Habitable Exoplanets: An Anthropic Principle

Artist’s conception of an extrasolar planetary system around the star Kepler 62 (Courtesy NASA/JPL-Caltech)

As the search for extra-solar planets continues to uncover potentially habitable planetary companions in the orbit of hundreds of stars across the universe, the ability to narrow down the potential target stars may save considerable time and resources spent on telescopy. Contemporary planet-finding technologies are close to reaching their full potential in terms of resolution, accuracy and the acquisition of data on extra-solar planets. Despite some future projects currently in development (such as the ESA’s Gaia Mission, set for launch in 2011), these indirect methods will most likely be superseded by the implementation of direct imaging technology in the foreseeable future, as is currently in independent and collaborative development at NASA, the ESA and other national space agencies. These missions include NASA’s Terrestrial Planet Finder (TPF) coronagraph and interferometer and the ESA’s Darwin projects (both are currently on indefinite hiatus due to budgetary constraints), as well as the James Webb Space Telescope (a collaboration between NASA, the ESA and the Canadian Space Agency) (NASA, 2010).

As the still relatively youthful discipline of exoplanet detection and analysis moves to the forefront of astronomy and planetary sciences, the budgetary constraints of the launch and operation of expensive space telescope missions ensures that these resources must be utilised efficiently and with the greatest possible scientific reward. Some exoplanets have already been shown to be orbiting within the habitable zones of their stars but they remain unlikely candidates for life because of their large mass and crushing gravity, a fact that should galvanise the scientific community and their sources of funding to accelerate the implementation of more advanced imaging technologies that would allow smaller, more habitable planets to be detected. Despite the fact that the detection of extrasolar planets, habitable or otherwise, adds significantly to our understanding of the formation, interaction and distribution of planets in the universe, the primary goal of exoplanet detection remains the discovery of possible life-harbouring planets that share our celestial neighbourhood.

Students and scientists are turning to increasingly novel yet basic means for determining where to begin our search for habitable planets. A good starting point is the habitable zone theory, a concept that has been well studied for several decades. Whilst the theory has some weaknesses it provides the best indication to date of the presence of ‘habitable’ conditions, operating within the biological and physical framework of our current understanding. Subsequent study and imaging campaigns directed at the chemical composition of atmospheres of targeted planets in star systems with a high probability of harbouring habitable planets using (as of yet undeployed) thermal and infrared imaging, coronagraph and interferometer technologies can then be carried out, saving valuable telescopy time and resources.

Probabilistic analysis of stars and their planetary systems of this kind also has wider implications for the philosophy of science and is relevant in the context of our species’ position in the Universe, both from an evolutionary and physical standpoint. Outside of the sciences, several theological and cultural schools of thought place our seemingly isolated position in high regard when considering fundamental questions such as the origin of life and the evolution of complex life and humans. These issues have also been addressed scientifically by cosmologists and astronomers and have historically been a point of contention between the physical sciences and traditionally-held religious or cultural beliefs. The traditional scientific viewpoint can be described by the Copernican principle, which states that the Earth’s physical position in the Universe should not be considered to be especially favourable or unusual and that humans should not think of themselves as privileged observers of the cosmos. Given that there is little reason not to assume that the Earth is a typical rocky planet orbiting a relatively normal star in an unexceptional region of a common barrel-spiral galaxy, and that the physical laws of science apply equally to the entire universe, it seems logical that the cosmos should harbour similar environments in which simple or complex life could have evolved, and that observable life should be common across the universe. If this is true, where should we be looking?

The habitable zone theory represents a good start in our search for extraterrestrial life, but it is possible that alien life may be so outlandish and dissimilar to the biota of Earth that our preoccupation with of the role of water, sunlight and carbon may be hampering our search, and we may have to broadly extend our parameters. Nevertheless, working within the limitations summarised by the habitable zone concept, the number of active extraterrestrial civilisations (that is, advanced life capable of the sentient observation and objective examination of their planetary and astronomical environment) in the galaxy has been tentatively quantified by a number of solutions to the Drake Equation, a probabilistic formula based on the mediocrity principle discussed above and several astrophysical variables, such as the rate of star formation. The solution to the Drake Equation was given as 10 (alien civilisations in the galaxy) by the original authors and has since been a driving force behind the growing field of astrobiology and the search for extraterrestrial intelligence (SETI) program. The apparent incongruity between the solution to the Drake equation and humanity’s lack of contact, discovery or direct observation of the alien civilisations, or indeed even basic extraterrestrial microorganisms that should be common across the universe given the evidence is known as the Fermi paradox.

The probabilistic approach to exoplanet discovery around low mass planets also has implications for biology and the evolution of life on these planets. The Anthropic Model of evolution suggests that the level of intelligence of life on a given planet is a function of the lifetime of the star (Watson, 2008). The model proposes that a number of increasingly unlikely critical transitions have to be surpassed in order to facilitate the evolution of advanced life. On Earth, these steps have been achieved at relatively regular intervals of ~1 billion years since the evolution of life roughly 4 Ga, and they include the evolution of prokaryotes, DNA, sexual reproduction and eukaryotes, cell differentiation and eventually the transition between primate and human societies (Watson, 2008; Szathmary & Manynard-Smith, 1995). The theoretical lifetime of the biosphere, as controlled by the increasing luminosity of the Sun and the ability of temperature regulating feedback mechanisms operating on the Earth to regulate the temperature to within habitable bounds, is likely to be 1 billion years from present, resulting in a total habitable period or biosphere lifetime of ~5 billion years (Watson, 2008). The Anthropic Model detracts from the principle of mediocrity mentioned earlier as what are considered to be the ‘critical steps’ in the evolution of the biosphere become increasingly unlikely to be achieved as time progresses, suggesting that whilst the evolution of basic life (populations of replicating molecules) may be common, the transition between basic and advanced life may be highly unlikely (Watson, 2008). Philosophically, the anthropic principle also suggests that humans, as isolated observers of the universe, may be biased in our understanding of the evolution and possible nature of life because of a logical fallacy derived from the fact all the observations of the universe made by humans must be compatible with the life known to currently exist within it (Barrow & Tipler, 1986; Carter, 1983). This bias ensures that human perspective is not tailored to consider the possibility that can life operate outside of the physical and biological boundaries that apply on Earth and that we have assumed are universal; organisms that exhibit alternative biochemistry and metabolic pathways, including the use of non-water solvents and non-carbon based life, differing chirality (the ‘handedness’ of biologically-active molecules, including amino acids and sugars) and alternative forms of photosynthesis that have evolved under different solar conditions (Pace, 2001).

If, with sufficient research, it transpires that the Earth is not typical of habitable planets and complex life is uncommon in the Universe it would contribute significantly to the ‘Rare Earth’ paradigm, and humans may have to reconsider our philosophical and biological position in the Universe as potentially isolated observers.

One comment on “Habitable Exoplanets: An Anthropic Principle

  1. Pingback: The Search for another Earth | the II-I- blog

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