Why Haven't We Found Alien Life Yet?

 by Max Harvey

The Milky Way (source: NASA)

Whilst usually I like to write about the search for alien life, talking about scientist's latest discoveries of exotic exoplanets or some exciting discovery of new chemistry that appears in the gas clouds of stellar objects, I wanted to focus this time on why it is that we have been unable to find alien life as of yet. Unsurprisingly, just as there are many theories as to where extraterrestrial life may first appear, there are just as many, if not even more ideas as to why we have been unable to find life elsewhere in the universe. The first big issue is to define what kind of life it is that we are aiming to look for. In this post instead of focusing on the immensely slim chance of our scientists being able to identify bacteria in distant galaxies or the long term goals to collect data using deep space probes, I would like to explore why we ourselves haven't been discovered by some other intelligent civilisation. Why has our solar system or Earth not been visited by interstellar explorers who came to document and analyse our movements and mannerisms? Or why have we not been contacted by some mystical deep space message from the inhabitants of another world like our own? I guess a more suitable title for this post would be 'Why haven't we been found by alien life yet?'. I believe that it is quite a sensible assumption to believe that if other intelligent life exists within the universe, that they would like us, wish to scour their own skies in search of some companions in the universe. So why have they not found us?

The most obvious answer to the above questions is of course that we really could be alone within the universe. Maybe we really are the only life in the whole of the universe and thus not only would it be impossible for us to be visited by alien life, but also for us to find any other life ourselves. However, this conclusion sounds pretty lonely and also doesn't make for a very interesting blog article so I myself choose to believe that this is not the answer and that there must be some other life out there in the rest of the universe.

Unfortunately, determining whether or not we are alone in the universe is not an easy task and the mere evolution of life is itself a disastrously complex process that relies upon many factors such as the availability of habitable planets containing the correct resources or the location of a planet within a stable star system in which the star is not going to suddenly go supernova and vaporise everything in close proximity. One useful method in trying to answer the question posed at the beginning of this post is through use of what is called the Drake equation (named after the American astrophysicist who coined the equation, Dr Frank Drake). The equation attempts to incorporate a number of factors that would influence the likelihood of a civilisation capable of detecting our presence in the universe arising and then tries to calculate how many civilisations of this kind there are in the whole universe. It looks a little bit like this:

Now although I could go into a lot of detail about each of these factors, I am only going to focus on a few of the most important ones. The output of the equation, N, is the number of civilisations in our galaxy with which communication might be possible. As you can see this equation is used just for one galaxy, but can be easily scaled up for the 2 million million galaxies in order to encompass the entire universe. The value of N, or in other words the number of civilisations that we might be able to communicate with, is dependent upon a number of factors which are the other terms of the equation. These factors are:

R - the rate of star formation within the galaxy

Fp - the fraction of stars that have planets

n - the number of planets per star that could potentially support life

Fl - the fraction of suitable planets upon which life actually develops

Fi - the fraction of planets with life where life evolves to be intelligent

Fc - the fraction of intelligent civilisations that develop technology that could potentially communicate with us

L - the lifetime of that civilisation.

Out of these factors, there are a number of particularly interesting ones. Firstly, the point that not only does life have to arise on these planets, but it also has to evolve into an intelligent form of life such that it can form a civilisation which then has to develop technology in order to potentially be able to communicate with us. This greatly reduces the chances of us actually being able to communicate with extraterrestrial life even if it does exist. Additionally, it's interesting to note that any civilisation that did become intelligent, would not persist forever, either due to exhaustion of its colony's resources or mutual destruction due to civil war, these intelligent civilisations will eventually be destroyed leaving only a window of time within which they are eligible for communication.

Of course we have no exact values for each of the parameters within the equation and so we must rely on estimates in order to predict the final value of N. The estimated values have large ranges and consequently provide resulting values of N which range from 9.1 × 10−13 to 15,600,000. So, not very conclusive. It is important to note that these values refer just to the Milky Way galaxy as an isolated galaxy and not the entire universe. However, the former of these values suggests that we are almost certainly the only intelligent contactable life within our galaxy whilst the latter suggests that the Milky Way is positively bursting with civilisations that we could contact.

One can also not ignore the possibility that maybe there is an intelligent civilisation out there that has detected us, but simply that they have chosen not to contact us as they felt it would not be beneficial to their development. Additionally, their transmission could be on its way but will take another decade to reach us due to how far away their civilisation is and thus the long time it would take for the light signals to traverse the separating distance.

Ultimately the takeaway is that we really have no idea whether or not we are alone in the universe or whether or not we could get contacted tomorrow by some extraterrestrial transmission. Whilst the Drake equation can help to provide estimates, the low accuracy of the parameters used within the equation lead to a large spread in resultant values. Hopefully over the coming years, the accuracy of these values can be refined leading to an overall more accurate answer for the value of N, and hence enabling us to say whether or not we really are alone with much more certainty.