Eco-habitable planets

Not so long ago, Earth science knew only the planets in our own solar system. Now we know about thousands, all on other solar systems, and the possibility that there were no aliens visiting us because there were no other planets in the galaxy has vanished in a puff of smoke. Since we know about so many, it makes sense to categorize them in different ways.

One aspect used for categorization that has been around for a long time is habitable, which, to an astronomer, means having a temperature where liquid water could exist. This is a bit fuzzy, as a planet could have these temperatures on a part of the planet, as Earth does, rather than everywhere on it, and it still qualifies. What about planets with liquid water during part of an eccentric orbit? What about liquid water buried deep? Many have discussed more specifications that might be added, and many have discussed ways in which such temperatures might be modified by various orbital aspects, atmospheric effects including circulation, and so on. Perhaps we will discuss these in a post, but not this one.

In this blog, we have talked about one categorization of planets, related to habitable, but much more specific: solo planets. These are planets which can originate some form of life, which are self-reproducing organisms. These are the planets which provide the minimum necessary to start the process of evolution. Since we do not know with certainty how life originates, or even how it originated here on Earth, we do not know the conditions to set when looking through the known exo-planets to see if they have them. So for now, the term solo planet is one in which the definition is not reducible to astronomic or exo-geological parameters. It is about as fuzzy as habitable in the non-astronomical sense.

The rest of the planets, not being solo planets, cannot originate life, by definition. However, they might be able to harbor life, if some were spread there. One idea which has gained currency is that perhaps life could originate on one planet, and then be blasted into space by a meteor strike or some such, and then the rock carrying it would someday land upon another planet, and fortuitously the life was still intact or at least capable of regenerating its ‘livingness’ upon landing in the right conditions.

The purpose of this post is to try and define a further breakdown of the non-solo planet category, into ones which can harbor life, and ones which cannot, and then take it a bit further. Let’s start from the back end. Define a planet as uninhabitable if even the smartest alien civilization cannot install living creatures there, themselves included, and have them stay alive. They might be too hot, have too high gravity, have horribly toxic, acidic atmospheres, constant volcanism, no land but just toxic seas, and certainly more. The fuzziness here is how long to survive there. A very hot planet could have a ship set down with thermal shielding and then take off again before the heat from the atmosphere penetrated into the ship’s interior. Life stayed alive on the surface. Venus has a cool band in its atmosphere, where a ship could float, buoyantly, for a long period, and not descend to the surface, where things are too hot. So, does Venus qualify as uninhabitable? Let’s grant ourselves the luxury of making definitions even before they are all needed. In this blog, uninhabitable means a permanent outpost cannot be set up anywhere on the surface, under the surface, in the atmosphere or in a body of water there. An uninhabitable planet cannot have a mining station upon it, and all mining would have to be done robotically or in an very expensive way, by landers going up and down within the tolerable time limits. Robotic mining might certainly be affordable, so perhaps inaccessible should be a subcategory of uninhabitable related to robots. An inaccessible planet cannot be used to provide resources of any kind, save by scooping the atmosphere’s upper bands.

This means, if we can define inaccessible in terms of planetary conditions, we know where not to look to find evidence of large-scale alien mining activity. Ones which are uninhabitable, but not inaccessible, would be excluded if the signatures we are able to detect relate to the presence of biological aliens.

In the large gap between solo planets and uninhabitable planets there are many varieties, and all of them might harbor life in a permanent way. Life might be anywhere for some planet in this gap, perhaps under the surface on an airless planet, perhaps under the oceans on a planet with tremendously high wind gusts, under ice on a snowball planet, in the atmosphere on a Venusian analog, or on the surface. There would be many possibilities for signature detection, some much less promising than others.

Let’s consider wide-spread life, which is more likely to be detectable. The title phrase, eco-habitable planet, is defined here to be a planet in which life can sustain itself using only the resources available on the planet. Quibbling about a two-planet solar system where each one is needed for some resource type, including energy, is not allowed here. So, on an eco-habitable planet an intelligent species can create an ecology, maybe consisting only of one intelligent alien species or type, that can survive for a long period, measured in star travel timescales, of millennia at the least. This can range from the sole inhabiting species, perhaps tailored for the planet, extracting energy and resources from the planet including using solar photons, which does not need imports to survive up to something where a monstrously large variety of organisms can live there.

The narrow end of this range might be exampled by an underground Mars colony, which mines uranium for power and finds every element it needs in various ores there, and does so in a positive energy way, so that all the building and mining and extracting and so on are paid for, energy wise, by the power gained from the mined uranium. Obviously if imported energy is needed to keep the colony alive, it is not eco-habitable by definition.
The wide end of this range might be exampled by Earth with a change. Let’s suppose we know the only way life can originate is by volcanic vents in a deep ocean, and in the Earth-prime, there are no such vents. The crust is a little thicker or the oceans a little thinner or whatever. So Earth prime can support life, if started in the right way. No chemotrophs living near the vent to give life a start, but if seeded with chlorophyll plants, enough resources can be extracted by the plants to produce an oxygen atmosphere and from then on, life can blossom in countless ways. There could well be a thousand eco-habitable planets in the galaxy for every solo world.
If the ratio of eco-habitable planets to solo planets is a really big number, like a thousand or fifty thousand, then the few solo planets that can generate intelligent civilizations can spread their civilization, with little impediment other than seeding, all over the galaxy. This situation would be reminiscent of what early humans found after evolving into tool use and finding a whole world of opportunity for their own use.

Right now, we do not have quite enough data to tell what this key ratio is. We only know it is extremely important in determining how we should look for signatures of life, and more so, in deciding what we want to do with ourselves.