Perhaps a better question is: why hasn’t extra-terrestrial life come looking for us? With an estimated 40 billion earth-sized potentially habitable planets in our galaxy alone, and estimates of up to one hundred billion galaxies in the universe, the chances that not one alien race exists among them is mind-bogglingly low. They’ve have had an enormous length of time to pay us a visit, and the Fermi paradox states that we should have received some sort of contact by now – if only some ancient written clues to their home world. It begs the question: is it us? Are the aliens laughing at us behind our cosmic backs? There have been many ‘solutions’ put forward to the Fermi paradox, including the suggestions that aliens are observing us cautiously from afar, or even hiding away from potential extra-terrestrial enemies.
But, whatever the reason, it falls to us to make predictions about life on other planets, and we can only make predictions based on what we already know. Thankfully, we know of one planet that not only has evidence of life, but also still contains it - our mother Earth. So let’s see what we know.
Firstly, Earth is slap-bang in the middle of our solar system’s habitable zone, where the heat of the sun is not too hot or too cold for life to start developing. Of course, our habitable zone also includes Mars - populated solely by robots - and Venus, where it rains molten metal, so that by itself is not a hard and fast rule.
Secondly, Earth has plate tectonics, where parts of the crust shift and move, causing mountains and trenches, volcanoes and geysers. It is thought that this is one of the main differences between Earth and Venus, most likely because the movement of the plates recycles carbon, taking it out of the atmosphere, and reducing the greenhouse effect. Venus, where, again, it rains molten metal, does so because at some time in the past its greenhouse effect grew exponentially, cooking the planet up to a hellish 460OC. Earth, however, is sitting pretty on a much kinder greenhouse effect, gently sautéing us to a lovely average 15OC, which is much more conducive to life as we know it.
Now, plate tectonics requires more than just pushing and shoving, it requires a little bit of lubrication (and maybe some Barry White) to get it going. Nature’s lubricant, as far as we can tell, is good old-fashioned water. And what would you know; water’s also more or less a necessary requirement for life! It’s almost as if it was intelligently designed to work out that way… Almost.
Finally, life - or at least all life on Earth - needs a source of carbon to build from. Carbon makes up the vast majority of all organic structures in your body, and on all life that we know of. On Earth, this mainly comes from the air, in the form of carbon dioxide. This is taken up by autotrophic organisms, like plants, and converted into sugars, amino acids, carbohydrates, and all that good life-making stuff. Humans and other animals then absorb these compounds when they eat vegetation (and each other).
It has been suggested that silicon could be used as a base molecule for life on other planets. Silicon forms much weaker bonds than carbon as it’s a bigger atom, meaning it can’t get as close to other atoms it wants to bond with as carbon can. It’s a bit like replacing the steel supports in a skyscraper with soft cheese – it sounds good, but it doesn’t work in reality. It could work in an environment where silicon compounds are more stable than on earth, but we’ve currently got little idea what that could be.
So, a planet that we think could harbour life would need to be in its star’s habitable zone. It would need to have some sort of tectonic activity, and an atmosphere that could maintain an ideal temperature. It would need to have liquid water on its surface, and it would need to contain carbon in its atmosphere. If we find one of these outside of the solar system, we call it an exoplanet, and so far, as I mentioned before, we’ve estimated billions of them exist in our galaxy alone.
Now that we’ve defined the recipe for life, let’s disregard it entirely, and look at the most likely suspects for life in the solar system. The most famous example is Jupiter’s icy moon, Europa. Despite Europa being neither a planet, nor within the temperate zone of our sun, it is thought to have tectonic plates and even liquid water. We also know it contains simple carbon compounds, as asteroid debris from the surface has kicked some of it up into the atmosphere for us to examine. Sadly, any life there would most likely not resemble an alien mermaid, but some sort of single-celled slime on a hot spring. However, we may get to see these creatures sometime in the next decade, when a European Space Agency mission known as JUICE is launched in 2022.
Another good candidate for life is another moon: Enceladus, orbiting Saturn. This could have really similar properties to Europa, maybe allowing life to develop there too. If both of those turn out positively, that’s three instances of life in one solar system, which bodes well for the billions of similar solar systems in our galaxy.
Of course, there is every possibility that life as we know it is unique in its own right. Perhaps it also exists as ominously glowing clouds, entirely confused about the possibility of life on rocky planets, or maybe it exists as hyper-intelligent shades of the colour blue, living on a plane entirely separate to our own.
Until we learn to read crop circles, we may never know for sure...