First written in April 2013.
In another triumph for NASA’s Kepler mission, two new planetary systems have been discovered. While such a discovery is not really a rare event anymore – 137 new exoplanets were found last year – a couple of factors make this finding particularly noteworthy.
First of all, some of the planets are “Super-Earths” – that is, they’re not too much bigger than our home planet. One of them, Kepler 62f, has a radius only 40% larger than that of Earth. What’s more, it is postulated to have a rocky composition.
Secondly, three of the planets are located in the so-called “habitable zone.” This is the region surrounding a star in which a planet could theoretically have liquid water on its surface. One of the planets, Kepler 62f, is the closest in size to Earth ever discovered in a habitable zone. Another, Kepler 69c, orbits a star that belongs to the same classification of our sun (G-type). Kepler scientists claim this is “a significant milestone toward finding truly Earth-like planets.”
While Earth-like planets seem like a logical starting point for our quest to find extra-terrestrial life, restricting our search to such a narrow range of environments may in fact be unnecessarily limiting.
There are many other factors to consider when it comes to deciding whether liquid water can exist on a given planet – from atmosphere, to stellar radiation, the age and type of the host star and of course an origin for water. Conversely, it is possible that environments conducive to life as we know it exist outside habitable zones – in fact, there is one such potential environment in our own solar system. Jupiter’s moon Europa is hypothesized to harbor a subsurface ocean similar to the deep oceans of Earth. The likelihood of this was bolstered recently with the discovery of an “unidentified and unclassified” bacterium in a lake buried under kilometres of ice in Antarctica, an environment postulated to be similar to that of Europa. Effects ranging from volcanic activity to planetary mass can facilitate conditions beneficial to life, and it is also possible that life may originate on a planet that starts in the habitable zone, but then ends up outside it.
The existence of extremophiles on Earth suggests life can arise in a diverse range of conditions – on Earth it can exist in volcanic vents and nuclear waste! The resilience of life is epitomized by tardigrades and bdelloid rotifers, peculiar organisms which are extremely resitant to harmful radiation and extreme temperatures. Tardigrades have even been shown to survive in the vacuum of outer space. A couple of other factors suggest that life may be more resilient than we expect: organic molecules are able to exist in the interstellar medium, despite the inhospitable radiation. Additionally, geological evidence suggests that life appeared early in Earth’s history – perhaps as early as 3.85 billion years ago. Earth back then wasn’t like Earth today: it was hot, with a toxic atmosphere of carbon dioxide, ammonia and other gases, and a multitude of volcanoes. Despite this seemingly uninhabitable environment, life managed to emerge.
The border between living and non-living is blurred, and exactly what constitutes a living being is a contentious issue. Biology has traditionally characterised life as matter that is able to metabolise, grow, respire and reproduce – but this definition is arbitrary and also problematic, as it leaves entities such as viruses in a grey area. Viruses possess genes, utilise natural selection and reproduce via self assembly, however they lack cellular structure, metabolism and cannot reproduce independently. These perceived shortcomings preclude them from the ranks of living things. Our definition of life has been inconstant, and has transformed over time. For example, the invention of the microscope opened up a whole new world of microbial life to human understanding. However, redefining life might not solve this problem – because definitions refer to words and meanings, which are inherently flexible and dynamic. What we need is “an adequately general theory of living systems” and this requires expansion of our conception of life beyond what we observe on Earth. One such theory has been proposed already, in which life is described as “the activity of a biosphere,” where a biosphere is a “highly ordered system of matter and energy characterised by complex cycles.” The originators of this theory, Feinberg and Shapiro, hypothesise that the minimum requirement for life is simply an entropy gradient (change in amount of disorder over time).
If we take Feinberg and Shapiro’s theory to be true, we can imagine that life might evolve in exotic places, such as the atmospheres of giant stars or on the surface of neutron stars. Of course, the kind of life that could survive in such extreme environments would be light years from our current notion of life – perhaps so much so that we might be unable to recognise it. In a similar vein, the human mind is unable to comprehend shapes in four dimensions, so it is entirely possible that bizarre forms of life occupy higher dimensions inaccessible to us. For example, in Greg Egan’s Diaspora, giant growing carpets composed of carbohydrate building blocks in shapes like Wang tiles appear one-dimensional but when Fourier-transformed, reveal an incredible “thousand-dimensional frequency space” rich with life. It’s all pretty mind-trippy stuff.
Returning a bit closer to home: maybe there are multiple biosystems on Earth that remain hidden from us, so-called “shadow biospheres.” Massive portions of the Earth remain unexplored – like deep within the soil – which represent potential locations for life founded on alternative biochemistry or physical processes. For example, life might be based on silicon instead of carbon, or differing chirality. Perhaps even life based on weak forces like Brownian motion might exist. Another interesting point to consider is whether life can only exist in baryonic form – that is, in the form of matter we can detect. Some cosmologists predict that baryonic matter only comprises 4.6% of the total mass density of the universe – the remainder consists of dark matter and dark energy. Maybe some form of life extends into these dark realms of the universe.
Overall, while it may be especially important to humanity to find extra-terrestrial life similar to us, I contend that any discovery of life beyond our own planet would be equally as extraordinary, and potentially more mind-blowing and enriching. Therefore I believe we need to let go of our obsession with the “habitable zone,” and think more outside the box when it comes to the notion of life.