The previous post started a discussion of how an alien civilization might voluntarily choose to abandon its home planet, not to seek a new one, but to live in space only, upon some starships. Just for a moment, consider that this was a logical and reasonable decision, and that all alien civilizations that get past asymptotic technology think about it and eventually decide to do it. It makes the hunt for aliens a lot different than if they were all like us, living on their home worlds and simply exploring and colonizing nearby exo-planets.
There are two currents in our society concerned with alien life. One is concerned with the origination of life on other planets, although the same considerations apply to seeded life. This current involves astronomers trying to figure out, with the help of other specialists, how to detect signs of life itself on other planets. This is the concept of biosignatures. The simple one is the chlorophyll signature, oxygen in the atmosphere. This blog has discussed the division of planets which harbor life into those which are solely full of chemotrophs and those where some surrogate of chlorophyll has taken advantage of the energy source provided by the solar photons that stream down on any orbiting planet. Chlorophyll produces oxygen, and some similar compounds may as well, and some more primitive photon-absorbers do not. We don’t seem to have the chemical knowledge yet to predict all of them and compute the relative advantages, but synthetic organic chemistry will one day get to this stage, and then we can more clearly define what the utility is of relying on oxygen in the atmosphere as a signature of photosynthetic life. For now, we look for oxygen, and it could be said the first current of alien life investigation is trying to find evidence of chlorophyll. These are the exo-plant hunters, who look at exo-planets in the habitable zone for indications of oxygen.
The other current isn’t hunting the other kingdoms, like fungi or animals. There doesn’t seem to be any biosignature arising from them that can be developed with current or near-term observational equipment. So the plant kingdom gets special attention. The other current is looking for intelli-signatures, to coin a word to mean something that can only arise from intelligent organisms. The hoary one is listening for some electromagnetic signal that sufficiently departs from natural sources that it can be seized upon as evidence of intelligent life. Another one is looking for evidence of very large structures, or in general, any gigantic engineering projects. Being so far from capable of doing such things, we don’t have much of a list to go on, and not much details for those things which are candidates for being on the list. Perhaps someday in the not too distant future we would be able to look for terra-forming, i.e., a planet which looks habitable from a temperature point of view, but shouldn’t for some reason. Or we might look for planets with climate control, which would require even finer observational equipment.
All this is rendered somewhat OBE, overcome by events, if the hypothetical assumption made at the outset of this post is true. A civilization will stay on a planet only for a few millennia, during which time it develops asymptotic technology and makes the decision to bid the planet goodbye, and become interstellar nomads. Ten millennia is such a short time to inhabit a planet, and an even shorter part of that time is when the planet might be emanating some electromagnetic radiation that could be detected. Once star travel is possible, the planet shuts down and turns the lights, and radios, off.
Our search for electromagnetics can only be successful with a nearby solar system having a transmitter, as the r-squared losses mount up and make any emanation from a distant planet too deep in the generic noise of the galaxy to be detected. Say over the lifetime of the galaxy there is a uniform probability of life originating on a habitable planet, and for the sake of the argument, assume it is high. Using ten billion years as the age of the galaxy, an intelligent alien civilization might emanate some EM transmission for a thousand, which is one ten-millionth of the total age. After that they are cruising the stars. So we would have a ten-millionth chance of listening when they are on, and maybe we could double that if we listen for a whole millennia ourselves. If there are a thousand habitable stars within the detection range of our best possible listening apparatus, whatever that might be, we would be up to two ten-thousands chance of hearing something.
So the concept of interstellar nomads, if it proves to have any merit, would essentially doom such an attempt at intelli-signature detection, at least of the type initially thought of. As for finding planetary changes, such as climate modification or terra-forming, well, they just aren’t doing that. They could care less what happens to planets, except insofar as they would be good for mining or energy sources. Building some large structure near a star? Also not interesting.
So it is worthwhile trying to determine if there could be any motivation that we have not yet appreciated for becoming interstellar nomads, as it conditions how we search for intelligent life. We would want to know if it is reasonable, and then what signatures, if any, there might be from a starship either making its way between stars, or hanging around some solar system, refueling and replenishing stocks of materials lost in the recycling process.
We don’t understand quite yet how useful it would be to mine asteroids. For an interstellar nomad ship, dropping down into the gravity well might be the last thing they would want to do, and so they would be most interested in those solar systems which have lots of the type of asteroids with things they have the largest losses of. Cleaned up solar systems might not be worth visiting, as the task of going from one to another demands a lot of resources as well as energy. One question, then, would be in what type of solar system would we be likely to find such a ship? At least then we would know where to look, provided we could develop the technology to find such systems. Right now, we can find larger planets and large dust disks. Finding small asteroids might be well beyond our capability now and in the near future. What to do?
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