The habitability of planets is an interesting concept to think about, as in doing so, we may shed light on the search for extraterrestrial life. This search has historically been one that has been conceptualized by many cultures, a few notable cultures would be the indigenous Australians, the American Navajo, Norse mythology, and modern day. There is a certain curiosity and mystery in thinking about whether there is life outside of what we know, and through thinking about that, we mix both creativity and logic, to form what is the essentials of scientific thinking and philosophy.
The conditions that make a planet habitable would be determined by the type of life that would exist, the most likely would be forms similar to us. Through an anthropologically biased point of view it is, to an extent, somewhat trivial to determine what is required for life as we know it to exist. As we know it requires energy, and water, and can only survive under particular weather conditions. Even exceptional cases of life, such as extremophiles still require conditions that are at least similar to that of others.
NASA’s Habitability Criteria
Let’s first consider what is probably the most notable criteria for determining whether a planet is likely to be habitable, NASA’s Habitability Criteria (Des Marais et al. 2008). This criteria was formed in terms of an anthropologically biased point of view, and thus is only valid for the consideration of life that exists as we know it. However, while that limitation does exist, it still makes sense given that in our experience, the only life forms that we have observed are those that we know of on Earth, and there is a pretty substantial sample size behind this observation.
So, NASA’s habitability criteria is defined as a planet that has extended regions of liquid water, that has conditions favorable for the assembly of complex organic molecules, and energy sources to sustain metabolism. These factors will be a major consideration for my hypothesis of habitability throughout the rest of this hypothesis.
Description of Life
In order to evaluate what a habitable planet would be, we must define the life that would be on it. While defining life may not aid us in understanding its origins (Szostak 2012), it does help us in knowing what we would expect to find. Defining what constitutes life could be seen as a philosophical problem, however, within the last few centuries there have been more scientific definitions. A relatively philosophical definition I liked is “the ensemble of functions that resist death” by 18Th Century anatomist Bichat, although, for a more conclusive way of hypothesizing habitable planets, I will use the more scientific definition written by Dorion Sagan, Carl Sagan, and Lynn Margulis (Dorian Sagan 2019), Life is matter that is able to:
- Transform Energy
Through this definition, we find it can be generalized to fit all life, and is trivial to see, for example, with bacteria, the respond to stimulus and move based presence of various chemicals, they grow, metabolism, transform chemical energy into kinetic, and reproduce through binary fission. Another interesting consideration is technology, over the past three centuries, we have seen the rise of computers, and even more recently, substantial advancements in Artificial Intelligence. Now, all technology is responsive, for example, a light turns on in response to a switch turning on and electricity being allowed to flow into it. Technology always transforms electrical energy into some action, therefore, that energy is metabolized for the purpose of the machine. Growth and reproduction of machines is arguably possible given the recent advancements in machine learning, that is, a machine could learn to optimize itself, and build others in order to solve a problem. I think that it is possible that there was life that created such technology, and thus there may be planets inhabited by technology. Such planets may not necessarily look inhabitable in their present state, but must have been in the past long enough for organic life to create said technology.
There is an interesting comparison formed in considering what would be sustainable for organic life as opposed to technological life. Let’s first consider how and what organic life would exist as. The most probable organic life to exist on an exoplanet would be microorganisms, life like bacteria, or archea. These would be unicellular organisms which would form large colonies also known as cultures, more primitive ones would be autotrophic, so, they would perform photosynthesis to produce energy. This is opposed to the potentially more evolved ones, which would be heterotrophic, so they would consume others for energy. If life on other planets managed to further evolve then there may be life more resemblent of what we have on Earth. Then the population distributions and morphology of such life would be subjective to what they have adapted to, similar to what can be observed on Earth. Life on other planets could potentially have alternate DNA compositions to ours (Dwayne Brown 2019), their DNA would probably still have the same abilities, yet may be chemically different to ours. However, the overall chemistry would be similar to ours, organic extraterrestrial life would be mostly composed of as the name suggests, organic chemicals, so primarily Carbon, Hydrogen, and Oxygen. The metabolism on such life would be largely dependent on whether it is autotrophic or heterotrophic, similar to what is on Earth, it will also be largely organic chemical reactions.
Technological life would be considerably different to organic life, in that it most likely will be primarily composed of inorganic chemicals, and will gain energy from electrical production. Such life would likely be harvesting its energy from the nearest star, such as with something similar to a Dyson sphere (Mann 2019). They could either be a massive population of many life forms with varying morphologies, or they could be one super life form. While they would not have a classical genetic composition, one could view the program that runs on them as their genes.
A habitable planet (Lissauer 2018) must almost always sit in a “Goldilocks” zone from its star, that is, it must not be too close to the star as the water will evaporate, it also must not be too far, otherwise the water would freeze (Johnson 2018). We need the presence of liquid water in some form on the planet in order to sustain life, that means even if the planet further from the “Goldilocks” zone, it may still possibly sustain life through planetary thermal energy. It is important that the planet orbits on its own axis in some form, otherwise, there will be one side of the planet that is too hot to sustain life, and the other will be too cold. Although, it has more recently observed that winds on such planets could balance the temperature, hence making such tidally locked planets habitable (NASA 2019).
A habitable planet will have liquid water in some form to support life, that may be in form like Earth as we know it, or it could be as a “snowball”. Where the planet is frozen over, but there is thermal energy inside the planet that supports life in particular hotspots. The most common chemicals within the universe are those with lower molar mass, such as Hydrogen, Carbon, Oxygen, Nitrogen, and Sulfur, this is due to the fact that nuclear fusion performed by stars (CSIRO 2019) will create more of the smaller elements in abundance before starting to create bigger elements, in fact it will use many of those smaller ones to create one bigger element. Since, planets are formed by the dust from supernovae the most abundant elements of a star, will be what the planets will be mostly composed of. It just so happens these small elements are what form organic molecules, and thus life.
We know that the the moon creates the tides on Earth, this gives extra dynamism in the movement of water bodies and thus is hypothesized to have allowed for the creation of life (Dorminey 2009). This impact can be observed in how with protocells, their chemistry is primarily of amphibiphilic minerals, minerals that would form on a terrestrial environment and are able to self-assemble into more complex structures when in water (Kee and Monnard 2016). Since water also has a major impact on the weather, water take a lot of energy to change temperatures, the tides formed by a moon will cause the water bodies to travel around the planet, spreading the water into many areas around the planet. Therefore, a moon will have an influence on the origins of life on the planet, and also on the weather and stability of weather on the planet.
The creation of a moon around a planet would be either due to a large impact which breaks a part of the planet off, or a an external object such as a meteoroid getting captured in the gravitational pull of the planet. In order to not eventually crash into the planet, or at least do so slowly, it must be a sufficient distance from the planet to form a geostationary orbit. The orbit of a moon determines the times and frequencies of tides of the oceans of the planet, it will also have an impact on the orbit of the planet itself. These impacts are dependent on the relative mass between the planet and it’s moons, a bigger moon creates more influence on both the tides an orbit of the planet, in fact, if the moon is of sufficiently size then the moon and planet would end up orbiting in an external point between them. There could also possibly be many moons around a habitable planet, of course, for stability in the environment, there still will be a balance between the mass of the moons, where this time it will be the cumulative mass of the moons. But, multiple moons introduces another factor, their positions to each other, if there are many massive moons far apart from each other orbiting the planet, the environment will end up quite unstable, whereas other configurations should be able to work given the aforementioned balance. Too many moons will also be detrimental for life on the planet as they will constantly move around the oceans of the planet, creating big amounts of friction and heating it up too much for life to be sustained on there.
Given the composition, presence of liquid water, and properties of weather, a habitable planet would be weathered primarily by erosion due water, and secondarily by the the volatile chemicals present, the air. Warmer areas will be impacted by the water, one would be able to observe layered sediments where the water washes the dirt across surfaces and depositing it at level points, like lakes or oceans. This means that the planet will have a constant yet steady change in topography, where there will be mountains, caverns, and craters, along with lakes, rivers, and oceans, albeit they may possibly be less extreme than Earths, as an habitable planet does not require tectonic plates. The hotter, drier parts of the planet will be weathered primarily by the air, this means that there will be more steady changes in the topography, and the minerals will only shift around, rather than deposit, this will form dunes that will steadily shift. It is also possible for life to be on the moons of moons (Worth, Sigurdsson, and House 2013), in this case, the Moon would have to have properties similar to the habitable planet, and it is potentially possible for said life to have been seeded between the planet and moon.
Heat is important for life to survive and thrive, in order for a planet to stay hot, there must be a layer of the atmosphere, like our ozone layer, that captures the greenhouse gases. The layer is also important to ensure that gases important for life, like Oxygen and Carbon Dioxide stay on the planet, such that it ensures respiration and photosynthesis occurs. Such a layer stabilizes the temperature of the environment, reduces the effects of cosmic radiation, and reduces the possibility of catastrophic events effecting the planet, like collisions with meteoroids. Given that a habitable planet would be composed of the common small chemicals within the universe, the atmosphere would be mostly composed of those that are volatile, such as, O2 and CO2. While the temperature is relatively stabilized by having protective layer, it will likely still fluctuate, this means in the events of “snowballs”, the life on the planet would only be located in the hot spots of geothermal energy. Whereas, when the planet becomes hot, life will be primarily located in the colder areas, such as those less exposed to the nearest stars like the poles of the planet.
CSIRO. 2019. “Stars & Their Energy Sources.” https://www.atnf.csiro.au/outreach/education/senior/cosmicengine/stars_types.html#startypeintro.
Des Marais, David J, Joseph A Nuth III, Louis J Allamandola, Alan P Boss, Jack D Farmer, Tori M Hoehler, Bruce M Jakosky, et al. 2008. “The Nasa Astrobiology Roadmap.” Astrobiology 8 (4): 715–30.
Dorian Sagan, Lynn Margulis, Carl Sagan. 2019. “Life.” https://www.britannica.com/science/life.
Dorminey, Bruce. 2009. “Without the Moon, Would There Be Life on Earth?” https://www.scientificamerican.com/article/moon-life-tides/.
Dwayne Brown, Elizabeth Landau. 2019. “NASA-Funded Research Creates Dna-Like Molecule to Aid Search for Alien Life.” https://solarsystem.nasa.gov/news/859/nasa-funded-research-creates-dna-like-molecule-to-aid-search-for-alien-life/.
Johnson, Michele. 2018. “Habitable Zones of Different Stars.” https://www.nasa.gov/ames/kepler/habitable-zones-of-different-stars.
Kee, Terence P, and Pierre-Alain Monnard. 2016. “On the Emergence of a Proto-Metabolism and the Assembly of Early Protocells.” Elements 12 (6): 419–24.
Lissauer, Jack J. 2018. “Habitable Zone.” https://www.britannica.com/science/habitable-zone.
Mann, Adam. 2019. “What Is a Dyson Sphere?” https://www.space.com/dyson-sphere.html.
NASA. 2019. “Warm Welcome: Finding Habitable Planets.” https://exoplanets.nasa.gov/what-is-an-exoplanet/how-do-we-find-habitable-planets/.
Szostak, Jack W. 2012. “Attempts to Define Life Do Not Help to Understand the Origin of Life.” Journal of Biomolecular Structure and Dynamics 29 (4): 599–600.
Worth, Rachel J, Steinn Sigurdsson, and Christopher H House. 2013. “Seeding Life on the Moons of the Outer Planets via Lithopanspermia.” Astrobiology 13 (12): 1155–65.
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