"There are infinite worlds both like and unlike this world of ours." So wrote Epicurus around the year 300BC. Giordano Bruno was burnt at the stake in 1600, in part for saying there are other stars and solar systems beyond our own. To avoid similar complications, Christiaan Huygens' book Cosmotheoros, in which he speculates about what life on other planets is like, was published in 1698 three years after his death.
The question of whether there is extra-terrestrial life is one that has preoccupied and fascinated human thinkers throughout history. We have only had telescopes for 400 years and space telescopes for 50 years, so most of our great philosophers and astronomers who have addressed the subject have been operating in the dark with little evidence to go on.
In recent decades however, technology has allowed us to move beyond pure speculation. We have sniffed for the tell-tale signs of water within our solar system, analysed incoming radio signals for patterns indicating alien communication and are now searching distant stars in our galaxy for rocky, Earth-like planets. While these experiments have yet to provide a definitive answer, they are generating data that are telling us more about the probability of there being other forms of life out there.
Most space scientists agree that based on the huge numbers of stars and planets out there, it is unlikely that life has only occurred in our little corner of the universe. Bookmaker Paddy Power is currently offering odds of just 16- 1 that existence of extra-terrestrial life will be confirmed this year. Leaving aside the question of dates, alien life is probably not in the form of the little green men so beloved of science fiction. Yet I do believe complex intelligent organisms in various forms are out there. This does not mean though that we should prepare the alien reception committee just yet. The question that is often overlooked is how likely is it that we will find them or they us?
The numbers are a good place to start. There are approximately 200 billion stars in our galaxy alone. Each of those potentially has a number of planets going around it. The Hubble Space Telescope Deep Field observation, in which light was gathered from a small segment of space for hundreds of hours, revealed that there are at least 100 billion galaxies, each of which contains billions of stars. That adds up to a lot of planets.
The Drake Equation, devised in 1961 by Frank Drake, Emeritus Professor of Astronomy and Astrophysics at the University of California, is a mathematical formula designed to estimate the number of detectable extra-terrestrial civilisations in our Milky Way galaxy. Note the important word detectable. The answer is the result of multiplying seven variables the number of stars in the galaxy, the percentage of these that have planets, the average number of life-supporting planets per star, the proportion of these on which life actually develops, the proportion on which intelligent life develops, the percentage with communication technology, and the proportion that release detectable signals in the right time frame for us to pick them up.
We can input values into the Drake Equation, with some based on solid science and others based more on guesswork but for which science may one day provide firmer evidence. We can start with 200 billion stars in our galaxy, with, say, 20% having planets, with an average of four planets per star capable of sustaining life, leaving us with 160 billion candidates.
Traditionally we have assumed that life will occur only on planets in a star's Goldilocks or habitable zone. This is the region in which liquid water occurs and conditions are thought to be favourable for life. NASAs on-going Kepler mission is one of the experiments that will help us reach more solid numbers to input into Drakes equation. Launched in 2009, it uses a space telescope with a very large field of view to monitor the brightness of more than 100,000 stars continuously for at least 3.5 years. When a planet crosses in front of its star, this is known as a transit, as in the recent transit of Venus. A terrestrial planet transit causes a small temporary drop in the brightness of its star, which tells us that the planet is there and allows us to calculate its size. Kepler will determine the abundance of Earth-like and larger planets in or near the habitable zone of stars, and help us estimate how many planets there are in multi-planet systems. So far, we've detected about 800 exoplanets, or planets beyond our solar system.
While peering into space is generating a great deal of new knowledge, we still have a lot to learn down here on Earth. In a BBC2 documentary I presented called Do We Really Need the Moon?, we looked at the theory that suggests tides might have triggered the precursors of DNA, the genetic blueprint of life, from the chemical soup that already existed on Earth. The idea is that these chemical precursors were formed as a result of interactions in a tidal pool, where they went through repeated cycles of being wetted by the incoming tide, dried out by the Sun and radiated. We dont know if this really was the process from which life emerged, but it potentially raises the question of whether you need the presence of a moon for life to occur.
Until not so long ago most scientists assumed that there was no life at the bottom of the ocean because of the lack of sunlight. While around 500 humans have flown in space, only three have been to the deepest depth of the oceans, the Mariana Trench, which lies at 10,994 metres below sea level in the Pacific; the most recent being film director James Cameron earlier this year. We have only recently begun to explore these environments and recent discoveries in the murky depths suggest we should think more broadly about the conditions in which life can develop. Some have suggested that hydrothermal vents, which bring very hot, mineral-rich water gushing through the ocean floor, could have provided the spark that triggered the creation of early life. Japanese scientists recently simulated the conditions inside a thermal vent and were able to generate an electric current. They think this could have helped convert carbon dioxide into complex organic molecules that formed the chemical precursors of life. The more we find out about the conditions in which life can occur, the more precise our calculation of the likelihood of extra-terrestrial life becomes.
Our imaginations are naturally constrained by what we see around us, and the conventional wisdom has been that life needs water and is carbon-based, but some researchers are doing exciting work, playing with ideas such as silicon-based life forms. Silicon is just below carbon in the periodic table, has some chemical similarities and is widely available in the universe. So perhaps we could imagine similar instructions to DNA but with silicon. Maybe life doesn't have to resemble anything like DNA at all.
Over the next 50 years we are going to identify many more planets outside our solar system, and, I believe, our conception of the forms life can take will also change. While we can see stars because they radiate a lot of light and heat, planets give off little radiation for us to detect.
However, new, more advanced instruments may allow us to analyse their atmospheres. Just a few weeks ago, the go-ahead was given for the construction of the worlds largest optical telescope. The European Extremely Large Telescope, to be built on top of a mountain in Chile, will have a main mirror measuring some 40 metres in diameter. This compares with the currently operating Very Large Telescope, a suite of interconnected optical telescopes with four primary mirrors measuring 8.2 metres across. Then there is the conceptual design for the futuristic Overwhelmingly Large Telescope, with a single mirror of 100 metres in diameter.
Over the next 50 years we are going to identify many more planets outside our solar system, and, I believe, our conception of the forms life can take will also change.
These telescopes with unprecedented power will allow us to probe the chemical make-up of the atmospheres of these exoplanets. We may find unusual results such as spikes of chemicals that we weren't expecting. Maybe well see spikes of silicon compounds, indicating silicon-based life forms, for example. Flowing methane and pools of methane have been identified on Titan, one of Saturns moons. It makes me wonder whether a form of life could be based on liquid methane. Some of the creatures we have found on the bottom of the ocean floor use thermal energy from the centre of the Earth rather than sunlight. This variant on life and the other possibilities being looked at lead me to believe we may need to extend our notion of where alien life might be found.
Returning to the Drake Equation, lets assume as our best guess that life develops on 0.5% of planets. I think most alien life will be in the form of a gooey soup of single-celled organisms and that multi-celled intelligent life only occurs on a few planets because more steps are needed to get there. Lets say that 1% of extra-terrestrial life forms are intelligent. At this point it might be prudent to consider whether we are talking about friends or foes. Stephen Hawking once said that we shouldn't be sending signals out into space because if there are life forms that are more advanced than us out there it might not be wise to tell them where we are. I dont believe we should be worried. If they are very intelligent I dont know what we have that they would want enough to attack us. We do have water on Earth but if they are travelling halfway across the galaxy to find us theyll bump into plenty of water on the way. They will probably see us as we see ants oddities that are perhaps worthy of study.
Having decided we do want to contact or be contacted by alien life, we have to multiply our remaining 8 million intelligent life forms by 5%, the fraction I estimate will have developed the technological capability to communicate. But if our intelligent aliens existed only during the time of the dinosaurs on Earth, its not much good to us, so we must also multiply by 0.001% to get to the proportion which coincide in time with us. This leaves us with an estimate of four intelligent alien civilisations in our galaxy with the means to communicate and overlapping in time with humans.
Nevertheless there remains a very big problem. Communicating across the galaxy is challenging. The Voyager 1 spacecraft, which is carrying a recording of greetings from Earth in different languages, has been travelling through the solar system since the 1970s and has only just made it into deep space. To get to our nearest neighbouring star, Proxima Centauri, would take it 76,000 years.
Even if we were able to develop technology to allow us to travel at the speed of light, which we are nowhere near doing, the journey would take 4.2 years, and most other stars are much further away. If one of our four alien civilisations is on the other side of the galaxy then it would take millions of years just for electromagnetic signals to travel that distance, and by then they may be too weak to be detectable.
The notion that we constitute the only little spike of life in the universe is a lonely one, and humans seem to have a natural urge to want there to be other life out there. Perhaps we can take comfort from the likelihood that there are all sorts of weird and wonderful other life forms out there. However the way the maths stacks up, those who like a flutter are probably better off backing horses of the traditional rather than the extra-terrestrial variety.