Life in The Universe & The Drake Equation
The Drake Equation. Our universe is suitable for life. How widespread will it be? We suspect that the vast majority of life forms will have the same basic chemistry as our carbon-based life. The elements that play a major role in our chemistry (carbon, oxygen, and nitrogen) are that first produced in stars and are thus very common. In addition, carbon has the most diverse chemistry of any element. It is not impossible that other life forms could exist based on phosphorus, arsenic, and methane, but these would, the author suspects, be far less common.
So, in most cases, we need locations where water is a liquid, such as the surface of a planet in its star’s habitable zone or perhaps an under-ice ocean of a satellite warmed by the tidal heating of a nearby giant planet. If we hope that other advanced civilizations such as our own exist then significant periods of time are needed – to allow the simple life forms that may arise a chance to evolve.
Drake Equation History
In 1960, Frank Drake, the found of Drake equation, who the previous year had made the first search for signals from an extraterrestrial intelligence in Project Ozma, gathered a group of eminent scientists to try to estimate how likely it was that other intelligent civilizations existed in the Galaxy and might perhaps be transmitting signals that we could detect by observing programs covered by the term SETI (Search for ExtraTerrestrial Intelligence).
This group produced what has become known as the Drake equation, which has two parts. The first part attempts to calculate how often intelligent civilizations arise in the galaxy and the second is simply the period of time over which such a civilization might attempt to communicate with us once it has arisen.
Factors & Condition for Life
Some of the factors in the Drake equation are reasonably well known; such as the number of stars born each year in the galaxy, the percentage of these stars (like our Sun) that are hot enough, but also live long enough, to allow intelligent life to arise and the percentage of these that have solar systems. Others are far harder to estimate. For example, given a planet with a suitable environment, it seems likely that simple life will arise – it happened here virtually as soon as the Earth could sustain life. However, it then took several billion years for multicellular life to arise and finally evolve into an intelligent species. So it appears that a planet must retain an equable climate for a very long time.
The conditions that allow this to happen on a planet may not be commonplace. Our Earth has a large moon which stabilizes its rotation axis. Its surface is recycled through plate tectonics which releases carbon dioxide, bound up into carbonates, back into the atmosphere. This recycling has helped keep the Earth warm enough for liquid water to remain on the surface and hence allow life to flourish. Jupiter’s presence has reduced the number of comets hitting the Earth; such impacts have given the Earth much of its water but too high an impact rate might well impede the evolution of an intelligent species. It could well be, as some have written, that we live on a ‘rare Earth’. How many might there be amongst the stars?
In addition, it has been widely assumed that once multicellular life formed, evolution would drive life towards intelligence, but this tenet has been challenged in recent years – a very well adapted, but not intelligent, species could perhaps remain dominant for considerable periods of time preventing the emergence of an intelligent species.
The final factor in this part of the Drake equation is the percentage of those civilizations capable of communicating with us who would actually choose to do so. Our civilization could, but currently does not, attempt to communicate. Indeed there are some who think that it would be unwise to make others aware that here on Earth we have a nice piece of interstellar real estate! Any attempts at communication have to be made in the very long term – the round travel time for a two-way conversation would stretch into hundreds or thousands of years. It would be hard at present to obtain funding for such a program. It is often cited that perhaps between 10% and 20% of civilizations would choose to communicate, but I suspect that this may well be highly optimistic.
Communicating with Extra-Terrestrial Life
The topic of ‘leakage’ radiation from, for example, radars and TV transmitters are often mentioned as a way of detecting advanced civilizations that do not choose to communicate. This, the author believes, unlikely. Any signals that could be unintentionally detected over interstellar distances are, by defi nition, wasteful of energy. Already, on Earth, high power TV transmitters are being replaced with low power digital transmissions, satellite transmissions are very low power and fibreoptic networks do not radiate at all. The ‘leakage’ phase is probably a very short time in the life of civilization and one that we would be unlikely to catch. It could be that airport radars and even very high power radars for monitoring (their) ‘near-Earth’ asteroids might exist in the long-term, and give us some chance of detecting their presence, but we should not count on it.
When all these factors of the Drake equation are evaluated and combined, the average time between the emergence of advanced civilizations in our Galaxy is derived. If we find it hard to estimate how often intelligent civilizations arise it is equally hard to estimate the length of time over which, on average, such civilizations might attempt to communicate with us. In principle, given a stable population and power from nuclear fusion, an advanced civilization could survive for a time measured in millions of years. Often a period of 1000 years for this ‘communicating stage’ is chosen for want of anything better. This length of time is critical in trying to estimate how many other civilizations might be currently present in our Galaxy. If, for example, a civilization arose once every 100 000 years – a reasonable estimate – but typically, civilizations only attempt to communicate for 1000 years, it is unlikely that more than one will be present at any given time. If, however, on average, they remain in a communicating phase for 1 million years then we might expect that nine other civilizations would be present in our Galaxy now.
The Drake Equation
When the Drake equation was first evaluated, the estimates of the numbers of other civilizations were quite high; numbers in the hundreds of thousands or even as high as 1 million were quoted. Nowadays astronomers who try to evaluate the Drake equation are far less optimistic. Many estimates are in the range of 10–10 000 but there are a minority of astronomers who suspect that, at this moment in time, we might well be the only advanced civilization in our Galaxy.
The truth is we just do not know. It was once said with great insight that ‘the Drake equation is a wonderful way of encapsulating a lot of ignorance in a small space’. Absolutely true, but an obvious consequence is that we cannot say that we are alone in the galaxy. SETI is our only hope of finding out.
The author’s own belief is that simple life will be widespread in the galaxy, but that few locations will keep stable temperatures for sufficient time to allow advanced civilizations to arise. Optimistically, their number might be in the tens to hundreds but it may very well be that none of these would choose to try to contact us so that we would remain in ignorance of their presence.