Aliens in Binary Star Systems

Can an alien civilization arise in a binary star system? This is not a relevant follow-on question to the principal one: Why haven't aliens visited us recently? It is one that is relevant to the hunt for alien civilizations from Earth, as if they won't come to us, we'll have to go to them. It is important to build some filters to separate out solar systems where aliens might be found, versus ones where they certainly can't have originated. After an alien civilization has mastered interstellar flight, they could go to any solar system they want, which makes the hunt more challenging, but if we are trying to find ones where they could have originated and specifically not where they might have seeded themselves, we can come up with some sharper criteria. So, could there be an alien home world in a binary star system? We don't want to spend precious telescope time on the impossibilities. First off, even if a binary or multiple solar system has a star which is suitable for origination, a G star like our sun, or sometime close to it, an F or a K star, that doesn't mean there aren't difficulties for life origination. When we see a binary, a physical binary of course not just a visual binary, if the companion star, or one of the companion stars in a multiple system, is a large star, we know that the age of the solar system is too young to have aliens, as these stars do not live very long. For example, even a mid-class F star, like an F5, doesn't last long enough for life, at least if evolution is as slow there as it was on Earth. Thus, both stars must be smaller than about a F7. If the other star is a white dwarf, this is also a bad sign, as white dwarfs are the end-stage of stellar evolution. It means that at some time in the past, they went through the red giant stage, then ejected most of their matter and collapsed to a white dwarf. A planet around a binary companion of this process would likely experience severe disruption, and any life that had originated on that planet would be either terminated or put through some severe extinction processes. While somehow life might re-evolve after this if the white dwarf process had concluded billions of years in the past, it would seem more fruitful to look at binary systems which have not endured the end-stage of stellar life. The next requirement is for stable planetary orbits. Three classes of orbits can exist in a binary system. One is where the two stars are close together, and the planet away from the pair of them by many times the inter-star radius. If you were such a the planet, you would see the two stars at once, circling each other. A second class is one where the planets are around one of the stars, and the other star is far distant beyond any planetary radii. The third is everything else. Your imagination can run wild here, with orbits making figure eight loops or some sort of modified oval around both of them. Clearly the discriminating ratio is the inter-star distance divided by the planetary radius, or for complicated orbuts, the mean distance over a long period of time from the planet to either of the two stars. If this ratio is very small, you have type one, very large, type two, mid-sized, type three. So far, it does not seem there has been a Kepler for type three orbits, and so we don't have a nice classification of them, along with the limits for stability. We hardly have the limits of stability for non-binary solar systems, so this is hardly unexpected. Type three orbits are better left ignored for now, although some computations could be done fairly easily to search to see if there are any weird orbits that are stable in this category. Type one orbits have a different problem. With two stars circling each other instead of a single star, a planet will fell much more of a tidal pull. In other words, two close co-orbiting stars will tend to transfer angular momentum out to the planet much quicker than a single star could. Since angular momentum increases with radius, this means the planets would be driven outward and eventually dispersed. Maybe that would be billions of years, but for life to evolve, a planet needs to be in a near constant orbit for these billions of years. The good-for-life situation is that a stars stays quiet and constant for eons and the planet is in a stable orbit. Alternatively, the planet could slowly drift outwards as the star becomes hotter with age; both of these processes happen quite slowly and fortunately go in the right direction. This matching is not something that would likely work with a type one orbit however. This means that we should look for planets hovering close to the star, meaning also that binaries of interest must be long-period binaries, the hardest to detect. In other words, if we already know a star is part of binary star, it is a poor candidate for an origin-of-life source because we can only identify short-period binaries with our current telescopes. Earth's astronomers have not identified many binary star systems yet, compared to the number of nearby stars, but somehow an estimate has been made that a third or half of all stars are in a binary system. Hopefully for the existence of aliens, these are mostly very distant binary systems. To use Earth as an example, we might have a binary companion star, maybe another F class, at 50 thousand AU, nearly a light year out, and it would not have prevented life from evolving here. At five thousand AU, perhaps it would have, and there is some boundary of influence that remains to be calculated, once we actually figure out how life originated, that is. To do a better job at identifying binary star systems in the neighborhood of our sun, we need bigger telescopes. Perhaps a verey large one at an Earth Lagrangian point could be used to develop btter parallax readings on nearby stars to get their distances and proper motions more exactly. One out at a Saturn Lagrangian point would be even better. There is little hope in simply watching far-separated stars to see if they circle on another. The type of orbits we are looking for, where a planet can be safe to originate life, means the two stars circle with orbital periods of the order of a million years. This is the limit of permanent connection. Stars cannot be in binaries at several light years distance from one another, as other passing stars will exert too much influence and destroy the orbital containment. So, distances of a tenth to a half of a light year are what to look for in a binary system where aliens can peacefully live and develop their civilization and hopefully star travel.