In 1961, Professor Frank Drake attempted to estimate the number of extra-terrestrial civilizations in the Milky Way with which we might come into contact by making several assumptions. The Drake equation [1] states that:

N = R* x Fp x Ne x Fl x Fi x Fc x L


N = the number of civilizations in our galaxy with which communication might be possible;


R* = the average rate of star formation per year in our galaxy

Fp = the fraction of those stars that have planets

Ne = the average number of planets that can potentially support life per star that has planets

Fl = the fraction of the above that actually go on to develop life at some point

Fi = the fraction of the above that actually go on to develop intelligent life

Fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space

L = the length of time such civilizations release detectable signals into space.

Drake gave each parameter the following values:

R* = 10/year (10 stars formed per year, on average over the life of the galaxy)

Fp = 0.5 (half of all stars formed will have planets)

Ne= 2 (stars with planets will have 2 planets capable of supporting life)

Fl = 1 (100% of these planets will develop life)

Fi = 0.01 (1% of which will be intelligent life)

Fc = 0.01 (1% of which will be able to communicate)

L = 10,000 years (which will last 10,000 years).

So that N = 10 × 0.5 × 2 × 1 × 0.01 × 0.01 × 10,000 = 10.

Recently, Professor Paul Davies has made a different estimate with a range of different values in the Equation [2]. His N is between 1 and a billion!

I find Drake’s approach strange. A more logical approach might be to ask how many stars there are in the Galaxy. If there are between 100 and 400 billion stars, if half of all stars have planets, if there is life on only one planet in each system, but if only one in a million of those planets develops intelligent life, then there are between 50,000 and 200,000 planets with intelligent life.

Of course the values chosen for the Equation are highly questionable; they are merely wild guesses. However, one can question some more than others. The guess that, where stars have planets, two of them will harbour life is hardly justified from the example of the Solar System, where, as far as we know, only one planet (Earth) carries life. Even that change could halve Drake’s estimate to five. More importantly, these estimates seem to overlook the circumstances in which intelligent life has emerged on Earth. In particular, the value given to Fi (that intelligent life emerges on only one in a hundred planets where life has developed) is questionable.

It is easy to assume that because we exist, intelligent life is common (see the popular belief in aliens). However, we should consider the peculiar circumstances that have allowed us to evolve. Although life appeared very early on Earth (at least only 500 million years after the planet’s birth), multicellular life did not emerge until about 600 million years ago (MYA), fish only 500 MYA, reptiles only 300 MYA and our species only about 500,000 years ago. So it may be that modern humans have existed for only about 0.1 per cent of the life of the planet and it is certain that our modern technological civilization has existed for only about 200 years (~0.00004% of the life of planet Earth). That is a chance of only 1 in 2.5 billion that anyone looking for an advanced technological civilization (ATC) on Earth between the planet’s birth and now would be successful. What does that say for our chance of finding another ATC now?

Then consider the possibility that such a civilization will destroy itself. Nuclear war could have destroyed our civilization in 1962, before we even began looking for signals from another Galactic civilization (although not before our radio, TV and radar signals leaked out). This could lead to the conclusion that the chance of finding another ATC at this time is vanishingly small (Paul Davies allows for fi to be zero).

The Equation does not appear to have made allowance for the fact that we owe our existence to the demise of the dinosaurs 65 MYA. It should not be assumed that such destruction does not threaten other planets, or that it does. Without that event, the dinosaurs, who had ruled for 180 million years would probably still rule the Earth. If life on other planets follows such a path, do we have to assume some equivalent calamity before intelligent life can emerge? If so, what odds do we put on it?

Another important factor is our Moon, which is unusual in being so large and influential. We already believe that the Moon’s birth was the result of a catastrophic collision been the proto-Earth and another planetismal the size of Mars. How typical would such a collision be and what odds do we put on it occurring in a planetary system? If the result is a moon such as ours and such a large moon is unusual, then perhaps such collisions themselves are unusual. But does that mean that we owe our existence, inter alia, to the Moon?

Professor Neil F. Comins asked himself what the implications would be if the Moon did not exist [3]. There would have been many differences, including a shorter rotation period and a different chemical composition, but those that might influence the development of life include the possibility of a different tilt axis and instability of that axis. The Moon, besides gradually slowing Earth’s rotation, also stabilizes Earth’s axis. The lack of the Moon would mean smaller ocean tides, perhaps making the transfer of life from the oceans to land more difficult. It may also have meant more bombardment of Earth by asteroids and/or comets (the Moon has shielded Earth to some extent). This may have interfered with the development of life. Comins also thought that a Moon-less Earth (he called it ‘Solon’) would have a different atmosphere, with such a large amount of carbon dioxide that ‘life as we know it may never have been feasible’.

It has already been observed that our civilization has developed in a balmy interglacial, but Professor James Hansen has recently drawn attention to the fact that (unusually) sea levels have been remarkable stable for the last 7000 years (the climate kept an ice sheet from forming in Canada but kept stable ice sheets in Greenland and Antarctica). He pointed out that, because our major civilizations have mostly developed on coasts, especially on river deltas, this may have contributed to the development of civilization. Repeated changes in sea level would have inhibited the development of civilization [4].

Most anthropologists agree that bipedal hairless apes (humans) evolved out of many other varieties of hominins due to fortuitous climatic changes. Some believe that these forced our ancestors out of the trees onto the African savannah (the ‘Tarzan hypothesis’) and some believe that we evolved our special characteristics, not least of all our large brains, in an aquatic environmental excursion (hardly a normal evolutionary experience) [5]. Either way, we appear to owe our emergence to random climatic fluctuations. How typical would that be of life on other planets?

Some point to the explosion of the super-volcano Toba (Indonesia) about 70,000 years ago, which may have led to the extinction of many rival hominins and severely reduced our own numbers and created a bottle neck in our evolution. This catastrophe may also have been the trigger for our migration out of Africa, which itself may have led to the development of civilization. It is fortunate for us that no other super-volcano has erupted since (the next one to do so may be the end of civilization).

Does it not seem that we have been lucky [6]? Or rather that we owe our existence to a series of fortuitous chance events that must be rare in themselves never mind in combination? If that is true, then we probably are a very rare phenomenon: an intelligent species that has developed advanced technology, even now venturing into space. My guess is that the chance of another such species emerging elsewhere in our Galaxy is almost nil and we may indeed be alone, even in the whole universe.


  1. See
  2. The Eerie Silence: Are We Alone in the Universe? by Paul Davies (2010, Allen Lane).
  3. ‘The Earth Without the Moon’, Astronomy 19:2 (Feb 1991); later in What if the Moon didn’t exist? by Neil F. Comins (1993, Harper Collins, New York).
  4. Storms of My Grandchildren by James E. Hansen (Bloomsbury, 2009).
  5. The Aquatic Ape Hypothesis by Elaine Morgan (1997, Souvenir Press).
  6. Lucky Planet – Why Earth is Exceptional – and What that Means for Life in the Universe by David Waltham (Icon Books, 2014).

Steuart Campbell

Steuart is a science writer, a member of the ASE and a regular contributor to the Journal.