Are we alone? Is there life out there in the vast expanse of space? Such questions have long been the domain of fantastical science fiction, and when we think of extra-terrestrial life, we think inevitably of tall green anthropomorphic aliens – the eponymous ET. But for nearly 50 years, the search for life in the universe has been a scientific pursuit too. In 1961, the field of astrobiology – the search for life beyond our terrestrial backyard – was born with the formulation of a simple equation.
Frank Drake, an astronomer and astrophysicist, was one of the first scientists to start looking for life in the universe. Using radio-astronomy, scanning celestial objects on radio frequencies, he chose normally quiet frequencies to listen for possible alien communication. Listening to two stars, both a similar age to our sun, for six hours a day over four months, Drake was confident that if there were communicating life forms out there, we would find them. When the vast data set was examined for patterns, all that was discovered was a secret military satellite. No, ‘Hello, we’re over here!’ from 11 light years away; no help beacon from a dying civilisation; no indication, in fact, that anything was out there.
So is that it? Does a lack of radio signals on a single frequency mean that humans are the only intelligent life form in the entire universe? Put like that, it seems far from conclusive, and in 1961 Drake attempted to quantify the probability of there being intelligent, communicating civilisations in our galaxy – cue the ‘Drake Equation’(see below). Its purpose is to break down all the factors necessary for a communicating civilisation to develop, apply a probability to each, and thus predict the number of civilisations we could list in our galactic phonebook.
This was an insightful, reductionist way of dealing with the problem. Unfortunately, many of the factors were either unknown or unknowable. The ‘lifetime’ of a communicating civilisation, for example, lies more in the social sciences, and cannot even be statistically tested with our current sample size of one. Nevertheless, even conservative estimates of each of the factors gives a number greater than one. As such, enthusiasm for the search for life in the universe has blossomed, giving rise to the suite of projects allied to SETI – the Search for Extra-Terrestrial Intelligence.
SETI projects have mostly continued to focus on scanning the skies for alien transmissions. The global following of the search is immense: the non-profit organisation The SETI League have created a global network of amateur-built radio telescopes pointed skywards, watching and waiting. Similarly, the SETI@Home project invites internet users to contribute computer power to analysing radio-astronomy data for signs of communication. Truly, the worldwide scientific collaboration is commendable. And what has this global search turned up? Nothing. It seems, then, that we are alone.
But stop there! OK, we haven’t found any other intelligent life forms that are communicating on radio frequencies, but are we not perhaps jumping the gun a little? Would it not be equally as enlightening to find life at all on another planet, whether it is intelligent or not? It would certainly give a more complete picture of how we came to be here on Earth. Discovering the range of interstellar biology would provide a ‘bottom-up’ approach to searching for more advanced organisms and, ultimately, intelligence. The modern incarnation of the field of astrobiology is concerned more with this, with the active search for life and its repercussions in the universe, than the somewhat stay-at-home approach of SETI. Astrobiology today is a broad collaboration between astronomers, cosmologists, earth scientists, biologists, chemists and engineers, with over 30 research groups working on different approaches to understanding the place of life in time and space.
How do you go about finding life if it isn’t actively trying to communicate? The first problem is what exactly to define as life. There are as many as 60 different definitions of life, depending on your point of view – for example the widely used biological MERRINGS (movement, excretion, respiration, reproduction, irritability, nutrition, growth) system, which is little use in testing fossil organisms, or atypical life forms, or in fact, anything we find in space. Astrobiologists choose to use the short NASA definition as a starting point: ‘Life is a self-sustained chemical system capable of undergoing Darwinian evolution. Working from this basic definition gives broad scope for investigation of early life forms across the many light years of space.
The first step in our search for alien life is to understand how to get life in the first place. Tying intimately into studies of early life on Earth, palaeontologists, geologists and chemists work together to discover the timing, likely environment and mechanisms of the origin of life. There are intermediate states of life that would seem very strange to an observer today, but were essential in the development of life as we know it. A cell with a fundamentally different metabolism to today was likely to be a common sight on the early Earth. Understanding these life processes may be particularly important in identifying newly emergent life on other planets.
Secondly, once life is established, it is the job of microbiologists and earth scientists to understand the limits of that life. On Earth, living things were thought to only penetrate to about 10 cm deep in soils, 10 m deep in water, and to die out as altitude increased. Now, however, we find life of one form or another pretty much everywhere we look. It can survive at temperatures from -20 °C to around 120 °C; pressures of up to 1060 MPa, equivalent to 50 km beneath the Earth’s crust; and extremes of pH (both acid and alkali) and salinity. Such information is invaluable in the search for life elsewhere in the solar system and beyond, as it extends the range of so-called ‘habitable zones’, the area around a star where it is believed that life can exist. Depending on the size and age of a star, the nature of the planets surrounding it, and the range of conditions that life could tolerate, the size and position of habitable zones within other solar systems may be considerably different to that within our own.
Having established how and where life could exist outside Earth, the search can begin for likely habitable worlds. The most obvious place to start is our own solar system, and there are cases for potentially habitable environments either now or in the past on Mars, Venus, the Jovian moon Europa and the Saturnian moons Titan and Enceladus. These bodies, although almost certainly not harbouring intelligent, advanced life forms, are important short term destinations for astrobiological exploration, including investigation by remote or manned missions.
Astronomers and cosmologists are also occupied in finding habitable planets orbiting other stars. Extra-solar planet searches turned up the first results in 1996 and have, at the time of writing, located 452 bodies orbiting other stars in our galaxy. Most are the size of Jupiter or greater, because of resolution limitations, but a number of planets of little more than a few Earth-masses have been found. It is thought that the Earth-sized rocky planets, thought to be more habitable than larger bodies, greatly outnumber the larger planets in the galaxy.
So what happens if we do find life? Whether close or far, simple or advanced, are humans as a race equipped to deal with the knowledge that we are not alone? Needless to say, any astrobiological revolution will deeply affect our philosophical and social outlook, as well as transforming our scientific goals and our view of the universe. Currently, despite the fact that we are yet to find conclusive evidence of life anywhere, there are reams of UN legislation and quarantine regulations to ensure planetary protection in the event of living sample return. Far from allowing a District 9-esque cohabitation, any alien life, whether microscopic or advanced and gigantic, will never leave a sealed container in quarantine at the landing site.
Clearly there are many theoretical and practical obstacles to be overcome in our continuing search for life in the universe. But the field of astrobiology is yet young. The first man-made object was launched into space only 53 years ago. Even in the short period of human history, this is just a blink of an eye, and technology is moving faster every day. In the words of the brilliant departed astronomer Carl Sagan: ‘How lucky we are to live in this time, the first moment in human history when we are, in fact, visiting other worlds.’
Leila Battison, 2010