A question that has puzzled us humans for a very long time; but the real puzzling question is 'Are we alone in the Universe?'. You might be thinking 'What's the difference?'.
When people think of aliens, they tend to imagine what they've seen in SF movies, on the Internet, read in books etc, like little green humanoids, humanoids with various ridges on their heads, different colored skin etc. But aliens can be simple bacteria, microorganisms that just happened to evolve on another planet. Or they could be something completely different, something we've never encountered before. So extraterrestrials can take on many forms, but I believe that what people really want to know is if we will ever encounter intelligent extraterrestrial life.
In this first part I will not talk about intelligent alien life but about the possibility of finding simple alien life which may not be so spectacular but would still represent one of the greatest discoveries of all time.
Primitive alien life
Primitive alien life means unintelligent extraterrestrials. This type of alien life could be quite common throughout the Universe. As we've seen here on Earth, simple organisms can survive in even the most extreme environments (deep in the oceans, frozen in the ice, in the hot arid deserts, some bacteria even survived for a while in space when they were brought there by accident and it is well known that many insects can survive intense radiation which is fatal for other organisms). So primitive lifeforms are very resilient and adaptable but does that mean that we can find them on other planets? Not necessarily. While it is true that some organisms from Earth could survive on other planets in our solar system (Mars for example) you would still need some special conditions for life to appear there in the first place. Conditions like those found on our planet.
So how did life on Earth appear?
Earth is located in what is called 'the Goldilocks zone' or 'habitable zone' of our solar system. More precisely it is located at a certain distance from the Sun to allow the existence of liquid water on its surface. It is a consensus among scientists that water is a fundamental ingredient for life. All known forms of life depend on water. It is believed that comets, which contain ice, crashed into our planet during its early stages of formation and brought water here. So we know of water but what else is needed for life to appear? Well, living beings are composed of organic molecules which contain the elements carbon, hydrogen, oxygen and nitrogen. These are light elements which are produced in abundance by stars in the process of nuclear fusion, and were present on primordial Earth. But having the elements and putting them together in the desired order are two very different things. In this case, the 'desired order' would be that of amino acids.
Amino acids are also essential for life, since they form the building blocks for proteins and have many functions in metabolism. So how did amino acids appear? Unfortunately nobody knows for sure but it is believed that the conditions on primordial Earth were good enough for it to happen.
An experiment carried out in the 1950s called the Miller-Urey experiment simulated the conditions of the early Earth. The result was very promising as 22 amino acids were formed.
From this point on, evolution takes over and, over a timescale of billions of years (Earth has been around for about 4.5 billion years and it is thought that life appeared 1 billion years later), things become more and more complicated as lifeforms begin to emerge and develop.
If this process happened here then it is very likely that it will happen (or has happened) in places where the conditions are similar.
There are many who believe that life didn't originally start on Earth but someplace else (for example Mars again) and somehow ended up here (via asteroid maybe). This theory is called panspermia, but it's just shifting the problem to another place. Somewhere somehow the conditions were right for life to appear and that is what's important.
It's important because in a galaxy of 400 billion stars and who-knows how many planets, like our own Milky Way, it is very likely that the conditions necessary for the formation of life can exist in more than one place. I am still talking about primitive life. In the 3.5 billion years since life exists on our planet, humans have been around for only 200,000 years (less than 0.005% of this time). This is what's called the 'Rare Earth Hypothesis', the belief that simple life is abundant throughout the Universe but complex life is not.
Based on this reasoning, in order to find alien life we must search for planets displaying similar conditions to Earth (Earth-like planets). This is why modern telescopes, using techniques such as radial velocity measuring, transit timing variation, gravitational microlensing detection etc, are searching for extrasolar planets within a star's habitable zone. Although many extrasolar planets have been found (almost 500), very few of these are Earth analogs (a notable planet is Gliese 581 d).
Modern planet-finding telescopes like Kepler and others that will be launched soon, show promise and may find many habitable extrasolar planets; though answering the question of whether or not there is any life on those planets may prove difficult without actually sending a probe there.
I agree with the Rare Earth Hypothesis and I believe it is only a matter of time until we discover primitive alien life.