We don’t know whether there are any aliens or not. But there are so many planets in the universe – some say more planets than all the grains of sand on Earth – that many scientists think it’s worth looking.
To help narrow our search, we often try to figure out what aliens might be like, and therefore what conditions they might require. For example, if we think aliens will be made mostly of carbon, like us, then we should look for planets that have carbon. If we think they will depend on liquid water, we should look for planets with liquid water, some of which we have found.
The problem is that figuring out what aliens will be like isn’t so easy. We have only one example of life – life on Earth – to learn from. To see why this is challenging, imagine we wanted to learn about butterflies.
Normally, we would look at as many butterflies as possible, and figure out from all those examples what things are true of all butterflies. We might learn that butterflies can come in a range of different colours and sizes, but that all butterflies have, for example, two antennae and six legs. If we only looked at one butterfly, say a Monarch, we might wrongly predict that all butterflies were orange and black.
Similarly, with only one example of life (life on Earth), it’s hard to know which features of life are universal and which are special to Earth. Here, all life is carbon-based and needs water. Is that true for all aliens, or a special feature of life on Earth?
Sometimes scientists make guesses based on a process called “convergent evolution” on Earth. This is when different traits, such as eyes or limbs, evolve multiple times in separate groups of organisms.
For example, eyes have evolved independently many times on Earth, so we might think it likely that aliens have eyes. The problem is that the different species on Earth are not independent examples, because they all descended from a single common ancestor. All life on Earth is related (literally!). Eyes may be common on a bright planet like Earth, but not a dark planet. Or they may be common in DNA-based life, but not some other kind of life.
Another option is to use chemistry and physics. Living things require lots of energy, and lots of chemical reactions. Liquid water is a particularly good place for chemical reactions. Similarly, carbon is especially good at forming the kinds of big, long, complex molecules that help support complex life forms. These are arguments in favour of looking for carbon-based life on planets where there is liquid water.
On the other hand, there are other liquids that are good alternatives to water, such as liquid methane. And there are other chemicals, including silicon, that can form complicated bonds. Perhaps we should be looking for silicon life on liquid methane planets. In fact, some scientists want to explore Titan, a moon of Saturn, which seems to be covered in oceans of liquid methane.
There is one thing, I would argue, that we do know: aliens, like us, will be the products of evolution by natural selection. Natural selection is the process by which some individuals have more offspring than others, and so the traits that led to having more offspring become more common over time. This is a major cause of evolution, and is the reason why organisms are well-adapted.
Universal natural selection
What sets life apart from non-life, what distinguishes a planet with aliens from just another planet with piles of rocks and sand, is life’s complexity. Living things are made of many intricate parts which work together for a common purpose – staying alive, replicating, eating. This kind of complex adaptedness can only be achieved by one process: natural selection.
This is probably the one thing we know for sure about aliens: they will be products of evolution by natural selection. It can be fun to think about what this means for aliens. Does the fact that they evolve through natural selection, just like us, tell us anything about what they will be like? Does it tell us anything about what kinds of conditions aliens need?
Like all good questions, this one led to more new questions than answers. But while you ponder those questions, remember: somewhere out there, just maybe, an alien – probably stranger looking than in our wildest imagination – might be pondering them, too.