“Why do astronomers look for Earth-like planets in their hunt for life in the universe. Why couldn’t life exist in all manner of ways that we are incapable of contemplating?”
I see this question, put in a variety of different ways, all the time. And to first glance, it is a valid question. Why should we focus on finding Earths? What if, by searching for only the ‘twin’ of Earthlife, we’re missing a plethora of friends and family? I hold the opinion that we’re not. That life formed of carbon, oxygen & hydrogen atoms suspended in water is in all likelihood the most common, and maybe the only, way that the universe is able to ‘do’ life. And that, while the habitable zone (the zone where an Earthlike planet would have liquid water on it’s surface) is not the only place in the universe that these ingredients can get together, it is both most likely and most likely to be detectable there.
So I will lay out my argument here in a series of posts that are almost certainly better explained elsewhere.
What is life?
A question famously posed by Schrödinger in the 1940s, it is also the first key question on our path: To find life, we need to know what we’re looking for…
Defining life may, to first glance, seem obvious. But the boundary between the living and the lifeless is hard to define – just ask any virologist. Definitions of life tend to focus on the fact that all lifeforms are structures capable of self-replication (ie reproduction), movement, growth, and the ability to evolve. But these properties, themselves, are not unique among the natural world. Crystals can replicate themselves and even respond to the environment. Bubbles of fat can inorganically grow and replicate. And anything capable of near-perfect self-replication is likely to feel the process of natural selection.
One of Schrödinger’s great insights about Life is that it does not die. It “evades the decay to equilibrium”, instead exchanging material with the environment to keep alive for much longer than an ordinary lump of matter might. In Schrödinger’s words, life feeds on ‘negative entropy’ (order in the environment) and produces positive entropy (in waste and heat).
But just how valid is this definition? Are we focussing too much on earth-life and leaving out a whole universe of possibilities. Could life be simple, rather than complex; electrical rather than chemical; or even gaseous rather than semi-solid? Give each point a bit of thought and the alternative, to my mind, sound implausible:
Complex – Picture an airless world covered in fine, unstable sand. One day, maybe thanks to a meteorite hit, a cavity is created and a small cascade begins, slowly eating its way into the surface like a wave. I can see this cascade steadily feeding off the gravitational energy of the raised sand, increasing entropy in the low sand left behind. It might even grow over time and then split into two or more such cascades. But is this system alive? My thought would be ‘no’, because of the final argument for life – that it must undergo evolution by natural selection. A single simple process such as this, while ticking all the boxes for life, cannot adapt. The cascade of sand is not determined by any in-built mechanics, but simply the result of disequilibrium.
There is also no easy way to turn one molecule into another and energy. Even the simplest chemical pathways have an energy barrier to get over, such that stored energy and catalysts are needed just to make more energy and avoid death. Then there is the apparatus needed to reproduce not just all that energy-making machinery, but also the structures that hold this all in one place. Life, by its very definition, seems to need to be more complex that the environment it feeds on to evolve.
Chemical – The universe has plenty of other ways of making and transferring energy, as the example above shows. But life requires not only creation of energy, but also to store and use that energy too. While electrical, thermal or even light energy could likely be used (indeed life on earth does), this energy cannot itself form its own storage facility; Light cannot conjure its own mirror. For that, it needs to interact with chemistry. Only the chemistry of complex molecules has the dexterity to perform these storage and reproduction tasks.
Semi-solid – Specifically, life must have a barrier to it’s environment. Just to avoid death and decay, all life on earth continually cycles through its own body weight in energy and resources once every day or so. This is only possible thanks to the fact that our bodies have a semi-solid barrier to the surroundings, with the molecules of metabolism able to leave by diffusion. If metabolism excreted solids (like FeS or SiO), these products would build up and suffocate the organism. If an organism were fully liquid or gaseous, all the important self-replicating machinery would drift away due to diffusion. Life needs to be self-contained, and it likely needs a liquid solvent in which to live…
I hope I’ve shown you that, by simply starting at the definition of life and applying physics, we can get a decent handle on what life in the universe must be like. It seems like that life must be more complex that it’s surrounding; it must be a chiefly chemical system and it must be self-contained. That is a surprisingly narrow conclusion, but surely there could be a myriad of ways to assemble a chemical system such that it fits the criteria for life? We shall explore that next in “The Ingredients of Life”.
Erwin Schrodinger – What is Life (pdf version here)
Nick Lane – The Vital Question (review)