It is often thought that by ‘astrobiology’ we mean the study of extraterrestrial life, because surely ‘astro’ refers to the stars? That is not correct. Astrobiology studies all life in the universe. So both terrestrial and extraterrestrial life. If you only study extraterrestrial life, you are doing exobiology. Astrobiology = terrestrial biology + exobiology.
In our time, astrobiologists are engaged in three major areas of research:
- Exploring possibilities for life on exoplanets: in planetary systems around stars other than our Sun.
- Fathoming life on our extraordinary and complex planet Earth, so far our only known form of life.
- The search for traces of extraterrestrial life in our Solar System.
Life on Earth
How can you efficiently search for life or habitability on other planets? You need to know what you are looking for, and where best to look. This is only possible if you get to know the phenomenon of ‘life’ itself well. Unfortunately, so far there is only one kind of life that we could get to know: Earthly. But fortunately, it is more than worthwhile to study this Earthly life. In this book, we will look for the most basic features of life on Earth, with the aim of learning to assess what we can reasonably expect to find on other planets or moons. How did life arise, and how likely is it? Why did life not just remain in its simple form or bacteria-like? Was it inevitable that complex life emerged afterwards, or just very strange? What major milestones occurred, and how accidental were they? And what characteristics must a planet actually have to contain life and sustain or allow life to develop into such complex organisms as plants, animals and fungi? No astrobiologist escapes the long search for answers to these crucial questions. But fortunately, this quest is an endless source of wonder and job satisfaction.
Searching the Solar System
The environment of our star, the Sun, and its eight planets is untold small in the gigantic universe. Then again, on a human scale, it is so vast that you have to be patient for years to decades each time you gather new knowledge via sent-out probes. Still, I am convinced that we live in a good time to make big discoveries in our Solar System. Meanwhile, we have been sending scientific missions into space for about 60 years, and all the planets have been visited at least once. No sign of extraterrestrial life has ever been discovered. So would there be no life to be found in our Solar System? That seems unlikely to me. We are getting better at learning where and how to search, and the technology of the instruments used continues to improve impressively. So it seems perfectly reasonable to hope that sometime between 2030 and 2060 we will find the first traces of extraterrestrial life! In this book, we will go over how the search for life in the Solar System has progressed so far, and why we expect a breakthrough in the coming decades. If there is extraterrestrial life in the Solar System, it is very likely to be in places that we have not explored on the ground so far, and that we will visit with future missions already in preparation today.
Searching outside the Solar System
And then there is the rest of the universe … Since the late 20th century, we have already discovered thousands of planets around stars other than the Sun, the exoplanets. We know those planets exist. There are countless billions of them, and we know some basic properties of the exoplanets that have been discovered. But today we do not yet have the means to observe or investigate those worlds in detail. I fear it will be a long time before we can do so. Nevertheless, there is continuous and little by little progress in science around exoplanets. And it is very promising, because if we really want to find a world with plants and animals or other forms of complex life, presumably it will only be through that. But in this book, I want to limit myself to the Solar System because it is exactly there that I expect breakthroughs in the 21st century. And because, as a biologist, I find it just a little more fascinating to talk about topics that we can explore and test in our own lives. As fascinating as research on exoplanets is, there will be plenty of other authors who will write a book on that.
I share this view with Bill Bryson, author of the book “a small history of almost everything”:
We are not going to interact with life outside the solar system in our lifetime. Even if, according to the current interpretation of the Drake Equation, we still have millions of intelligent civilisations in the galaxy, the average distance between 2 civilisations is 200 light years. Too far, in other words.
What’s the point of finding alien bacteria?
Colourful monsters with three eyes and six arms are purely subjects for the film industry for now. If our generation finds the first alien life, it will more than likely be bacteria-like life or possibly more complex microscopic organisms. The reasons for this will become clear when you finish reading this book. But then, is finding alien microorganisms really that special? Why would it be revolutionary for us earthlings?
Well, for most people, it’s not even that special. It just depends on what you like to do. For some people, it would have a giant impact: for example, for biologists – who want to understand life – and for philosophers – who want to assess how unique we are. In any case, the discovery of alien microscopic life will help answer many outstanding questions:
Did life arise multiple times in the Solar System? Does life arise easily when the ‘basic conditions’ are there?
The answer tells us something about how difficult or easy the phenomenon of life can arise. If it arose more than once, we can rule out that the emergence of life on Earth was an extremely exceptional coincidence. In that case, we can likewise expect life to be numerous on billions of other planets in the universe.
Did life on Earth originate in hot springs in the deep sea or just at the water surface, like in warm tide pools?
The origin of life in deep-sea hot springs is the most favourite hypothesis among (astro)biologists today. Its biggest competitor is the emergence of life in warm shallow pools that continually dry out and flood again with seawater. Our search for extraterrestrial life focuses mainly on Mars (where deep-sea hot springs and shallow pools both once existed) and on subterranean oceans in the outer Solar System (where only deep-sea hot springs occur, but no surface water with drying pools). Thus, if life is discovered in the underground oceans of ice moons, we can immediately conclude that Earthly life most likely also originated in deep-sea geysers.
Is there only 1 kind of life, or is there a different biochemistry from Earth’s?
If we study life on Earth in detail, it seems to have very logical characteristics. Characteristics that almost necessarily arise from the physical and chemical properties of our environment. But perhaps numerous variants are conceivable after all.
Suppose we find extraterrestrial life that uses the same biochemistry as Earth’s (DNA, mRNA, ATP, Cell membranes from phospholipids,ribosomes, etc.), then where we found it plays an important role. If we find such life on Mars, this proves very little about whether or not alternative biochemistry exists. Because the chance that Mars and Earth contaminated each other via meteorites when both planets had liquid water on the surface is very real. In other words, it is possible that the first life originated on Earth or Mars, and then contaminated the neighbouring planet. This is not the case with the ice moons around Jupiter and Saturn. The chances that life from Earth ended up there, survived, and could then infect the ocean beneath the thick ice crust are virtually zero. In other words, a finding of life on the ice moons that uses the same biochemistry as ours would very strongly suggest that only one successful form exists. Or at least at least within our Solar System. Provided we are sure that this find on an ice moon could not have come from contamination by a human spacecraft on that ice moon.
What level of complexity can life achieve in less ideal conditions? And how likely is the emergence of complex (or therefore possibly intelligent) life?
A good century ago, people still dreamed of Venus as a kind of biblical paradise, covered with rich forests and tropical fauna. That dream is far behind us, and the first missions in the 1960s proved once and for all that this planet is a lifeless hell. Within our Solar System today, Earth is clearly the richest and largest imaginable paradise for life, if not the only place where life can thrive. Yet certainly in our time, as we live in the climax of ecological richness in our planet’s history. Every other place in the solar system is less suitable, and will not exhibit such biodiversity or develop such breathtaking ecosystems. But has there never been such a thing as ‘complex life’ outside Earth? Are there perhaps small animals swimming in the ocean of the moon Europa? Or has there ever been an oxygen-rich atmosphere on Mars that stimulated the development of this Solar System’s first animals? Theoretically, all this is possible. But honestly, today we have no idea how realistic these thoughts are.