Astrobiology is the study of life on planets or moons other than the Earth. But, to search for life, we must first know what life is. And that might not be as simple a task as you’d think.

It turns out that ‘life’ might just be an arbitrary definition that humans have come up with in an attempt to understand the world. In fact, there is much debate over the actual definition of life. Have you heard of the 7 characteristics of life? We learn these in school to be the signs that something is living, but many also occur in non-living things. For example, salt crystals left in the open air can grow and expand. Computer programs can replicate themselves – reproduction. Fire consumes wood. There are also examples of things that appear to be alive but lack one or more of these characteristics – such as viruses. Viruses cannot replicate themselves but require a host cell to do it for them.

A bacteriophage – a virus which invades a bacterial cell and hijacks its DNA to create more copies of the virus. Is it alive?

So, while we don’t have a perfect definition of life, we can look for similar characteristics that living things share. But where should we be looking? In the absence of any known earth-like planets thus far, we are studying extremophiles – organisms that thrive in the extreme environments of earth -to see how life could appear in the harsh conditions that are present on other planets in our solar system. If we work out the limits of life on earth, we can rule out planets unlikely to host life and find planets that could. So, we calculate the extremes at which life cannot survive; temperature, pH, salt concentration. The places where life exists are called the biosphere, and we are still learning the limits of that biosphere, as we continue to find microorganisms in more and more extreme places.

A penguin is a very cute and cuddly example of an extremophile.

This gives us ideas of where we might start to look for life on other planets. We can rule out those planets that don’t, and have never had, any liquid water on them. This requirement is so key to life as we know it that we have defined a habitable zone around every star as the range within which liquid water can exist. This habitable zone is also sometimes called the ‘goldilocks zone’ as it is not too warm, and not too cold, but just right. Other solar systems have varying sizes of habitable zones, many with one or more planets within that zone. In fact the Trappist-1 system consists of a small, cool star (so the habitable zone is much closer to the star), with seven planets orbiting it. Three of these planets are in the habitable zone, and all of them are earth sized.

The TRAPPIST-1 system compared to our solar system. The green area represents the habitable zone.

First, though, we should start close to home, at planets within our solar system. We start here because it is within the realm of possibility for us to send a rover to these worlds within a lifetime, and in some cases we already have! This way we can find out more about these worlds in a way that is not just a prediction based on vague data.

Our solar system. Not to scale!

So what are the prime candidates within our solar system for life? Well, it has to be ‘habitable’. We’ve already mentioned that there is a habitable zone around each star, where the temperature is right to host life (this could be a combination of the distance from its star and the amount of greenhouse gases in the atmosphere), and we know there must be water present (and the water must be liquid). Also needed is energy for life to carry out metabolic processes, the correct nutrients for life to build structures and grow, and an active geo-chemical cycle that recycles nutrients for reuse. All these conditions must be present on the planet in the long-term, and not disappear regularly or after a short time.

We can find planets on our solar system that have some of these characteristics, or at least had them in the past.

Venus, from Mariner 10 images

Venus may not seem like the ideal place to harbour life, with clouds of sulphuric acid in the atmosphere, an atmospheric pressure 92 times that of Earth, temperatures reaching 450°C, and a day that lasts 243 Earth Days. Hopes were dashed of an Earth-like planet harbouring life in the 1960s, when a number of probes relayed news of an extreme climate. All hope is not lost, though, as there could be some life in the high altitudes of Venus’ atmosphere, which has a mild climate. There are chemicals in this area that are hard to produce without life of some kind, so some are hypothesising large colonies of microorganisms high up in the atmosphere, which could potentially use UV light as an energy source. This is where our study of extremophiles comes in handy, as we can predict what life might look like in these extreme environments and predict where the conditions are just too extreme for life.

Venus is an example of what will happen to the Earth if we don’t get our act together and stop burning fossil fuels. There is the possibility that liquid water once existed on Venus, before the runaway greenhouse effect took over and heated the planet to an oven-like environment. Due to its hostile environment, there hasn’t been much analysis of the surface of Venus, so it is entirely plausible that there is evidence of past life ripe for the discovering.

So, Venus is Earth in the future. Another potential future (if we don’t actually manage to kill the Earth entirely) is shown by our other next-door neighbour, Mars.

Mars, taken by Rosetta

Mars has been a source of fascination for centuries as a potential inhabited planet. There are countless stories of life on Mars, the most famous being ‘War of the Worlds’ which has Martians with advanced technology coming to Earth with fighting machines to conquer humanity. Schiaparelli saw canals on Mars in the late 19th century and assumed that there must have at one point been beings capable of creating an infrastructure on the red planet (these were later learned to be an optical illusion). This inspired many other stories about life and civilization on Mars, and life was speculated and expected from the 17th century when the ice caps were discovered, right up until the 1960s, when the Mariner 4 landed on Mars and showed evidence of a barren rocky desert.

Mars’ Ice caps – made of frozen carbon dioxide, not water

While Mars may not have many of the conditions required for life nowadays, it likely had them 3.8 billion years in the past, based on current understanding. Mars is no longer geologically active but used to have volcanoes and there is evidence of past cycling of chemicals from the interior to the surface of the planet. The planet has an extremely thin atmosphere, and so no liquid water can exist on the surface. There is water ice at the poles of Mars, but any water that becomes liquid will eventually evaporate under the low pressure. Water likely once covered 36-75% of the planet, but with too high a salinity to support most Earth-like life. Here, though, is another area where our study of extremophiles comes in handy, as there are species of archaea that exist on Earth in hypersaline solutions.  There is hope that we will find fossilized bacteria-like life forms on Mars, and some argue that we may have already found it. There are sediment samples and meteorites from Mars that have been confirmed to contain various organic molecules, which could indicate that life existed in the past. However, the general consensus is that these organic compounds could have been produced abiotically, and researchers arguing that these compounds indicate life are controversial. So, no confirmed life on Mars thus far, and we’re unlikely to find any extant life as is. Life on the other planets in our solar system seems unlikely as well based on their chemistry. Dwarf planet Ceres may have water on it (in the form of ice), but little is known about it so far.

Ceres, showing bright spots that might be water ice.
Alien life…

Some other bodies, though, hold promise in the search for surviving life. Tune in next time for another astrobiology article featuring some of the moons of the solar system which may harbour alien life…


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