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Why do we always think that aliens would need similar life resources as we need on Earth for humans? The term “alien” means it’s unknown to us. Shouldn’t we change our thoughts about aliens? They might not need conditions like Earth – Arushi, 11, Houston

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.

What alien life might look like. ©Helen Cooper, Author provided

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?

Big guesses

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.

Read more: Evolution tells us we might be the only intelligent life in the universe

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.