Whenever the topic of extraterrestrial life comes up, there's usually an almost unanimous agreement that life probably does exist elsewhere in the universe. The odd thing, however, is that there's absolutely no evidence at all that it does!
Why do people believe in alien life despite the lack of any evidence?
Well, people are aware of the vastness of space, the limits of current technology, and the unlikelihood of life existing so conveniently close to us that we could easily detect it; so the lack of evidence need not be due to the lack of extraterrestrial life, but may simply be down to our inability to discover it.
The lack of evidence, however, means that we can only make rational inductive arguments on the issue. i.e. we can supply reasons (premises) that support our conclusion (alien life exists, for example) but the conclusion doesn't necessarily follow even if the premises are true.
When looking at the reasons people put forward to support their belief in extraterrestrial life, they tend to fall into three categories:
- The principle of plenitude
Plenitude means abundance. This is the most common reason people cite when arguing for belief in alien life. The arguments are normally expressed using phrases about the size of the universe or the sheer number of stars and galaxies out there. The idea being that even if the formation of life is a very unlikely event in the universe, the fact that there is such an abundance of matter means that there being more than one place in it which is favourable for the formation of life is bound to arise.
In nature it is usually the case that if something can happen it will - given an abundance of chances.
- The principle of uniformity
This is based on the fact that the laws of physics/nature are the same throughout the universe. Stars and galaxies, no matter how distant, all show the same composition and exhibit the same properties. This implies that the laws of nature that produced life on Earth are equally as valid throughout the universe therefore the physical mechanisms that produced life on Earth could also produce life elsewhere.
This point is usually implicit, rather than explicit, in people's reasoning: without this principle, the principle of plenitude would cease to be a good supporting reason for the argument for extraterrestrial life as it wouldn't matter too much how many stars and galaxies there were if they were completely different from ours.
- The principle of mediocrity
We orbit an ordinary, average star in an ordinary galaxy, in an ordinary position in space. No other Earth-like planets have yet been discovered, but given the abundance of Iron, Nickel, and silicate material in space, smaller rocky Earth-like planets (as opposed to gas giants like Jupiter) will most likely be similar to Earth.
The implication of that is: if we are not in a special place in the universe and the physical and chemical environment that we are in is not special also, then the resulting biology that has arisen is not special in any way either.
These three basic principles are the basis, in one form or another, on which most arguments for the existence of life on other planets are based.
However, the three principles alone are not sufficient to make a strong case. There's also the complication of how likely it is for life to arise in the first place.
How did life come about in the first place?
There are also three main ideas of how and why life originated.
This puts the formation of life down to one supernatural event. An event that, however briefly, transcended the laws of nature. This could involve the act of a deity or an anomaly or singularity in the laws of nature. This is the explanation held by many religious organisations. It implies that we are alone in the universe, although see the comments on panspermia theory (below).
- Rare chance
This is another point of view that states that life happened in one very unlikely event, but one in which no natural laws were broken. Life formation is an extremely unlikely event, which would require many unlikely steps along the way. So unlikely is it to have happened twice, that we are probably the only place in the universe where life has formed.
This view is probably the one best supported by our current state of knowledge (i.e. we exist but there's no evidence anyone else does) but it's not the most popular view.
- Natural process
This point of view argues that formation of life is pretty much inevitable as long as all of the starting conditions are right. A warm planet which is orbiting a stable star, and which has water, carbon dioxide, and simple organic molecules for example, is likely to form life given enough time for evolution to work. The limiting factor is the number of suitable planets available. The phenomenon of self-organisation is often cited with this argument.
Self-organisation occurs in physical systems that are driven far from equilibrium by an energy source - such as the hot conditions in the early years of an Earth-like planet's existence. Simple starting conditions produce complex results. Complexity arises much more readily than would be expected by chance alone.
Most people who believe in life on other planets opt for the third option as it strongly supports the three principles. Even if we did discover extraterrestrial life, there is the complication of panspermia however.
Panspermia theory states that life (in the form of micro-organisms) is capable of being transported through space, perhaps protected inside rocks/meteorites, and can spread from one region to another. This means that life may have originated only once (rare chance) but is spreading throughout space.
An example is: it is entirely possible that life originated in our own solar system on Mars as it will have cooled earlier than Earth. Through meteorite impacts, micro-organisms may have been picked up and eventually deposited onto Earth some time later when Earth was cool enough for life to thrive.
How could we tell if alien life was independent from us?
One way we could possibly tell is by examining the DNA (or its equivalent) against our own. If it had DNA that was different from ours then it would suggest that life had formed independently more than once. If, however, a life form were discovered (in a meteorite for example) that had DNA that was identical to ours it would suggest that life on Earth and the alien life shared a common ancestor.
This would be an important factor should extraterrestrial life be discovered in our solar system (perhaps bacterial life existing in hydrothermal vents on Jupiter's moon Europa). If the bacteria have the same DNA as Earth life then it will be most likely that it shares a common ancestor with us and got there via panspermia (perhaps from Mars!)
The question of whether extraterrestrial life exists is an interesting one not only from the perspective of understanding our place in the universe but also as a thinking exercise in itself.
A strong rational case can be made for extraterrestrial life existing (by invoking the three principles above); however, the three principles do not compel you to reach the conclusion that it necessarily does. For example, if you accept that life arose by a miracle or 'rare chance' then the three principles will not invalidate your conclusion.
No conclusion on this issue can be deemed right or wrong (that can't be decided until or unless we get some evidence for extraterrestrial life), but we can make rational arguments about the issue.
Earth life may have come from Mars