Definition of Planetary Habitability

| Jun. 2022
Cand. Doctor in Earth Sciences and Astrobiology

Main factors that define the ability

Habitability is a phenomenon that depends on the presence, interaction, magnitude and stability of a group of factors. So far, the following have been proposed as the main factors: radiation from stars in their main phase and nearby supernovae; the radiation emitted by our star (Sun); the presence of giant planets that protect the other planets, by attracting comets or meteorites due to their greater gravity; the terrestrial movements, both orbital, rotational and of inclination of the axis (where the Moon exerts a stability); water, nutrients and Energy sources profitable for life; as well as plate tectonics and volcanism, which recycle and maintain the atmosphere.

These factors, in turn, maintain a generation of resources and climate stable, in which life can proliferate, while the small or medium variations in them, allow the evolution and adaptation of life. Climate, resources and evolution are also affected by the interactions of life with itself and with its environment, so these factors define the capacity of a place to allow the origin and/or to sustain life, that is, if said place is habitable and how habitable it is, for one or more species, or for life in general.

Weather habitability

When "a certain time" is mentioned, it refers to a period of time that can be from minutes to billions of years. In the case of a life originating on a planet or one whose precursors (DNA, RNA, amino acids, lipids), are transported to a certain place or places on a planet or moon, since it is not yet understood how life originated on Earth, from the non-living, a time of habitability cannot be accurately assigned.

However, this time could be delimited between the moment in which the first organism metabolizes, excretes, grows, interacts-reacts, reproduces, adapts and evolves, until a species is extinguished (genetic-environmental habitability), or all the life of said planetary body (habitability global). To measure the above, the process would have to be observed and studied during its course, i.e. find a planet where the precursors of life exist or are carried, and study the process of origin and evolution; or the other option would be to find and study the biosignals that life left after its extinction.

In the case of a life that was transported by some natural or artificial means, the habitability time would start from the moment a species or a group of them is introduced (to create a biome), and would end again until the extinction of that species or of the entire biome. Obviously, the artificial introduction requires prior work on the design and implementation of a technological habitat (colony modular or dome), or from a semi-natural habitat (planetary terraforming), or from morpho-physiological modifications (genetic engineering) of the organisms that will be introduced, so that they can adapt and proliferate in the natural or modified atmospheric conditions of said planet or moon.

It should be noted that human beings have already started, either by mistake, or as a consequence of our ignorance or technological reach, to develop and study this type of habitability, from the moment in which the astronauts who landed on the Moon left their stool; or when the Sojourner and the other rovers rolled, touched and drilled the surface of Mars, seeding archaea, bacteria and terrestrial fungi; also since the Israeli Beresheet probe crashed on the moon, seeding thousands of tardigrades on its surface; or since the Cassini probe entered the atmosphere of Saturn.

This has probably happened with all other probes that have crashed or landed on the surface of the Moon, Venus and Mars, because no matter how strict the sterilization protocols of the probes are, in their manufacture, assembly and departure from Earth, they carry with them microorganisms resistant to cleaning, decontamination and filtering protocols, and very likely we will continue to do this, since life She has shown her way through, despite the most terrible cataclysms, so humans don't seem like much of a challenge to her yet.

How much do we know about planetary habitability?

On the one hand, the study of this phenomenon is quite recent, barely a couple of decades, and has been carried out by a new multidisciplinary field of scientific knowledge called Astrobiology, and on the other, the only place we currently have to study it is the Earth, so it is a subject in which science has little information and experience. However, their study has led to important advances in biology (study of extremophiles), in astrophysics (study of exoplanets), in biochemistry (study of possible sources of Energy for life), in engineering (development of devices and probes, for planetary exploration), among other advances.

Throughout these two decades, multiple specialists from various universities, institutions and government agencies around the world have A great effort has been made to design and publish the definitions that adequately explain this phenomenon, from the physical, chemical and astrophysicist.

Abiotic and biotic planetary habitability

To synthesize this information, I have designed and published two working definitions, one that encompasses all of this effort (abiotic) and an innovative one, which gives life (biotic) a fundamental role within this freak.

Planetary habitability from an abiotic view

It is the capacity mediated by the stellar, planetological, physicochemical and thermodynamic conditions, of a place or of diverse places (on the surface, in bodies of water or in the subsoil), of a planet or moon, to favor the origin of life or shelter it (in case it is transported to it), and to allow it to proliferate for a time certain.

Planetary habitability from a biotic view:

It is the ability or property of life, indigenous or transported, to adapt and persist through changing various conditions of a planet or planetary satellite, and that facilitates biological diversification or the establishment of other introduced species, for a time certain.

Even both definitions can be joined to better explain the phenomenon, since we do not know with certainty if it is only an effect of the abiotic system or if it is a net property of life, or if parts of one definition and the other necessarily have to be fulfilled, and in what proportions, or even, if some other component or crucial condition is missing that we have not yet analyzed or it is understood. Only the life we ​​know has many secrets that biology, even after centuries of study, has not been able to reveal, such as the place, time or conditions where it originated; what are its limits of survival, and of adaptation, etc.

Incredible as it may seem, both the lack of information, as well as the new discoveries about organisms, and biological and mineral materials, and the generation of complexes circuits and software, have caused that there are more than 100 different definitions of life, which complicates being able to understand what generates it, how it is supplied, which they can be the limits of its habitats and ecological niches, how it diversifies, reproduces and evolves, etc., increasing the complexity of the habitability phenomenon planetary. For this reason, the role played by Astrobiology is crucial, by integrating the information generated by all the disciplines that study or relate to life and its habitat (including human life and its artificial habitat), and try to generate an explanation or definitions increasingly precise.