Download C472 Continuous Assessment: Essay #2

C472 Astrobiology
Nick Shuttleworth
C472 Continuous Assessment: Essay #2… Extra-Terrestrial Life
“The total chances against the evolution of man, or an equivalent moral and
intellectual being… will be represented by a hundred million of millions to one.”
Alfred Russel Wallace – Man’s Place in the Universe (1904)
Wallace’s reasoning for his rather profound statement about the uniqueness of the
Earth and its inhabitants was based around the view that the Milky Way was
coextensive with the Universe at large and that the Sun lay at its centre, in a modern
day re-hashing of pre-Copernican ideas. However, fifteen years later (and several
after Wallace’s death) the Sun was given its proper place towards the outer reaches of
the Milky Way, which in turn was discovered to be one of many, many galaxies in the
Universe, placing the Earth in the un-envious position of being as unique as every
other planet in the Universe.
Although the model on which Wallace based his study was proved incorrect, the
hurdles which he argued to be encountered by potential life are still accepted as
accurate today. In 1961 Frank Drake developed an equation for the rough calculation
of the prevalence of communicating intelligent life in the Universe for the first SETI
conference. The now famous Drake Equation considers, amongst others, the factors
discussed by Wallace, as a series of diminishing probabilities in an attempt to quantify
the number of stars in the Milky Way galaxy, the fraction of stars with orbiting
planets, the number of planets per star capable of sustaining life, the fraction of
planets where life evolves, the fraction of evolved planets evolving intelligent life and
several factors relating to communicative ability. The first of these factors is relatively
easily found to be around 100 billion, however the values of later factors are not so
clear cut.
The results of searches for extra solar planets currently show that around 10% of solar
type stars have orbiting planets. However, this is very much a lower limit, as the
methods used are biased towards more massive planets, and measurements have not
been being performed for long enough to detect Jupiter analogues or smaller. Thus
C472 Astrobiology
Nick Shuttleworth
current estimates of the number of stars with orbiting planets are above this exceed
measurements, spanning a range between 20% to 50% of all stars.
The question of the number of planets per solar system capable of sustaining life is
complex, mainly because the requirements for life are complex, but also because we
must generalise from a single case and assume that the rules of Earth also hold true
elsewhere in the Universe. One thing generally accepted to be necessary for life is
heat, because if the temperature is too low then the kinds of chemical reactions
necessary for energy generation become impossible. Working from the terrestrial
model, this heat comes from a near-by star and is dependent on both the star’s
luminosity and the star-planet distance. For a given luminosity there is a distance
range over which the heat will be compatible with life and any planets outside this
range will be unsuitable. The next requirement is that there be some mechanism of
producing energy. On Earth this is done in numerous ways from aerobic respiration to
fermentation to redox reactions, and it can be assumed that these mechanisms can also
be in place on other planets, so the necessary reactants would have to be present. The
third major vital consideration is the existence of a medium in which chemical
reactions can occur, the terrestrial version being water. Although it is possible that
liquids such as methane could be utilized, water remains the most accommodating.
Taking these factors into consideration current estimates put the number of planets per
star that are compatible with life at between one and five.
Another contentious question is whether, given that the circumstances necessary for
its evolution, life is the inevitable result. Theoretical models of the evolution of life on
Earth show a gradual change from a world in which only chemical reactions occur to
one containing living organisms. The start of this process is thought to be the
formation of the cell-like structures necessary for reactions to occur and not be diluted
by the outside environment. Certain molecules with separate hydrophilic and
hydrophobic parts are capable of forming such very simple cells. Moreover,
experiments have shown that the formation of these molecules, and others necessary
for basic evolutionary processes to begin to occur, is possible but minimal. It is
feasible, therefore, that life could get underway in a similar way on other earth-like
planets. However, evolution to such an advanced stage as that seen terrestrially is not
necessarily inevitable, once this stage has been achieved. The perpetuation of life is
C472 Astrobiology
Nick Shuttleworth
limited by the fact that the basic parameters allowing life are dynamic. For example,
potential stars have limited lifetimes and their luminosity, and therefore the orbital
range compatible with life, changes as they move off the main sequence of stellar
evolution. Another factor hindering evolution to larger or more intelligent animals is
the risk of mass extinction events, such as asteroid strikes. Collisions, such as that
believed to have killed the dinosaurs on Earth, would likely destroy the more complex
forms of life. In the case of the Earth, Jupiter is thought to have acted to remove the
asteroids in this solar system, allowing for a reduced terrestrial collision rate which
increased the opportunity for evolution to progress. If, however, a large mass planet
like Jupiter is required to be in close proximity to a life compatible planet, then the
percentage of planets suitable for the evolution of advanced life diminishes still
further. The evolution of intelligent forms of life seems likely, but only if the
circumstances allow life to evolve to a sufficient level of complexity for it to be a
selective advantage.
It seems from the above points that the evolution of intelligent species is extremely
unlikely because the probability that the necessary conditions for its creation,
evolution and consistent prosperity decreases with each factor considered. However,
this alone does not give a clear picture. The most important factor is the fact that there
are billions of stars in a galaxy and billions of galaxies in the Universe. Even the
smallest probability when applied to such an enormous sample must surely yield more
than simply one circumstance in which all the necessary ingredients for life come
together and intelligent beings result.