Astrobiology can be defined as ‘the study of the origin, evolution, distribution and future of life in the Universe’. Striving to understand the emergence of life on our own planet and the possibility of it surviving in extraterrestrial locations is necessarily a very broad area of multidisciplinary investigation, reaching across aspects of astronomy, planetary sciences, geology, biology, and the design and engineering of space probes. This field of science is sometimes also called exobiology, xenobiology or bioastronomy.
The first question that must be addressed before attempting to understand the origins of life on our own planet, and searching for it on other worlds, is in defining what ‘life’ actually is in the first place. Although it may be relatively easy to point to things in the world today and classify them as living or dead, a precise, unambiguous definition that will work even for life forms we have yet to encounter is a little tricky. All life on Earth is built from long chains of carbon atoms and requires liquid water, with cells using DNA to store their operating instructions, and proteins to carry out most of the crucial biochemical processes. But what makes astrobiologists think that life elsewhere would necessarily be carbon-based and in need of liquid water? Could life be silicon-based or depend on solvents other than water? The simple answer is we don’t know, although we do know it is definitely possible with carbon and water, so looking for this kind of biochemistry seems to be the best bet for now. A more conceptual definition of life is that it is a self-replicating system containing a coded description of itself, able to extract energy from the environment to maintain its own complexity and reproduce – this defines life by what it does, not by what it is made of.
It makes sense to start by looking for the kind of life we know about, and that means searching the solar system for locations with both liquid water and interesting carbon-based ‘organic’ chemistry.
The prior existence of lie in ancient rocks on Earth, long after the cells have been destroyed, can be concluded by the presence of molecules not produced by abiotic chemistry. As well as the chemical components of the cells, we can detect life by the effect it has on its environment. Life produces distinctive features, such as the high percentage of oxygen in Earth’s atmosphere, steep chemical gradients set up in lake waters of sediments, or the biasing of one isotope over another (the elements come as different isotopes, differing only in the number of neutrons in the nucleus – photosynthesis, for example, preferentially uses carbon-12 over carbon-13). In general, ‘biosignatures’ are any telltale molecules, structures or features that can only be produced by biological action. Proving that a distinctive aspect can only be produced by life is extremely difficult, however, and debate rages on over the earliest signs of life on Earth, and claimed evidence for Martian life within the meteorite ALH84001.
For over a decade, astronomical techniques have been sensitive enough to detect planets beyond the solar system by the tiny effect they have on their parent star. These are called extra-solar planets or exoplanets, and more are announced at an ever-quickening rate.
At the time of writing a total of 775 confirmed new worlds have been discovered. An up-to-date list of all known exoplanets can be found here. None of the exoplanets discovered so far are thought to be potential abodes for life – due to limited sensitivity we have mostly discovered gas giants (which have no habitable surface). However, a few do orbit within the parent star’s habitable zone and if they possess large moons these may well be suitable for life. As our instruments improve, and especially with the launch of sophisticated space telescopes, we will be able to find smaller terrestrial planets. Once alien Earths have been discovered, there is even the ability to assess the make-up of their atmosphere and check for signs of life. The combination of high levels of oxygen and gases such as methane in the air, which would react very rapidly, suggests the presence of widespread photosynthetic life keeping levels topped up (as on Earth).
One of the central research areas within astrobiology is finding out the extent of conditions that terrestrial life can survive. Extremophiles are organisms that can tolerate what are considered to be extreme environments.
For example, some organisms thrive in the extremely salty water of the so-called Dead Sea, others survive the highly acidic conditions within volcanic pools such as Yellowstone National Park, and other specialised cells tolerate temperatures over 100°C and the crushing pressure around deep-sea ‘black smoker’ hydrothermal vents. However, the concept of ‘extremity’ is obviously in the eye of the beholder – many forms of life would find the cold and low-pressure of Earth’s surface, awash with toxic oxygen and bathed in lethal ultra-violet radiation from the sun, to be an extreme environment. Complex life, including land pants and animals like us, tends to be much more fragile than simpler cells like bacteria, and so many habitats on Earth are populated only by microscopic single-celled organisms. These hardy kinds of lie are also the most likely to be discovered in extraterrestrial environments.
Since the only forms of life we know about require liquid water, it was generally thought that the best chances for the emergence of life are on a planet that orbits a star from a particular distance. Too close in and the heat boils away any oceans; too far away and all water freezes solid.
What is needed is a planet that is not too hot, not too cold, but just right – this range of habitable orbital distances has been dubbed the ‘Goldilocks Zone’. But stars get hotter as they age, and so a planet must orbit at a habitable distance long enough for life to develop, giving us the concept of the continuously habitable zone (CHZ) around a star. It has now become clear, however, that liquid water is possible much further out from the sun, such as in moons orbiting the gas giants Jupiter and Saturn.
In addition, some astrobiologists have been arguing that not only must a planet orbit its start at the right distance, but the star itself must orbit the galactic centre in a particular way. Rocky planets like Earth might only form around stars in certain regions of the galaxy, and astronomers can think of many potential hazards in the cosmos, including close shaves with other starts, exploding stars, and great dark dust clouds. So for various reasons, it might be that life only has a fighting chance on planets within the galactic habitable zone, especially more complex, or even intelligent, life.
Of all 160 or so planets and moons in the solar system, Mars has so far received the most interest from astrobiologists. There are many signs that very early in its history Mars was much like Earth, with large areas of liquid water, a thick atmosphere, ample sources of energy for life and a good inventory of organic molecules.
But the surface of Mars today is a bitterly cold desert, its atmosphere largely stripped away. Did life ever get a foothold on the planet? Could cells be eking out a living deep underground even today? Or has it all fallen extinct leaving only scattered biosignatures as traces of its prior existence? The only way we will ever know is by sending probes and eventually human explorers to find out.
We are unlikely to ever find hints that Venus hosted life before the ancient catastrophe of runaway global warming, but there is a chance life continues high up in the clouds. Here the atmosphere is cool enough for droplets of liquid water, and there is the possibility they support microorganisms acquiring energy from the sun’s rays.
Another planetary body in our solar system that astrobiologists are getting more excited about is Europa, the second moon of Jupiter. There is good evidence that this moon hides a great ocean of liquid water beneath its icy surface. One problem facing life on Europa, however, is that sources of energy might be severely limited. Perhaps Europan microbes thrive off gas bubbling out from the crust, or hydrothermal vents like on Earth’s sea floor, or even nutrients created in the surface ice by ionising particles and subducted down into the ocean – ecosystems not living off sunlight like on Earth, but indirectly eating space radiation.
A fourth possible habitat for extraterrestrial life is Titan, the huge moon of Saturn. Titan is the only moon in the solar system with a thick atmosphere, but it is still far from the sun and too cold or liquid water on the surface. Instead, hard-frozen water builds the hills and rocks seen by the Huygens probe, and clouds, lakes and rivers appear to be made of methane. Could chemistry become complex enough for life in such a frigid environment, wet with methane instead of water? Another possibility is microbes within pools of water at geothermal hotspots – volcanoes of molten water rather than silicate rock as on Earth.
The most recent addition to potential habitats for life in our solar system is Enceladus, and stems from the discovery of great plumes of water, ice and organics spurting from cracks on its icy surface. Additional measurements, taken by Nasa’s Cassini space probe, revealed that Enceladus possesses a thin atmosphere made up of water vapour, carbon dioxide, methane and nitrogen. These observations tick several boxes for habitability; it has liquid water, complex organic compounds and a source of internal heat. Although the source of this heat is still unclear, it is thought to support a subterranean liquid ocean. If lifeforms are present on Enceladus, the observed plumes would greatly facilitate our detection of them, making this target a very attractive prospect for astrobiological exploration in the coming decades.
This is the theory of ‘panspermia’. The idea of living spores being blown between stars to spread life through the galaxy was first proposed in 1908. It has since become clear that unprotected microbes could never survive the harshness of space for long enough to be transported between stars, but there is rapidly growing evidence that cells can indeed survive the journey between planets in the solar system. Microbes could be ejected from their homeworld as stowaways within lumps of rock blasted off by a nearby impact, transfer through interplanetary space, and touchdown on another world falling as a meteorite. Experiments show that a proportion of cells can survive all the main hurdles of panspermia protected within their host meteorite: the heating and pressure-wave of being thrown off their planet; exposure to vacuum, freezing cold, and cosmic radiation whilst in transit through interplanetary space; and then the heat and shock of smacking into their destination. And it only takes a single bacterium to survive the voyage, to reawaken, grow, divide, spread beyond the impact crater to infect a lifeless world, and subsequently evolve over billions of years into the vast diversity of organisms of a planet-wide ecosystem.
Lewis Dartnell, the ASB’s Secretary, wrote a great blog post on this question. You can read it here.