Using what we have learned from life on Earth, what can we say about alien life? While it would probably be vastly different from life on Earth, alien life would probably adhere to certain universal guidelines, as the widely varying life on Earth does. These guidelines or ground rules include the following:
Alien life would be governed by laws of physics and chemistry.
Alien life would be based on some type of chemistry (eliminating the sci-fi concept of pure-energy beings).
- Solvent - On Earth, the solvent for all of our biochemicals is liquid water. Other chemicals could be solvents as well, such as ammonia, methane, hydrogen sulfide or hydrogen fluoride.
- Temperature - Alien life may require temperatures at which its solvent can remain liquid.
- Pressure - Alien life may require environmental pressures (and temperatures) that allow solvents to exist in three states of matter (solid, liquid, gas).
- Energy source - Living things require energy to remain organized. This energy may come from a star or from chemical or geothermal energy (as in hydrothermal vents and hot springs). On any alien world, there would have to be some source of energy to sustain life.
- Complex molecules - Living things on Earth are organized and made of complex, carbon-based molecules that carry out biochemical functions. Carbon is a versatile atom that can form bonds with up to four other atoms, in many shapes, to make molecules. Although not as versatile as carbon, silicon can also form up to four bonds with other atoms and has been proposed as a basis for molecules of alien life (silicon-carbon hybrid molecules have also been proposed). It is likely that alien life forms would have some type of complex molecule to carry out similar functions.
- Informational molecule - In Earth organisms, deoxyribonucleic acid (DNA) is a complex molecule that carries genetic information and directs the formation of other molecules in order for life to reproduce and function. Because a characteristic of life is that it reproduces, it seems likely that alien life forms would also have some type of informational molecule.
Alien beings that are larger than microbes would have some equivalent of cells. As an organism gets larger, its internal volume (cubic function) grows faster than its surface area (square function). This places a limit on the organism's size, because substances from the outside of the organism must pass into and throughout the organism by diffusion, which depends upon large surface areas, short distances and differences in concentrations. As an organism grows larger, the distance to its center increases and diffusion gets slower. To maintain workable diffusion distances, an organism must have many small cells instead of one large cell. So, an alien would be multi-celled if it is larger than a microbe. (We would not expect to find a light-years wide, single-celled organism like that portrayed in the original Star Trek episode "The Immunity Syndrome.")
Alien life would evolve and adapt to its surroundings by the theory of evolution as previously explained.
The physiological make-up of a multi-celled alien would be most suited to its environment. Organ systems would be adapted to environmental conditions such as temperature, moisture and gravity.
- The alien would have some way of bringing solids, liquids and gases inside its body, distributing them to every cell and removing waste products (equivalents of heart , blood vessels and kidneys, for instance).
- The alien would be able to take in energy from its surroundings, extract the energy and eliminate wastes.
- The alien would have senses (such as sight, sound, touch) to obtain information from the environment and respond to stimuli (while we use vision as our primary sense, this may not be true of aliens). They would also have some type of brain or nervous system to process information.
- The alien would have some means of reproduction, either sexual or asexual.
Alien organisms would probably have similar ecological structures to life on Earth.
- Population sizes would be limited based on the predominance of food, predators, disease and other environmental factors.
- Alien life forms would exist in food chains and food webs in their native environment, like life on Earth. Producers will make food, consumers will eat producers and/or other consumers and decomposers will recycle atoms and molecules from dead organisms back into the environment.
- Alien life forms will be integrated with their habitats and ecosystems, like life on Earth.
As you can see, life of any kind is intimately tied to its environment, so the characteristics of the planet would be extremely important in determining the characteristics of the life form.