Mineral Planets

Let's use the term mineral planet for planets that an alien species could turn into a sustainable habitat. These are a far cry from an origin planet, which is one which could give birth to life by evolving its own first cells. It is a far cry from a seedable planet, which is one which could not evolve its own starting cells, but which could take a seed of some sort of cells and have them multiply and eventually evolve into something interesting, like an alien civilization. Instead, a mineral planet is one where an advanced civilization could establish mines and habitats, on the surface or below it, and thereby produce enough resources, energy and minerals, to sustain an alien colony without any continuing support from the home planet. It has to persist for a long period. 

There may be very few origin planets in the galaxy, and somewhat more seedable planets, and maybe a huge number of mineral planets. One implication of such a lopsided ratio would be that mineral planets can be stepping stones for an alien civilization to cross the galaxy. Note that some or all alien civilizations may adopt the goal of seeding as many seedable planets as they can, following a philosophy that life is its own goal, and that just like planet-bound species try to disperse as much as they can, alien civilizations try to spread life as much as they can. Traveling 300 light years from a civilization's origin planet to the nearest seedable planet might be simply too much to do, and so finding a network of mineral planets in the general direction of that seedable planet would allow them to gradually work their way over to it, and when close enough, to accomplish the seeding effort with more payload and duration in orbit that they could have if they had to travel 300 light years.

Reliability might play a role here. If a speed of 1% of the speed of light is used as a guess of the maximum speed the civilization might attain with its colony ships, this means 1000 years of reliability is necessary to go to the nearest mineral planet, but 30000 years would be necessary for the closest seedable planet. If the probability of enough equipment lasting 1000 years can get raised to 98%, a risk the civilization might be willing to take, the same equipment has a probability of the same quorum still working after 30000 years of travel of only 55%.

Monitoring a seedable planet is also easier from 10 light years away than 300. It might be that seeding a planet is necessarily a very chancy situation, and multiple visits are the only way to accomplish it and verify that it has been accomplished in such a way that a billion years of evolution or two can follow without total extinction. Maybe seeding can only be handled by landing a small colony on the planet, and staying there for a long period. This could also be accommodated better from a nearby solar system than from a distant one.

Is there anything which can be credibly said about the prevalence of mineral planets? The formation of stars seems to leave a disk of matter revolving around it, which can turn into planets. This is a matter of the disposition of angular momentum, and how hard it is to collect it all in a central body. Everywhere we look we see planets, and our ability to find them is not so great right now, so there are probably many more per cubic light year than we have discovered in our locality. If there are several planets on the average per star, how likely is it that at least one of them is a mineral planet?

To be a mineral planet, the planet has to be mineable and habitable. Planets too close to the star are too hot on the surface to establish a colony, and the temperature below the surface would be higher than the average temperature at any latitude. The orientation of the planet would indicate the spread of temperatures over the planet, from pole to equator, and indicate if there was any latitude above which a colony ship could land and stay without thermal damage. Phase-locked planets provide a different criteria, but if the north pole of a non-rotating planet is designated as the closest-to-star point, then again, there may a latitude beyond which the ship could land.

Too much atmosphere would interfere with colonization, and planets might be excluded on this basis. Since smaller planets cannot long hold onto the atmospheres they have at formation, size is an indicator of this problem. The maximum size depends on the distance from the star, as it is easier to hold onto an atmosphere if the planet is far from the star and the atmosphere is very cold. Cold gases evaporate much more slowly.

Another question to be asked is the radiation level. If the star is a very active one, the colony ship would not even be able to come in close to it, unless some sort of shielding was build into the hull. Perhaps advanced engineering could figure out a way to get a mine dug, and alien colonists down into the mine without receiving too much radiation. Once under the surface, all the radiation is absorbed before reaching them. This is an interesting project to be considered.

With all these factors eliminating candidates, how much might be left? Our surveys of exo-planets are too limiting to calculate this number, but it might be that 90% are no good, meaning one in three stars, of middle class, has a candidate. There is more to being a mineral planet that simply being mineable with a surface not too lethal. There has to be the right mix of minerals.

An alien body has certain needs for elements, and alien technology has a different set of requirements for elements. Together they comprise the shopping list of elements, or rather minerals from which the needed minerals can be extracted. Some small molecules might also be extracted, principally water and carbon dioxide, maybe some others. The distribution of elements on a planet is a result of the original composition of the gas cloud, which comes from the effect of nearby supernovas in the cloud's history. Then there is the condensation question and the diffusion question, with minerals forming as elements and condensing into dust, and then being filtered by the solar wind and light output from the star over millions of years. After that, when the planet forms, geology plays a role in determining which minerals are at the surface.

The only planet we have any experience with is Earth, and it can provide us with a model problem. Suppose there was a planet in a state just like modern Earth but without any atmosphere, without any fossil fuels, no life, and of course no people, meaning no mining. Could an alien colony ship find the right minerals, in accessible form, so that it could produce a sustainable colony here? Perhaps U-235 is the key. We can mine uranium ore, refine it, enrich it, build a reactor, and extract more energy than was needed to construct the reactor and keep it fueled. Alien reactors should be even more efficient in the use of fissionable and fertile fuel than ours are, as we have had only a few decades of experience with fission power. Perhaps the guess of one solar system in three having a mineral planet is not too far from the truth.