One set of things we might look for in the skies as evidence of aliens are starships. Seeing one would answer our questions about are we alone in the Sagittarius Arm or are there other life-forms here. It would tell us that intelligence is not our prerogative and somewhere else under probably far different conditions it evolved. It would tell us that star travel is possible, should we choose to do it. Even if we determine that we can detect the signatures of biological life, we might not be able to detect the signatures of an advanced alien civilization that lives buttoned up in hermetic cities. And even if we surmount that obstacle, knowing if star travel is possible would be a tremendous triumph in its own right.
Previous blogs have already addressed two motives that the alien civilization might have for sending up not one, but multiple starships to cross the galaxy to another planet. One is a scarcity planet. This is a planet that has little resources, just barely enough for an intelligent civilization to evolve and learn science. Life would be hard there compared to many other planets, and the dominant theme of the planet would be getting the hell out of here. A planet just beyond the Scarcity Great Filter line might only be able to afford the resources to build one ship, once they advanced enough to be able to do that with confidence that it would arrive at its destination. A little bit further beyond the filter line, and they might be able to send several.
The civilization might allow its population on the home world to diminish, move everyone out of one of the cities, and then salvage its resources to build the next ship. Desperate civilizations do desperate things. After that they would keep going after that until the salvage process ran out of momentum. So there might be a line of starships, leaving the scarcity planet, all heading to a new home. They might be on a voyage of a thousand years, separated by twenty-five years each.
The other motive is stellar death, or some other galactic peril, that would force the evacuation of the planet. It could be a beautiful home with all the comforts an advanced civilization could offer its members, but when the volcanism starts, it’s time to leave. With advanced notice of the peril obtained by their advanced scientific abilities, they too would be sending ship after ship, spaced out in time, heading somewhere in the galaxy where the perils are not present and the planet’s geology is quiescent.
Here on Earth, if we build the right instruments, we might be able to tell which planet is short of resources and which planet has just had something very unpleasant happen. In short, we might be able to deduce the origin planet of the line of starships, which would give us a head start in finding them based on their own signatures. It would be even easier if we knew where they were going. A line from planet A to planet B is pretty simple to scan, as compared to an expanding volume around planet A. So, where would they go?
By the time we are able to detect planetary distress or the type of planet and therefore its likely resources, we can also tell a lot about other planets near the origin planet. Frankly, some planets are losers, and we can simply strike them off the list as possible candidates.
We have already made some baby steps in finding loser planets. Knowing the mass tells us something about the gravity, and too much or too little means the alien civilization would either get squashed or bounce too much when they walked. Detecting the atmosphere, the type and depth, is on the near horizon, and a yuccy atmosphere is sure to put a planet on the losers list. The list would also contain all planets which are not stable, or which have such inherent high wind velocities that life there would be impossible. There are likely several more criteria which we might just be able to detect.
Of course, being in the habitable zone is a definite requirement, as without being habitable, it is not going to get inhabited. Recall that the ships from the scarcity zone are looking for a place to settle, not a place to terraform. They don’t have the resources for this. The ships from the planet in distress likely did not have the time to prepare the massive amount of supplies and equipment needed to do this, and neither of them wants to hang around in orbit for millions of years waiting for the planet to improve.
What they mean by habitable is not necessarily what we mean by habitable. If the origin planet has methane for an atmosphere and ethane seas, they will be setting forth to another one like this, which is probably well beyond what we think of as the habitable zone. But if they come from something in our type of habitable zone, then our search for their destination planet should be restricted to this type. We probably by this time already would have an extensive catalog of planets in the habitable zone, along with information about their atmospheres, the type of rock they might have on the surface, and hopefully, lots of other very interesting information.
The travelers certainly have observational instruments far in advance of ours, and knowledge of planetary everything far better than ours. If we can see a biosignature, they can too. If we can simply detect the biosignature, they might be able to obtain much more information about what was living on the planet, and feed that into their selection process for a target planet. As noted in another post, alien civilizations probably do not have anything like a reverence for life, but they assuredly do have a reverence for their own life. They would not head toward a planet with an advanced civilization, bristling with defensive weapons, which could shoo them away. Heading for a shoot-out is not the most efficient way to pack up your civilization and move. But anything lower down the scale from a civilization of the same capability as theirs, ranging from bugs up to semi-intelligent creatures, might be fair game. Desperate civilizations do desperate things. If a few squirts of a lethal virus would make the planet more hospitable, perhaps that’s what they would do.
We can use the information discussed here to eliminate the loser planets they would not head to, and possibly narrow down the list a bit further to perhaps a few, or anyway not too many that would overwhelm our instrumental time, and then search the connecting lines for the emigration train. It would be a grand celebration here on Earth if we answered our long-held questions about intelligent life and stellar travel by detecting such travel in progress. Unless, of course, what we detected was the nose cones.
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