When we think about whether or not aliens exist, we generally imagine them on a vaguely Earth-like planet circling a distant star. We do not normally think of them living out in space itself.

But maybe that is not such a ridiculous idea. In April 2016, researchers reported that some of the key building blocks of life can be produced from simple substances under harsh conditions mimicking those of interstellar space.

Cornelia Meinert at the University of Nice, France and colleagues showed that a mixture of frozen water, methanol and ammonia – all compounds known to exist in the vast “molecular clouds” from which stars form – can betransformed into a wide range of sugar molecules when exposed to ultraviolet rays, which pervade space. The sugars included ribose, which is a part of the DNA-like molecule RNA.

This suggests that the fundamental molecules of life might be formed in outer space, and then delivered to planets like Earth by icy comets and meteorites.

The finding is actually not surprising. We have known for decades that other building blocks of life can be formed from chemical reactions like this, before being incorporated into comets, asteroids and planets.

However, there is a more intriguing possibility. Life itself might not have needed a warm and comfortable planet bathed in sunlight to get going. If the raw ingredients were already out there in interplanetary limbo, might life have started there too?

Ideas about the origins of life do not often consider this scenario. It is hard enough to figure out how life could have begun on the early Earth, let alone at temperatures close to absolute zero and the near vacuum of interstellar space.

There is no fundamental reason why life might not arise far from any star

Making the basic building blocks of life, like sugars and amino acids, is the easy part. There are lots of chemically-

plausible ways to do that, starting with the simple molecules found in young solar systems.

The hard bit is persuading these complicated molecules to assemble into something capable of life-sustaining processes like replication and metabolism. Nobody has ever done this, or come up with a completely plausible way it might happen, in the nurturing environment of a warm, rocky planet – let alone in space.

Still, there is no fundamental reason why life might not arise far from any star, in what is often regarded as the barren desert of interstellar space. Here is how it might happen.

First, we had better agree on what counts as “life”. It does not necessarily have to look like anything familiar.

The broad basis of life on Earth – that it is carbon-based and requires water – “reflects a universal norm”

As an extreme case, we can imagine something like the Black Cloud in astronomer Fred Hoyle’sclassic 1959 science-fiction novel of that name: a kind of sentient gas that floats around in interstellar space, and is surprised to discover life on a planet.

But Hoyle could not offer a plausible explanation for how a gas, with an unspecified chemical make-up, could become intelligent. We probably need to imagine something literally a bit more solid.

While we cannot be sure that all life is carbon-based, as it is on Earth, there are good reasons to think that it is likely. Carbon is much more versatile as a building block for complex molecules than, say, silicon, the favourite element for speculations about alternative alien biochemistries.

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