The ten most important questions for astrobiology: Number 5

The next question relates, as they all do, to previous questions. It may not represent where cutting edge progress is being made in astrobiology, whether it be in microbiology or exoplanetary science, but it's a perennial query. I've lost count of how many times it's been raised after a few drinks and left hanging there casting a slightly embarrassing shadow over the conversation, so here's Number 5:

Could there be life in the universe operating on entirely different principles?

So what exactly does this mean? Typically the definition of 'life' here is pretty broad and a bit fuzzy. It ranges from stuff we'd recognize by its actions (that rock just ate my sister!) to stuff we'd have to kick ourselves about later (that cloud not only looks like a rabbit...). As for 'principles', well this too has a broad range of meaning.

At the simplest end we're talking about fiddling with terrestrial-style biochemistry. But to operate with different principles there have to be some fairly radical alterations - more than just substituting arsenic for phosphorus or silicon for carbon, assuming any of that can work. A key factor has to be the capacity to store information in a way that is both robust and yet readily accessible. Our biochemistry uses complex polymers and a great array of support structures and molecules, from histone spools to enzymes. Various array-like, or quasi-periodic molecular forms occur in other parts of nature, in clay lattices and crystals. The read-write issue is a tough one though, to read out something like DNA, to convert that information into function, requires unzipping the molecule - a dangerous exercise. Equally, the process of writing to that storage device can be both very subtle (the filter of natural selection over short to long timescales) or quite brutal (viral RNA insertion). As we look at the enormous complexity of our own biochemistry it becomes increasingly hard to blithely suggest a plausible alternative - which is not to say there couldn't be one, but it seems like an almost insurmountable challenge to invent one with just theory. The closest we can get may well be our own version of artificial life, the push and pull of electrons in semiconductors.

But what if the different principle is really different? Here we firmly enter the realm of science fiction where 'beings of pure energy' serves as a useful scriptwriter cop-out. All that we have in this case is speculation. For example, is something like a galaxy alive? They involve mutual interactions of smaller structures (stars, molecular clouds, globular clusters), large scale connective processes (density waves, magnetic fields), they have speciation, they avoid reproduction as we know it by simply being eternal, they feed off smaller structures and on occasion each other, they call out to their brethren like leviathans in the deep - their booming voices carried across the cosmos as waves of gravity. They are also full of microbial life, most stellar cells carrying a swarm of simple chemoautotrophs, little spherical bodies of rock or gas processing atoms into complex molecules - fed by the mitochondrial-like warmth of nucleosynthesis.

It makes for a good story, but therein lies the problem with this type of speculation. The story is far more compelling than anything we might realistically stand to test or learn from even if it were true.

So the answer to question 5 at the moment seems to be 'possibly, but we really have no good idea what it would be'.