The fragility of life - are the aliens dead ?

Figure 1: Scenarios A) B) and C)

Despite the realisation that there are literally billions of stars, exoplanets, planets and other celestial bodies in the Universe, the perplexing reality is that proof of life on other planets has been impossible to obtain despite decades of effort. Programs such as SETI and observational work through telescopes (Hubble, Spitzer, Kepler and in the future TESS and James Webb) has done little to establish the existence of life on another planet. 

Chopra and Lineweaver, in a controversial paper, have proposed a Gaian bottleneck theory to explain the low or non-existence of life while making the telling observation that archaeological excavations have not unearthed alien spaceships and optical and radio searches for extraterresttrial intelligence have not been successful.

The Gaia hypothesis, (theory or principle), contends that organisms interact with their inorganic surroundings on Earth to form a self-regulating complex system that plays a critical role in maintaining the conditions for life on the planet. Within this paradigm are key components such as the way in which the biosphere and the evolution of life forms affect the stability of global climate, ocean salinity, oxygen in the atmosphere and other environmental variables that affect the habitability of the planet.

According to Chopra and Lineweaver, a Gaian bottleneck exists whereby if life emerges on a planet, it only rarely evolves quickly enough to provide activities which regulate greenhouse gases and albedo, thereby maintaining surface temperatures compatible with liquid water and habitability. This bottleneck theory therefore suggests that first, extinction is the cosmic default for most life on the surfaces of wet rocky planets and second, rocky planets need to be inhabited to remain habitable. Almost a Catch 22 situation.

The emergence of life's ability to modify its environment and regulate initially abiotic feedback mechanisms is termed 'Gaian regulation'. As far as Chopra and Linewear are concerned, without rapid  evolution of Gaian regulation, early extinction would be the most common outcome for planetary life. As continuing efforts are made to search the universe to locate and identify life, the Gaian bottleneck model suggests 'that the vast majority of fossils in the Universe will be from extinct microbial life.'

Three scenarious are shown in Fig 1 above - A) Emergence bottleneck, life rarely emerges even on rocky wet planets; B) No bottleneck, life emerges with high probability and lasts for billions of years; C) Gaian bottleneck, life emerges but goes extinct within a billion years.

So, the chances of finding any form of life, under this model is close to zero and what evidence may be found will have long been dead. 

Chopra and Lineweaver's paper can be accessed here:

The Case for a Gaian Bottleneck: The Biology of Habitability

Gas and dust form planetary disks - formation - image courtesy NASA