If planet-bound fusion proves to be impossible, every alien civilization has the same problem: how to power their civilization. Unless that is solved, the civilization will not be able to rise up to asymptotic technology or determine how to best travel from solar system to solar system. What are their alternatives?
The options we on Earth have thought of might not be a complete set, but they do provide an excellent starting point. There is an extraction of energy from solar photons striking the Earth, either by transforming them into heat or electricity directly, or by gathering up the energy as it diffuses into other modes of energy storage, such as in rain by capturing it at some elevation and combining it with gravity to make electricity, or in fluid motion, either the atmosphere or the ocean. Combustible materials, such as fossil fuels or vegetation are another way solar photons are accumulated. Precipitated methane may exist on the planet, with origin unknown. Flying some sort of direct photon capture in space near their home planet or in solar orbit might be an option as well, depending on how far they get with technology before they need to do this.
Gravitational energy is another option, either via the thermal energy stored and still present from the condensation of the planet itself or via the tidal interactions of a large satellite. No one on Earth seems to think much about the gravity between the sun and the planets and planetesimals, but there is energy there. Disturbing planetary orbits might be bad for the stability of their home planet in the very long-term, but perhaps something which further circularized the orbits of some gas giants might be something they would consider.
We on Earth had a coincidence of the invention of electricity and a mass expansion in the use of fossil fuels, and this seems to have resulted in the movement of electricity around the planet from source point to use point. Perhaps a difference in timing might have resulted in Earth using hydrogen as a medium for transporting energy, and alien planets might find this method rather than having metallic wires stretching all over. Hydrogen can be easily generated from water, and combusting it does not produce anything other than water, so there are some good reasons alien planets might use it.
The other possible source of energy is uranium or thorium fission, which may produce part of the heat in the earth, mixed in with the residual heat from the condensation of the planet. Fission can also be done in a specific site, by concentrating these elements, and the assumption that fusion is not feasible, for the purposes of discussion, has nothing to do with the various possibilities that exist for fission. We on Earth are still in the throes of early design of fission plants, having only several decades of work on the subject, but an alien world with centuries of experience would certainly master those aspects which trouble us. These include safety and security of the plants, recycling the fuel efficiently, being able to cycle the plant on and off to cope with changes of demands, and costs, in terms of energy returned for energy invested in the plant.
In order to power an alien civilization, even partially, with nuclear power, there has to be enough uranium and thorium available to them. Is it obvious that all solo planets would have this, or is it possible that some alien worlds would be devoid of much of these two elements? Some elementary astrophysics answers this.
We make the assumption that the beginning of the known universe, back in the time of the formation of galaxies, had the whole mass in the form of hydrogen and helium, and it was the formation of the first generation of starts, which astronomers like to call population III because they were the first to come into existence, that produced the heavier elements. Heavy stars have short lifetimes, so the age of the universe can incorporate many generations of heavy stars, each starting with more heavy elements and each producing even more when they detonate as supernovas. Of course, all stars produce heavier elements, as that is the nature of fusion which powers them, but stars which don’t explode don’t spread out what they have produced nearly as prolifically as supernovas.
Simple nuclear physics tells us that iron and elements around it have the most stable nuclei, measured in terms of energy per nucleon, so if the universe ran down from hydrogen, the least in energy per nucleon, to the state of maximum energy per nucleon, it would be making iron only. Perhaps this will happen tens of billions of years from now, in processes not yet initiated in the universe and not envisioned, but for now and in the time before now, elements heavier than iron are produced in abundance by stars.
This might be thought to be because in a stellar interior of a heavy star, nuclei are all in equilibrium distribution, and heavier elements, being higher energy states, would be populated in some sort of Boltzmann distribution. Perhaps that does exist somewhere, but it is much easier to make heavier-than-iron elements at somewhat lower temperatures, because of the one-way street that exists with nuclei. In a star, with plenty of protons and alpha particles flying around at high energy, iron nuclei get hit with some, and they form a different element, a heavier one. These elements form as isotopes which are stable, or else they emit alpha or beta radiation and become one. Stable isotopes simply last for a long time, and in fact, collect another proton or alpha particle, pushing their atomic number higher. The long chain of stable isotopes that stretches from iron up to uranium is responsible for the accumulation of heavier elements. Then, when the heavy star goes supernova, all this good stuff gets blasted out into space for later generations of stars and planets to pick up. The supernova process may even do some more pumping up of atomic number with all the hot particles involved in it.
So, the generation of elements beyond iron is a certainty, as the stars which generate iron also generate them, admittedly in lesser numbers as the atomic number goes up. The actual numbers depend on the cross-section of the individual isotopic nuclei for the particle energy distribution that exists in the star, which of course depends on the depth. Isotopes change in numbers depending on the input and output to them, which depends on some particular cross-sections for neighboring isotopes. That detail is not very relevant to simply figuring out if alien worlds would have uranium and thorium – they would. We have another case of interstellar convergence, as the laws of physics are the same all over the galaxy. There is an aging question, however.
There might be an additional question of whether the uranium on other planets is buried more deeply than on our planet, or even more shallowly, and would it be in ores of lower or higher concentration. This is a geophysics question. The presence of an element on the crust, in ores of different concentrations, is a question of something like relative solubility. Since elements and simple compounds like to bind with elements and simple compounds of the same type or the same size and, for compounds, shape, ores form, and they form on Earth the same way they would form on other planets of the same size and location in a solar system. Density also plays a dominant role. Iron, nickel, and other elements which like to dissolve in them form heavier density solids or liquids, and sink downward. There is a gradation of density starting with the center of a planet. Thus, what is left on the surface should be largely the same on planets which are similar in size and composition.
We do not know how close an alien planet would have to be to Earth to have this phenomena occur, where the ores here match the ores there. Is there some threshold at 1.2 Earth mass, or at 0.85? Extraterrestrial geology has not yet mastered these questions, as we are so new to the exo-planet game. However, it would be surprising if there was not some reasonably wide range around Earth mass where planets were similar in ore availability. Since life is likely to originate on such planets, we have a coincidence that favors aliens as much as it favored us.
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