonization present themselves and those that do would be scooped up early in the possible window for colonization.
If this is the situation, it provides a shifting ground for answering the original question of this blog, where are all the aliens. The answer might just be they are on their home solar systems, some being solo worlds where intelligent life originated and formed civilizations and others being ones which were colonized. All of them are out of resouces and are making due with what solar power they can utilize, along with non-scarce resources. None have the capability any more of doing any more colonization. In this picture, Earth escaped being colonized because its window of colonization opened too late for any nearby civilizations to take advantage of it.
No comments:
Post a Comment