It would be nice to know just what conditions a solo planet actually has to have. Recall that we use the term solo planet for one in which life can originate without being seeded by an alien civilization. It is not the same as a habitable planet, which is one on which life can exist in some way. There are grades of habitability, ranging from ones where the aliens who migrate there can simply drop in and live on the surface, all the way down to ones where they could only live underground in sealed chambers. There are also worlds which can be transformed by an alien civilization. These worlds have their own conditions, typically less rigid than those of a solo world.
Recall also that solo worlds come in gradations as well. They all have the conditions in which simple microbes could form, and the more fortunate ones eventually get to evolving chlorophyll or a surrogate, and then probably land animals, and finally intelligent something-or-others. At this point in astronomy on Earth, most scientists simply use surface temperature as a measurement for ‘habitability’, but this is because we don’t understand much about the origin of life, so we can’t come up with conditions for a solo world, and we don’t understand all that much about the evolution of life, so we can’t come up with conditions for the germination of intelligent life.
This post speculates about both of these in connection with two orbital parameters, axial tilt and rotation rate. Axial tilt is of course the angle between the orbital plane and the rotational plane. On Earth, axial tilt is about 22 degrees, which makes life even more fun here. We have two polar areas where there is no sunlight during part of the year, as well as a tropical zone around the equator where the sun is directly overhead at least once a year.
Consider three alternatives. Suppose the axial tilt is zero, meaning that the two planes are completely aligned. As a second alternative, consider the axial tilt is ninety degrees, meaning they are perpendicular, and as a third, something in the middle, which is where Earth is.
When the axial tilt is near zero, there are no seasons except for the changes caused by the eccentricity of the orbit. If it is nearly circular, as are most orbits we know of, there are no seasons and the year is pretty much the same. There is almost nothing to designate a year. If you were sitting on the planet, it would be hard to tell when a year had passed. There is nothing about the hypothetical sequence of events that were discussed earlier that depend on seasonality. So the question is if there are any second order effects which would affect the origination sequence.
Unfortunately for us, the root causes for tectonic activity are not known. A star exerts a gravitational gradient on the planets that would fluctuate with the passage of a day, but if the day is short, there would be an averaging effect and the daily fluctuations of the gradient would be washed out. Having an elliptical orbit would add to this effect, as would axial tilt. It might also be that even the planet’s year is too short to allow any stellar gravitational gradient to have an effect on magma flow or the separation of different minerals in the lower part of the planet’s crust. These are the phenomena which might be implicated in the generation of rich mineral flows into the planet’s oceans, which is one of the conditions for life origination.
Another condition for the origination of life, specifically the earliest form of a membrane with some DNA inside, is a soup of amino acids, which would, after an eon of time, fit together to make a self-replicating chemical. Amino acids are created everywhere, including interstellar clouds. What creates them in larger numbers. One hypothesis is that lightning leads to random chemical links between carbon and other biologically important elements, and this produces some amino acids. Lightning comes from cloud formation, and this might not occur with anything like the amount on earth if the planet had no axial tilt and instead was extremely stable in heat flux. So, perhaps, planets with zero axial tilt have lower probabilities of formation of amino acids and lower occurrence of geothermal eruptions in the seabed, and therefore the probability of the origination of life there, defined as self-replicating cells, is much lower, and perhaps so low that it would not happen in the age of the universe or the planet.
The other extreme is a ninety degree axial tilt, and in that situation, one pole would receive intense stellar thermal flux for half a year, and then receive none at all for half a year. This is a recipe for the build up of ice and snow and eventually permanent ice cover for both poles. The feedback effect of ice and snow having a very high albedo tends to perpetuate the cover, and both poles would develop this over time, if other conditions were conducive to it. If the planet were too close to the star, it could not happen, for example. But having axial tilt at ninety degrees or close to it would lead to more glaciation than other axial tilts, and therefore less areas in which life could form. The slow variation of heating would also tend to make the atmospheric flow more stable, meaning perhaps less lightning.
Thus, an axial tilt away from both extremes seems likely to promote the origination of life, if certain conditions are correct and if the hypothesis forwarded previously is close to correct. Rotation rate is another matter. For planets with moderate axial tilt, say from ten degrees to sixty degrees, having a very slow rotation rate would lead to more stability, less atmospheric activity, less lightning, less amino acid concentration, and lower probability of the formation of life. If the rotation rate was very high, of the order of an hour, there would be an averaging effect and the same results might be expected.
Thus, having a moderate axial tilt and a rotation rate that is slower than the time taken for major atmospheric events to form, yet significantly faster than a year, would tend to make the probability of the initial origination of life more likely. Life origination is a gamble, and none of these extreme cases can be excluded. Only if a large number of solo planets were discovered could any statistics be generated.
Once life originates, evolution takes over, and the process of generating ever more complex life forms begins. It is not clear why any of the extreme cases would be less favored for any stage of life, with the exception that a highly glaciated planet would have less area in which it could happen. Even the generation of chlorophyll seems to be independent of these two parameters. The emergence of life from the seas onto land surfaces seems to be independent as well. There is no apparent bar on the emergence of trees, which have figured into the emergence of grasping extremities from the two parameters. Thus, once a planet forms the original cells, most planets with appropriate temperatures would be able to evolve higher life forms, and perhaps lead to an alien civilization that can travel the stars.
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