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.
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