How Will Aliens Look?
Science can help us predict how complex alien life will be different from us.
Every living being on planet Earth is unique. Each species in existence today has evolved in accordance with their own environmental surroundings. The Earth is host to a vast array of varying environments, almost all of which have been explored and inhabited by living beings, complex or otherwise. This is what inspires natural diversity.
Many differing factors are at work in determining the diversity of life here on Earth. Things like temperature, water availability, terrain, natural resources, and daylight abundance are all varying elements of Earth’s biosphere. All of these elements must be addressed in the traits of each evolving life form in order for that species to propagate. Because these factors change so dramatically across the surface of the Earth, life has evolved in countless forms in each environment. In this way, all life on Earth is vastly different from each other.
However, there are also several vital elements about life on the Third Rock that remain true for every living species here today (to a degree). Things like the strength of gravity, the sun’s output, atmospheric pressure, atmospheric content, seasonal changes, and the presence of a large, luminous moon are all generally constants for every living creature. All life on Earth has grown into these constants, and we all together cope with their existence in one way or another. In this way, all life on Earth is surprisingly similar to each other.
The idea of extraterrestrial life is not a new one. Both scientists and dreamers have fathomed the existence of otherworldly beings for thousands of years. No one is certain whether or not alien life exists somewhere in the endless expanse we call space, but if it does, it is sure to have developed attributes in accordance to it’s home planet’s unique environments, the same way life has done here on planet Earth.
With the activation of the Kepler space telescope, NASA has discovered that exoplanets are both abundant and extremely diverse, each with unique planetary attributes. I have divided all of these differing planetary attributes into three general categories regarding a potential species’s home planet and star system, in order to attempt to discover what life on an alien world may look like. I am no biologist, but I will attempt to use these planetary attributes to predict how complex life will look by making comparisons to life that already exists in similar conditions here on Earth.
Because we only have access to one data point in regards to the evolution of life (i.e. the Earth), I will be forced to make some assumptions in my analysis. Since all life on Earth is carbon-based, I will make the same assumption for life on an alien world. This is not to say that other life forms are impossible, but simply to address the fact that it is the only form of life we are certain is possible to exist. I will also assume that life on this alien world started in the same way it did here on Earth; in a chance chemical reaction induced by natural factors. I will also assume that the process of evolution on this alien world would take place in the same way it did here on Earth. Finally, I will assume that this life is reliant on the same natural resources that Earth’s life relies on; water, air, and nutrients.
With all of this in mind, it is now possible to speculate on the traits and appearances of complex alien life on worlds that are vastly different from our own to help us answer the question, how will aliens look?
1) Planetary Mass
- Gravitational acceleration
The mass of a planet affects the strength of gravity on said planet. A more massive planet will have stronger gravity, and vice versa. It is likely that a larger planet with increased gravitation would result in shorter, stockier builds for complex life forms. This trait both lowers the potential energy for falls, as well as creates more stability against the higher gravitational acceleration. Bipedal life may not even evolve on large enough planets, due to the shear danger of being that high off the ground. Higher gravity would also take a greater toll on things like bone joints. This means that life on a larger planet may have more muscle-oriented structures versus our largely skeletal-oriented physique.
- Atmospheric content/pressure
Our atmosphere on Earth mainly composed of just two gasses; oxygen and nitrogen. Of course CO2 is also present, and required for plant life (as we know it), but it is in much lower abundance. A larger planet, having larger gravity, would attract a thicker atmosphere. This thicker atmosphere would also be host to several other gasses we don’t find here on Earth; gasses that are too lightweight to remain gravitationally bound to our planet against solar pressure. Life on a larger alien world would be forced to adapt to these foreign gasses (like we have done with nitrogen). Alien life may require an advanced respiratory filtration system, or perhaps even a form of dry gills, in order to filter out all of their air molecules not necessary for respiration.
- Terrain & water availability
The powerful gravitational force associated with a planet of a larger mass would aid in tearing down mountains and filling in canyons over the course of millions of years of erosion. This would make the presence of flatlands and prairies much more common on a large world. Similarly, ocean floors on such a planet would would be flattened out, creating expansive but shallow oceans. In fact, a massive alien planet’s surface may be covered in up to 90% water, or more. Marine life would be much more common on such a planet, and terrestrial life would not need to be outfitted so heavily for maneuvering difficult terrain. On a small planet, the opposite would be true.
2) Planetary Orbit, Rotation, and Axial Tilt
- Equilibrium temperature
Assuming the life is carbon-based, the temperatures required for life to succeed are between 0 and 100 degrees Celsius, which corresponds to the freezing and boiling points of liquid water respectively. Here on Earth, our planet’s equilibrium temperature is surprisingly close to the colder barrier; about 15 degrees Celsius. However, liquid water is possible anywhere between 0 and 100 C, and in fact, in the presence of a thick atmosphere, possibly even higher temperatures. So long as life on an alien planet has evolved in these temperatures for millions of years, it is possible that life could be conformable closer to the hotter barrier for liquid water. Such life would require an advanced natural thermal control system for protecting against excess heat, such as a highly reflective, low absorbance exoskeleton, or perhaps even a bodily fluid-based heat radiating mechanism.
- Length of seasonal changes
If a planet orbits a cooler star, it simply must orbit a little closer to its host in order to maintain equilibrium temperatures comfortable for life. This, however, would entail a shorter orbital period, and thus shorter seasons. Shorter seasons would encourage more rapid temperature and pressure changes in the course of a year, spawning more frequent storms and weather phenomena. Life here would be forced to adapt accordingly. The opposite is true for a planet orbiting a hotter star, say an F-type white star. On such a world, winter could last the better portion of an Earth year, and blistering summers would be equally as long. Life around such stars would be forced to better cope with a wide range of temperatures for long periods of time.
- Severity of seasons
The length of a planet’s seasons are determined by its orbital period, but the actual severity of the seasons would be determined by that planet’s axial tilt. With a tilt of more than 23 degrees, temperatures between summer and winter here on Earth can vary by as much as 60 degrees C. If an alien planet’s tilt were much greater, these annual temperature swings would be much more pronounced. Accordingly, life would need to learn how to evolve to these greatly differing temperatures, especially if their planet’s orbital period was short. There may even be a theoretical limit to the severity of temperature swings which can be endured by evolving life induced by a planet’s axial tilt.
- Length of day
The length of a day on a planet is determined by its rotational speed. Planets with a more rapid rotation will have shorter days. Rapidly spinning worlds will have turbulent atmospheres and substantial temperature changes, inducing larger and more frequent weather phenomena. The opposite is true for slowly rotating worlds. A slowly rotating planet would be more atmospherically calm. Instead, these worlds would have very long days, which would force life to evolve with certain stealth attributes in order to hunt (or avoid being hunted) during the extended nights. Planets around small stars may actually be tidally locked to their host star, so that the same side of the world always faces the star. Life on this half of the world may not even know the definition of darkness, and their eyes may be permanently adjusted to the daytime light. If there was life on the dark portion of the planet, the opposite would be true.
3) Home Star and System
- Solar output/wavelength
Our sun emits a constant light source at a peak wavelength of around 500 nm. It is no coincidence that life on Earth has evolved to observe light that is at and around this wavelength. Aliens on a planet orbiting a smaller, red star may evolve longer wavelength vision that, for us, may be confined entirely to the infrared. Their eyes would also probably be more sensitive to bright light, because M-class stars would appear larger and less intense than our sun appears in our sky. Smaller stars are also known to be more active, and emit more deadly flares than their calmer, larger counterparts. Life around a small star would need to be better outfitted to this radiation as well, lest face extinction.
- Natural resource abundance
A planet’s home system is also a major factor in the abundance and type of natural resources that would be available for life that evolves on it. An older star system may be more pure, and be composed of less heavy elements. Rocky planets would be less abundant than their gas giant cousins, and the number of rare elements would be greatly reduced. On the other hand, a newer planetary system may hold the natural resources of several previous solar system remnants, and be rich in natural resources. Life on Earth relies on some of these heavy elements, such as calcium, potassium, and magnesium, to function. A world with less of these resources may have trouble forming life in the first place. On the other hand, a planet with more of these resources may be able to support a larger population and diversity of biomass on its surface.
- Presence of a moon
Our moon plays a huge role in the evolution to life on Earth. It is bright enough to illuminate the ground for most of its cycle, exposing potential predators or prey to each other. Without our moon, the night sky would be detrimentally dark. Because our moon formed in a very chance collision, it is likely that an alien world would not have such a sizable moon, or perhaps no moon at all. Terrestrial life on an alien planet with no large moon would require that it evolve in such a way to be able to see better in the darkness, or develop their other senses better in order to accommodate. Our moon also drives the tides here on Earth. A planet with no moon would have no such tides; another thing that life would have to adapt for.
- Being a moon?!
There is also the potential for the alien planet itself to be a moon of a larger gas giant, tidally heated enough to support the presence of life. This possibility throws many other planetary attributes through a loop. For example, because exomoons are likely to be tidally locked to their host planets, the length of their day is determined by the length of their orbit around their host planet, and the length and severity of their seasons would be determined by the time and eccentricity of the host planet’s orbit, respectively. These worlds would also exhibit multi-day eclipse periods of darkness when they fall behind their host. Life would have to adapt to all of this. Some scientists actually believe that life on the surface of an exomoon may actually provide a more suitable environment for life than the Earth, due to the presence of the host giant’s large magnetosphere. This massive shield would protect its children moons from harmful solar radiation. Exomoons and their parent planets could also be farther away from their violent host stars because such moons could instead be heated by tidal forces, instead of just solar radiation.
Conclusion
If alien life exists, it is sure to appear much different than life which has evolved here on Earth. Life is diverse, and seems to reflect whatever environment it evolves in. Perhaps even the strangest and most hostile environments of universe’s planets could be conquered by the driving power of native evolving life. There is, of course, the chance that life has evolved on an alien world in a way that no one can predict, or perhaps even in a form that we can’t comprehend, in which case everything which I have just analyzed here would be completely invalid.
This, however, does not make the speculations any less wonderful. Even if alien life doesn’t exist elsewhere in the cosmos, it is at the very least a reminder of how unified life is here on Earth, and how much we all share in common. Sometimes, it is important to contemplate these larger scale questions in order to see how small our effect on the greater universe really is. Amidst all of this natural diversity, we still can’t seem to get along with each other on the most elementary of issues. Perhaps the discovery of life originating from elsewhere would finally make us realize this fact.
Every living being on planet Earth is unique. As would be every living being on each of the different planets of the universe.
