Planetary Configurations and Stellar Reflection

Tides are interesting things. Tidal force transfers angular momentum from a central body to something orbiting it. If the central body is spinning faster, it adds angular momentum to the orbiting body, moving it gradually outward. Earth's moon is slowly receding from the Earth, as Earth's day is much shorter than the orbital cycle of the moon. If a planet is being driven in toward its star by interactions with other planets further out, there may come a point where the planet is gaining as much angular momentum from the star as it is losing to the outer planets. It is undergoing what might be called a reflection, although this situation has nothing to do with the reflection of light from matter. The sun, at the equator, is rotating about four times faster than Mercury in its orbit, meaning Mercury, from solar tides, is being pushed outward. 

If a planet is at the point of stellar reflection, it is still receiving angular momentum from the sun, and passing it along to the outer planets, who do move outwards. Once they have done this for a while, their effect on the innermost planet will diminish, allowing the tidal force from the star to move this planet outward as well. The innermost planet is temporarily acting as a conduit for stellar angular momentum.

Among the thousands of planets which have been detected by the various astronomical instruments, there are some groups which revolve around the same star, making up exo-solar systems. A few have multiple planets and it might be thought this can show us something about the various patterns that different solar systems can take. While this might be interesting, the important thing is that these distant solar systems might not be stable of long periods, commensurate with the age of the solar system, but might be in the process of slowly rearranging themselves, through the swapping of angular momentum between one another and with the parent star.

The location on at least one planet near the star is advantageous is accelerating the approach to stability, or even to make it possible without the ejection of one or more planets. The interaction of the star with a close planet slowly and continuously transfers angular momentum to the planet, which then transfers it to other planets. But this transfer is associated with a transfer of energy as well, whereas interplanetary interactions largely conserve energy. It is likely not possible for a set of unstable planets to find stable orbits without some energy transfer, and so the stellar interaction mediates that. Science should attempt to figure out just how much assistance planets of different masses at close locations to their star help in this regard, and then they might serve as a semaphore for the posssibility of a stable planetary configuration, which means a planet could stay in a liquid water zone for long enough to evolve land life and maybe even an intelligent alien. Otherwise, there is simply no opportunity.

Earth has been stable in its orbital location for the billions of years of its existence, as otherwise evolution could not have occurred. Evolution from pre-cells to now lasted three to four billion years, and this would have been terminated had the Earth been outside the liquid water zone during the first portion of this period. It can certainly have wandered around inside it, as any planet in a resonant orbit relative to its planetary neighbors would, but the wandering has to be limited in extent.

This alludes to the main point of the search for life via the detection of exo-planets in the habitable zone. They need to have been there for a long, long time, meaning that only long-term stable configurations of planets need to be extensively investigated. A planet which has sat in the liquid water zone, even assuming everything else was optimal, for a hundred million years would not have recognizable life on it. Thus, stability of planetary configurations should be the first thing that is investigated. Luckily enough, that can be investigated without any need for a giant telescope or other astronomical instruments. It simply needs a mountain of computation, or some brilliant theory which obviates the need for patterns to emerge from the data. The brilliant theory could be checked in much less computational time than would be used for an exhaustive search over all possible combinations of planets and their parameters.

The very long time needed for evolution has two effects, a bad and a good one. The bad one is that if we search the sky for exo-planets with a new generation of telescope, one which does not have such strong selectivity effects for close-in planets or ones whose orbital plane lays within a very narrow band, and therefore comes up with thousands of right-sized exo-planets in liquid water zones, we might have to throw ninety percent of them out immediately. These would be the ones where the planet was just passing through the liquid water zone on its way to a more stable orbit further out from the star, or maybe toward the stellar reflection radius. Motion in a not-quite-stable planetary system might take millions of years or even more to occur, and thus finding some planet in the right location might mea absolutely nothing at all.

The good thing is that, in rare instances, planetary radial drift can be just what a planet needs to keep it in the liquid water zone. Hotter stars evolve faster, and an otherwise just perfect planet might find the liquid water zone moving away from it long before it evolved life. But if there was radial drift going on at the same time, the planet, with a large dose of good fortune, might find itself staying within that zone, even as the zone moved from the effect of stellar aging. So, a slightly large class of stellar spectral types can be searched to find planets that might have alien civilizations.

Ice Ages and Alien Civilizations

While it is true that the location of non-habitable planets doesn't matter much in a solar system where a possible planet for an alien civilization exists, there is a significant exception: Climate effects. When a planet co-exists in a solar system with other planets, they exchange angular momentum, which translates into a change in the semi-major axis of rotation. This means that in a stable, resonant planetary system, planets will slowly drift in and away from the star, not very far, but far enough to possibly trigger climate change. And climate change caused by this oscillation in average radius may be almost negligible, or it may be near a threshold where an instability in the climate system on the planet flips from one condition to another.

The obvious one is Ice Ages, but it is not the only one. It is, however, an easy example to discuss. Ice Ages happen because the absorbed solar energy is less because the average rotational radius from the star, averaged over a hundred thousand years or so, is a bit larger than it had been before the Ice Age started, plus there are important feedback effects. Energy absorption depends on the amount of energy emitted and its spectrum, the reflection and absorption in the atmosphere of the planet, and the fraction absorbed by the surface. 

Feedback effects come from the average albedo of the planet, which measures the fraction of energy reflected. When the planet gets a little colder, ice forms, which has a high reflective coefficient for visible light, where much of the energy in a mid-level star exists. A little ice means less absorption, and a colder planet, and more ice – starting the feedback loop. The final state of this loop is the peak of an Ice Age. This final point might be a saturation point, where almost all the available water is frozen, or where the convection of heat away from the warmer equator is insufficient to cool the equatorial regions down to freezing. There might also be a timing question, where the time necessary to freeze the planet is of the same order as the time for orbital change.

Consider a second example. The second feedback effect is much more difficult to detect eons later and has been given no name at all by Earth scientists, and may not be recognized as such. It is when the albedo of a planet is increased not by snow and ice, but by cloud cover. Clouds also reflect solar energy more than rock or other terrain, and once they begin to cool a planet, a “Cloud Age” might occur. Clouds occur at altitudes where there is sufficient moisture, but insufficient heat. High clouds do not lead to rain as low clouds do, but they do reflect well. Unfortunately for science, clouds leave little geological record.

Atmospheres with different compositions may have different types of clouds, and these may also have albedo effects. Atmospheric compositions on exo-planet has not been thoroughly examined, but it seems likely that some constituents might be present in an atmosphere and still have life supported.

Quite naturally, the opposite to an Ice Age might occur when the planet gets shunted in a bit closer to the star, all the ice having been long ago melted and all the high-altitude clouds dissipated. Life starts dying because of thermal effects, and when animal life is migratory but vegetation is not. If vegetation dies off over large areas, one area, a desert forms and less carbon dioxide is recycled back to oxygen. With more carbon dioxide in the atmosphere, heat is trapped near the surface, raising the surface temperature more, killing off more vegetation. Good numbers for this effect might well show it is much less powerful that the other two.

The effects that an Ice Age, and possibly a 'Cloud Age' and a 'Heat Age', to coin a term, would have on an emerging alien civilization are evident. These phenomena would lead to a large reduction in the surface area able to support vegetative life. If a civilization was still dependent on agriculture for its nutrition, or if there was a pre-civilization, perhaps at the pre-city stage, it could be doomed. For the Ice or Cloud Ages, if there was some small area near the equator still not completely blocked off, the numbers of survivors would be limited, and civilization would be stymied. Ice ages on Earth have lasted millions of years, and no civilization could be expected to survive this. Heat Ages would force population to the poles.

But the time scales are not commensurate. Civilizations can be estimated to last of the order of a million years, meaning that they could come into existence during the interregnum of a climate extreme survive, flourish, come to visit Earth if they wish and find it possible, and then disappear before the next extreme starts. These epochs are controlled by the interactions of planets, shuffling angular momentum between one another and slightly shifting their orbital radii. A question which might be answered in short order is: In which type of configuration of planets would there be periods this long between these catastrophic ages, and in which type of configurations, if any, would they be too short to permit a civilization to develop? Knowing how to translate the duration between climate extremes to planetary configurations would immediately tell us which configurations are worth examining in great detail on our hunt for alien life.

These types of calculations can be done now, but there is no clear understanding yet of what to look for. This means that large numbers of scenarios would have to be computed, until a pattern emerged, and the ensemble of possibilities could be winnowed down. Maybe the key variables involve the existence of two gas giants in some particular relationship, orbit-wise, and once this is established, other planetary resonances correspondingly exist and can hold planets. If one of these resonance bands is of the right thermal characteristics, and a potential host of other variables are within some ranges, we might be able to say that some particular planet, out of the millions which will be detected over the next century or two, are 'life-possibles', and deserve a great deal of specialized attention. 

The Formation of Solar Systems

The basic outline of the formation of planetary systems is well-known. A cloud of mostly hydrogen gas is rotating, and subject to its own gravity, it starts to condense. It may not be spherical, but nonetheless, there will be a point of maximum condensation, caused not by gravity at first, but by the pressure generated by the overall gravity pulling everything together. This maximum point will start attracting gas near it and a steadily shrinking ball of gas forms. The center point of the ball will be the densest location, and temperature will mount due to compression happening faster than the ball can cool itself, which happens when the ball becomes dense enough to become opaque. Temperature and density continue to rise, and finally thermonuclear ignition thresholds are passed, and it begins to fuse at the center. Gravity doesn’t stop, and neither does condensation, and larger layers pass the ignition point. At this point, there is a star.

There is a direction of cloud angular momentum, meaning that in general, the cloud is rotating around this axis. Centrifugal force pushes the cloud out, especially near the plane intersecting the new star, while the lack of that force in axial directions leads to a compression there. A disk of gas forms, and it becomes thinner with time. On a tiny scale, condensation into molecules and dust particles happens in the disk, and if the star is large enough, a solar wind begins and blows lighter molecules out further, leading to a partial differentiation of materials in the disk.

The disk is unstable to ring formation, so instead of a smooth disk, with time some dust and gas rings develop, and they continue the condensation process. The particle size increases in the inner part of the disk, and in the outermost part, ices form into particles as well. These particles congeal, and in the middle region, blobs of gas start to form, as the rings are unstable to planetoid formation.

At this point, resonant interactions start to form, and the dominant gravity will be from the densest part of the disk, which is somewhere in the middle. The gas blobs may be in resonance with each other, or anti-resonance. If anti-resonance happens to be the situation in a solar system, the innermost gas blob, now turning into a planet, loses angular moment to the outermost one, and they drive in and out respectively. If a large gas giant planet is driven into near the star, passing by the planets and planetoids between its original formation radius and the star, they will be strongly perturbed and may wind up anywhere in the system. Similar things happen with a large gas giant which is driven outward. The ice giant planets will be scattered and can wind up anywhere. If they go inward far enough, they will lose some of their mass from thermal effects.

The alternate situation, where there is only one large gas giant or two of them fortunately in resonant orbits relative to one another, the solar system is divided into bands, being resonant or anti-resonant. Two gas giants in this situation will exchange angular momentum between each other, but not secularly, only with a to-and-fro situation which keeps both of them near their resonant band’s centers. Smaller planets inside and outside of this which are in antiresonant bands will be scattered out, winding up almost anywhere, while ones in resonant bands will simply engage in moving around within the resonant band. Two of them in the same resonant band will result in one expelling the other, or a violent merger will happen. These types of collisions occur with much less relative velocity than any other interaction in the solar system, as two planets in one resonant band are moving with close to the same orbital speed, not much eccentricity, and a seeming repellant effect. When one of the two planets comes close to the other, it speeds up, changing its orbital parameters, and may pass by the other without doing more than being distorted. This can continue to happen until the two of them get closer and closer in orbital parameters, when a merger finally happens with minimal velocity of impact.

The existence of resonant bands and antiresonant interactions helps to explain why the solar system zoo which we are gradually discovering is so diverse. Rings condense with no resonant interaction, just anywhere, depending on the radial structure of the gas and dust disk, but once they condense, they become subject to another instability leading to planetoid condensation. They might be in resonance with some other planet or anti-resonance, meaning they can travel in radius from the star, ending up in some strange place. This happening with many planets at once can lead to the lack of clear patterns of solar system planet location.

What does this mean about where to best look for aliens? The time scale for planetary rearrangement should be relatively short compared to the time needed for evolution of cells, so if a suitable planet, with the right gravity, atmosphere, and composition, shows up in a thermally favorable location, having other planets in the solar system in strange locations should not affect it. It might be that a solar system which has gone through a period where antiresonant effects took place would have many less planets, so the numbers might be against a planet holding an alien civilization, but if one exists, the other planets should not prevent origination and evolution of life. They will, of course, perturb the orbit over millions of years, but antiresonant effects should be over and the perturbation will be back-and-forth, simply creating a bit more interesting planet to evolve upon.

If there was a veryhot Jupiter and a very cold Saturn in some solar system, and one nice habitable planet in the middle with all the stuff needed to originate life, a civilization arising there might have no interest at all in interplanetary travel, which is the learning stage for interstellar travel, assuming it is possible at all. It is actually quite entertaining to try and think of what life might be like in an alien civilization in the variety of solar systems we are now discovering, bit by bit.