When considering life in outer space, we have been bathed – dare I say conditioned - for over a century now with stories that mainly involve land-based aliens evolving on terrestrial planetary bodies such as Earth-like planets and moons; from Marvin the Martian to Klingons, from E.T. to the heptatods in the film Arrival, intelligent life as imagined by storytellers almost always moves around by walking on a pair of legs or more.
While this has more to do with the practicalities of setting up alien species that can interact with humans in fictional settings (books or low-budget films), it comes loaded with assumptions. One of these is the fact that intelligent alien life almost always originates from another solar system; in other words, from an exoplanet (or exomoon) illuminated by a parent star.
This is based on the idea that to support life as we know it, life originates from planets that must have liquid water and orbit in the right place in their solar systems, not too close and not too far from their star.
Yet, I’d like to advance the idea that, if complex alien lifeforms exist - and that is a big if - these will be dominated not by those with legs and a sunhat but by those that have evolved in aqueous environments far from the warmth of a star.
This idea is based on a simple observation. In our Solar System, only one planetary body has liquid water on its surface, while five moons* have already been confirmed to hold global oceans of liquid water under their icy surfaces: the so-called ocean worlds (and the topic of my first book). Additional moons and possibly dwarf planets are most likely part of this ocean worlds group; we just don’t have enough data to confirm their existence yet (I’m thinking of Pluto and Dione, for example). Even more remarkable, two of the five ocean worlds in our solar system might be habitable for life as we know it: Enceladus and Europa.
When contemplating the thousands of other solar systems we have discovered so far; it is hard not to consider that they are also populated with a multitude of ocean worlds, some possibly harbouring habitable environments as well. Viewed this way, we can assume that our galaxy is awash with liquid water and that most of it is not resting on rocky planetary surfaces but locked deep within icy bodies.
Yet, we should also consider rogue planets, planets that have lost the gravitational pull of their parent star and are now wandering aimlessly through interstellar space. It is hard to figure out how many of these planets exist; with no star to illuminate them, their small sizes and the vast distances that separate us from them, they are undetectable. This hasn’t stopped researchers from trying to place a figure on the population of rogue planets in our galaxy. Estimates vary widely, with one that puts the number of rogue planets at over 50 billion in our galaxy alone. This figure is staggering. Rogue planets as big as Jupiter and Saturn might act like mini solar systems supporting numerous icy moons, which in turn leads to the possibility of ocean worlds.
So, in addition to the likelihood of ocean worlds existing around the numerous stars that populate our galaxy, we have to add those that might be present around countless rogue planets as well.
Given the vast numbers involved, it seems likely that, in our galaxy alone, habitable environments within the subsurface oceans of small planetary bodies far outnumber those on the surface of rocky exoplanets orbiting stars.
Therefore, we might be the exception rather than the norm. When pondering about alien life and intelligences, think more cephalopods and less Spock. *Callisto, Titan, Ganymede, Europa and Enceladus.