Tag Archives: life

The Cosmic Zoo, by Dirk Schulze-Makuch and William Bains: a book review

The universe appears to be silent. Even though astronomers have been making incredible advances — building vast new observatories, developing better techniques and analytical approaches, opening up entirely new windows on the universe — none of this has turned up any evidence of extraterrestrial intelligence. This silence is becoming deafening. Indeed, the most important question in science — to my mind, at least — is: are there any intelligent, technologically advanced civilisations out there in the universe? Or is it just us?

One can take many approaches to this question, of course, but it can be useful to think in terms of “barriers”. What barriers must be overcome in order to get from a soup of abiotic, organic chemicals to a technologically advanced species capable of disturbing the universe in a way that distant astronomers can detect? This is an interesting approach because, depending on where you place the barriers, you end up with quite different universes.

For example, you might argue that abiogenesis represents a major barrier. In other words, that the transition from non-life to life takes an unlikely sequence of events — so unlikely that life itself is vanishingly rare. This is certainly plausible. If this argument is correct then we are living in an empty universe (“empty” in the biological sense).

Or you might argue that abiogenesis isn’t an insuperable hurdle, but that the transition from a simple prokaryotic type of cell to the more complex eukaryotic type of cell represents a significant barrier. If that argument is correct then, when we go out and explore new solar systems (or when we use the new generation of incredibly powerful telescopes to study exoplanets), we’ll encounter worlds rich in microbial life but lacking in complex life.

Or you could argue that a barrier lies ahead of us, that technological civilisation itself is a self-limiting phenomenon (and it’s depressingly easy to imagine why that might be the case: nuclear holocaust, biowarfare, climate change as a byproduct of technological life…) This dispiriting thought gives rise to a quite different universe, one in which there might have been many civilisations at a similar stage of technological development to our own — but which died out before they reached the stars.

So, as you can see, it’s interesting to think about the location of barriers on the path from non-life to technological civilisation. The Cosmic Zoo: Complex Life on Many Worlds (Springer, 2017), an ambitious book by Dirk Schulze-Makuch and William Bains, is one of the best studies in recent years on this topic.

Cover of "The Cosmic Zoo"

Cover of “The Cosmic Zoo”

Schulze-Makuch and Bains look at what life does — how it functions — and make a strong case for saying that if life starts on a world then complex lifeforms will at some stage inevitably arise on that world. Many possible barriers that scientists have suggested — the development of photosynthesis, for example, or the rise of multicellularity — are minor speed bumps in the road rather than tall brick walls: the particular biochemical pathway that life took on Earth might be rare, but many different biochemical pathways are available that lead to the same functional outcome. In other words, if abiogenesis is not particularly rare event then when we start to explore the universe we’ll find worlds teeming with complex life forms. The universe will be a cosmic zoo! It’s an enthralling possibility (albeit one that highlights the possibility of a barrier lying somewhere in front of us: a universe that’s silent because it’s rare that technological civilisation survives long).

The Cosmic Zoo is clearly written, mercifully devoid of jargon, and concisely covers a vast amount of material. Every reader, having read this book, will come away with a better understanding of the current state of astrobiological thinking – and also with a much deeper appreciation of the wonders of life here on Earth.

The most habitable exoplanet?

Pressures of work mean that I’m way behind on my reading. I’ve just got round to reading the December 2011 issue of Astrobiology. One interesting article in this issue is A Two-Tiered Approach to Assessing the Habitability of Exoplanets, by Dirk Schulze-Makuch et al.

The authors suggest the use of a two-tiered classification scheme in order to assess exoplanet habitability. The Earth Similarity Index (ESI), as its name implies, ranks planets based on their similarity to Earth in terms of mass, size, temperature and so on. The Planetary Habitability Index (PHI) ranks planets according to the presence of a stable substrate for life, available energy, appropriate chemistry, and the potential for the planet to hold a liquid solvent. The authors have formulated both indices in such a way that they can be updated as our knowledge advances; this is particularly important for the second tier of the classification scheme, the PHI, since that index requires more information than currently exists for any exoplanet.

The fun bit of the paper, though, is the appearance of a “top-10” list of objects as given by the ESI and the PHI.

The planet with the highest Earth Similarity Index is, of course, Earth. More interestingly, the object with the second-highest Earth Similarity Index is Gliese 581g (Earth has an ESI of 1; Gliese 581g has an ESI of 0.89).

The planet with the highest Planetary Habitability Index is, again no surprise, Earth (which has a PHI of 0.96). Titan, Mars and Europa occupy places 2-4 on the list. The exoplanet with the highest Planetary Habitability Index is, once again, Gliese 581g (with a PHI of 0.45).

So – is Gliese 581g the best place to be looking for alien life (perhaps, as has been suggested, by analysing reflected light from the planet in a search for biomarkers such as the presence of chlorophyll). Maybe. But it’s worth pointing out that it’s not at all certain that Gliese 581g even exists! The Lick-Carnegie Exoplanet Survey ‘discovered’ this exoplanet in September 2010; but the planet did not show up in an analysis of data from the High Accuracy Radial Velocity Planet Searcher. As things stand today, the existence of the planet is unconfirmed.

Perhaps the most habitable exoplanet, as of today, will turn out to have been a mirage.