Read more: “Instant Expert: Astrobiology“
From the Epicurean Greek philosophers more than 2000 years ago to fiction writers of the late 19th century, people have speculated about the possibility that there might be other worlds which are home to alien life. Yet it is only with the gigantic telescopes and interplanetary probes of the space age that we finally have a realistic hope of answering the question. As we learn more about our own planet and the evolutionary history of terrestrial life we feel a stronger urge than ever to put it into context. Embarking on the search for extraterrestrial life pushes our technology and scientific understanding to the limits, but the quest is one we should not shirk. Finding life beyond our own planet would teach us things about ourselves we might never otherwise learn.
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What is life?
This seemingly simple question is surprisingly difficult to answer, not least because we risk being blinkered by the nature of life on our own planet.
Life on Earth is based on the uniquely versatile chemistry of carbon. The size of the carbon atom and the configuration of its electrons allow the creation of chains of carbon atoms, and the formation of varied bonds that can link with other elements. It is this that permits the formation of the vast range of carbon-based or “organic” molecules found in nature.
The building blocks of terrestrial life, including proteins and DNA, are composed mainly of carbon, along with hydrogen, oxygen and nitrogen. Together with phosphorus, sulphur and traces of iron and a few other elements, these form the molecular machinery in all known living creatures on Earth. And none of this diverse and complex biochemistry could function without a solvent with unique properties: water.
This does not mean life elsewhere will be the same. So how do we search for it, without knowing what it will look like? Modern astrobiology hopes to sidestep this conundrum by focusing on aspects of life that should allow us to recognise it, whatever form it takes.
Key to this is the fact that life cannot exist in isolation. Living organisms extract energy and raw materials from their surroundings and release waste products. As organisms grow and reproduce, they alter their surroundings. Indeed, life is an amazing geoengineer: over billions of years, living creatures have completely altered our planet’s environment. Life also changes the make-up of our planet on the timescale of months and years. Photosynthetic organisms modify the appearance of land and sea, and in the atmosphere the concentrations of gases such as carbon dioxide fluctuate on an annual cycle. Alien life should influence its environment in similar ways, and astrobiologists will seek out such changes as a signpost for life.
Aliens on Earth
Even on Earth, there is more to life than meets the eye. Living things are divided into three great branches or domains. We belong to the domain of complex-celled organisms called the Eukarya, but it is the Bacteria and the other microscopic domain, the Archaea, that dominate the biosphere.
These single-celled microbial organisms, which have a lineage extending back almost 4 billion years, occupy every nook and cranny of our planet. Those living beneath the continental and oceanic regions in rock and sediment may account for most of Earth’s biomass and genetic diversity. Many utilise energy sources that seem alien to us, with metabolisms that exploit the chemistry of hydrocarbons such as methane, sulphur compounds, or the elements manganese and iron. Some, known as extremophiles, thrive in conditions of very high or low temperature or pressure, or extreme acidity or alkalinity. The toughest can handle temperatures of over 120°C. Others can survive radiation levels 1000 times what would be deadly to us.
Such organisms are found in environments like those of the volcanic calderas of Yellowstone National Park in Wyoming (right), or around hydrothermal vent systems at the mid-ocean ridges, often at great depths and cut off from sunlight. As well as hinting at how life might survive elsewhere, these organisms represent the core genome of terrestrial life that has transformed the planet over billions of years, providing the oxygen we breathe and the soil we grow food in.
The modern Earth provides only a snapshot of the flora and fauna this planet could support. How alien would it have seemed 2 or 3 billion years ago? Questions like this are vital in helping astrobiologists look for the habitats and life forms that could exist elsewhere.
Where should we look?
Our home provides a rough template for the conditions needed for life to form and survive. As a small rocky planet, Earth has many characteristics that appear critical for the long-term survival of life. These include plentiful water; an atmosphere and surface whose rich chemical composition is constantly refreshed and recycled by geological activity; incoming solar radiation of about 90 petawatts across the globe, averaged over the year, that provides energy for photosynthesis and other chemical reactions, as well as maintaining a temperate climate; plus environmental stability on timescales from hundreds to millions of years.
We don’t know how important each of these factors is. Several moons in the solar system, including jupiter’s Europa and Ganymede, may have huge oceans beneath their icy crusts. Saturn’s Enceladus may have subsurface pockets of liquid water. But these moons receive little sunlight and may have few carbon-rich chemicals in their depths.
When seeking exoplanets – worlds in other solar systems – with an Earth-like environment, a basic rule is to start by looking for a body with a surface temperature between 0°C and 100°C. It can’t orbit too close to its star and be too hot, or orbit too far away and be too cold; it must be in a “Goldilocks” or habitable zone.
The location of this zone depends on the brightness of the star and on the greenhouse effect of a planet’s atmosphere – how effectively the atmosphere holds in heat. Using mathematical models of planetary climates and knowledge of stellar radiation we can decide which planets are worth inspecting.
Yet this is not the whole story. Mars is just outside our solar system’s habitable zone, which starts around 125 million kilometres from the sun and extends outwards by roughly the same distance (see diagram). Yet we know the planet had liquid surface water in the past. Around the smallest stars, 1/10th the mass of our sun and a thousand times fainter, the habitable zone is so close to the star that gravity should lock any planet’s rotation so that one side is permanently day and the other night. The environment on much of the surface of such worlds will be extreme, but liquid water may still exist in some regions.
Read next article: “Astrobiology: The hunt for alien life“
