What could life on other planets be like? This page looks at the extremities of similarity and difference, from biochemical make-up to major substantial and macrostructural differences.
- Why Life Could be Similar to that on Earth
- Alien Life
- Robot Life: Artificially Intelligent Satellites, the Ambassadors of Man, and the Tiny Robots of Life
- Extremely Exotic Life Forms
Why Life Could be Similar to that on Earth
Given that life cannot exist on all planets and that it does on ours, without further information we have to conclude that life is more likely to exist on a planet similar to ours than on those that are dissimilar. Life requires stable conditions to enable reproduction and then evolution. It requires a stable and consistent mix of chemicals so that life can start. Without this stability, life can never evolve to be able to cope with instability.
Carbon is the most likely source of organic material on most planets due to the ease at which it is formed during planet formation and its readiness to form complex and predictable molecules. Given that carbon is likely, so are the rest of the necessary molecules such as hydrogen, oxygen and nitrogen. It can be noted that all the molecules of life are also the most abundant in the atmosphere in which we live.
Life is probably on stable planets with an atmosphere (or local areas of atmosphere) that contain these chemicals. It will, just like life on this planet, evolve to use the most efficient methods possible for its own reproduction. Life on this planet, over millions of years, has sought to do the same thing. We can draw the conclusion that as nearly all higher cells use similar methods that these reactions may be the most efficient way of fuelling carbon-based life form's cycles. Therefore it seems that similar metabolic paths will exist in alien life as in ours; similar sugars could be used. Similar excretion could exist, as will a similar food chain.
One can be confident in stating that life on other planets will utilize many of the same reactions as we do in our cells - both driven by the fact that those who are most efficient sooner or later dominate the population. There is no doubt that aliens would have efficient organs and organelles that carry out specific tasks, like our cells do.
Life could indeed by quite similar to that which we see here. It won't have evolved the same species or phenotypes but we could be certain of seeing 4-legged animals, birds, fish, etc, the same basic classes that we have on Earth, and even a similarity in cellular (or pseudo-cellular) structure.
The foremost evolutionary biologist, Prof. Richard Dawkins, explains that while on Earth DNA molecules are the unit of natural selection, in alien worlds this may not be so:
“The laws of physics are supposed to be true all over the accessible universe. Are there any principles of biology that are likely to have similar universal validity? When astronauts voyage to distant planets and look for life, they can expect to find creatures too strange and unearthly for us to imagine. But is there anything that must be true of all life, wherever it is found, and whatever the basis of its chemistry? If forms of life exist whose chemistry is based on silicon rather than carbon, or ammonia rather than water, [...] If a form of life is found that is not based on chemistry at all but on electronic reverberating circuits, will there still be any general principle that is true of all life? Obviously I do not know but, if I had to bet, I would put my money on one fundamental principle. This is the law that all life evolves by the differential survival of replicating entities. The gene, the DNA molecule, happens to be the replicating entity that prevails on our own planet. There may be others.”"The Selfish Gene" by Prof. Richard Dawkins, p191-2
DNA came to be the unit of self-replicating life because its constituent atoms happen to be frequent enough on Earth to facilitate reproduction. In other environments the chemical mix may well be different, meaning that the fundamental building-blocks of life could be different. So although the chemical mix might be the same and life that is broadly similar- made of amino acids and proteins, using DNA expressed through RNA - there is good chance that it'll be different. We start off by looking at life that is different in its specifics, before imagining intelligent life that is so utterly different we might not even recognize it.
Alien Life
Life could be surprisingly similar to that which we already know. But the potential differences are even more amazing. Major differences could arise from even small differences between the alien planet's solar system and ours.
“The initial gases, and the energy sources, are common throughout the Cosmos. Chemical reactions like those [duplicated by researchers in laboratory experiments] may be responsible for the organic matter in interstellar space and the amino acids found in meteorites. Some similar chemistry must have occurred on a billion other worlds in the Milky Way Galaxy.
But even if life on another planet has the same molecular chemistry as life here, there is no reason to expect it to resemble familiar organisms. Consider the enormous diversity of living things on Earth, all of which share the same planet and an identical molecular biology. [...] There may be some convergent evolution because there may be only one best solution to a certain environmental problem - something like two eyes, for example, for binocular vision at optical frequencies. But in general the random character of the evolutionary process should create extraterrestrial creatures very different from any that we know. [...]
Some people - science fiction writers and artists, for instance - have speculated on what other beings might be like. I am skeptical about most of those extraterrestrial visions. They seem to me to rely too much on forms of life we already know. [...] I do not think life anywhere else would look very much like a reptile, or an insect or a human - even with such minor cosmetic adjustments as green skin, pointy ears and antennae.”
"Cosmos" by Carl Sagan (1995), p49, 55
What do Martians Look like?
Apparently a really good book on the potential biology of alien life including gas-life, etc, that normal people wouldn't often be able to imagine and that we would find very difficult to recognize as life. (I.e., self-replicating living electromagnetic life). No doubt will include Gaia Hypothesis material.
The Sun
The Sun is the basis of our entire ecological system. Due to the radiation given off by our local Sun we find that the chemical chlorophyll is most efficient at absorbing energy from it. Chlorophyll absorbs all visible light except green, which it reflects. If a sun gives off different levels of radiation, chlorophyll could be useless. Plants and such may not even use what we call the "visible spectrum" at all. If the basis of the ecosystem does not use visible light, i.e., it used IR radiation, Gamma Radiation, Radio-Waves, whatever, then this would affect the entire planet's life profile.
With a star with even slightly different radiation emissions to our sun the local life could use a visible spectrum completely different to ours. It would mean that we could not view their "colors" as they do, and they would see us completely differently. We might find all the colours we use to communicate are invisible to them without scientific equipment, and visa versa. They may not see any of our color coding, it would appear different perhaps just as noise, depending on the material's other emissions. In communications we would have to avoid using any specific range of colors under the assumption that they can see them. A piece of metal with a story printed on it could appear to us as a plain piece of metal because by eyesight we cannot detect the same range of electromagnetic outputs as they.
If the Sun emits a different pattern of radiation, microbial and plant life may evolve to use completely different biochemical pathways to ours. Their fundamental biochemistry could be vastly different. A different sun implies different metabolisms, different consumption patterns, different amino acids. Proteins, amino acids and the fundamental basis of living life could be completely different. In a hundred science fiction films, few have ever really come up with life that is fundamentally different to ours. The possibilities are said to be largely beyond imagination.
Communication
The communication of data could use very different methods to those that we're accustomed to. One good example of difference is to contrast our vocals with that of whales and dolphins. Due to the different quality of the medium they live in, species will develop locally effective ways of communication. We could pass life right by without ever realizing that what we view as white noise, or what we can't detect, is in fact rich communication. Think of the hormone- and pheromone- rich world in which some animals live... and all of it is completely undetectable to us. How much more undetectable and unobvious could alien communication be on a different planet? Unimaginably obscure!
Robot Life: Artificially Intelligent Satellites, the Ambassadors of Man, and the Tiny Robots of Life
Daniel Dennet points out that we do not consider chemicals to be alive. Even self-replicating crystals and clever nanobots are not "living"; we consider them to be completely mechanical. Yet we are made of these non-living, "mechanical" chemicals. Not only are we made of robots, and we still consider ourselves to be valid conscious life, but the evolution of all life is from such mindless robots. As life is based in biology, and biology in biochemistry, chemistry is based on the cold, non-living laws of physics.
“We are made of robots - or what comes to the same thing, we are each a collection of trillions of macromolecular machines. And all of these are ultimately descended from the original self-replicating macromolecules. So something made of robots can exhibit genuine consciousness, because you do if anything does”"Kinds of Minds" by Daniel C. Dennett, p24
It is easy to imagine that we are capable of building computers that have massive lifespans. It is also very possible that machines and robots outlive the species - and planet - that created them. A homeless, wondering robot or machine might be the sole survivor of an intelligent species. Impossible, you think? Well consider the Voyager satellite and space probe; our creation is now speeding through space far, far away. In how many thousands of years will it be discovered by another intelligent species? In a million years? Or if somehow it does not fall into a star, a billion year in the future, will a spacefaring race stumble across it, in an era when Human beings have long died out? What a melancholic representative such a robot would be.
Given the current pace of development, it is easy to imagine artificial life venturing out into the universe and leaving us fragile humans behind. It may even be more likely than human-first space exploration. If it is more likely for us, it may well be more likely for others, too. A robot handshake is likely to come many thousands of years before a biological one.
Extremely Exotic Life Forms
Life in other places in the universe could easily be utterly different to ours. Our life is based on biochemistry: this is not known to be the only way that life can exist. Where any pattern can reproduce itself, life could evolve. In complex echoes in sound waves, in complex reflections of radiation... in the movement of waters we find repeating patterns. Life could be gaseous, exist purely in invisible radiation or anywhere. We might find that such life exists on our planet and we simply don't see it. Planets could be alive, complex patterns in our atmosphere could be alive, actively evolving and thinking about things that are utterly alien to us.
Science fiction has presented nearly all of the above examples in some form or other. John Wyndham writes of living planets, early Star Trek episodes contained gaseous and electronic life in various forms, and multiple films contain psychic life of unknown mediums interacting with Humanity in some way. Alien life could be extremely different, unrecognizable. We might be as invisible to it as it is to us. Bacteria do not know what they grow in, yet we know of bacteria.
We must face the truth that as life as we know it is derived from the cold, unthinking laws of physics it is possible that quite exotic life forms could emerge through similar processes, but a different beginning.
Examining exotic forms of life from Gaia, the consciousness of the Earth, to Artificial Life born from computers and the life forms of science fiction such as conscious interstellar gas, we find that they all share things with real life as we know it. If life is based on biochemistry and therefore the cold laws of physics, and is a result of complexity and pattern-preservation, then these forms of life could actually be possible.