by Max Harvey
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| Planet Kepler e orbiting the Star Kepler (Wiki Commons) |
Whilst earlier this year I wrote a post detailing how part
of our failure to find extraterrestrial life could be because we make
assumptions that any life would be similar to ours and hence based upon carbon,
water and sunlight, it has come to my attention that there could be many other
reasons behind our failure.
Although predicting which planets could be habitable is a
complex matter that spans across all disciplines of science from biology to
physics, the fundamental process of this procedure is rooted in mathematics.
Assuming that in order for life to exist there must be liquid water, we can
quite sensibly state that the temperature of the planet must be in a region
between 0ºC and 100ºC. It turns out that there is a very helpful law that can
aid in our task of determining where water can exist and it looks like:
Although this equation may seem quite complex, the value of
σ is simply a constant known as the Stefan-Boltzmann constant, and the value of
4pi is also constant. R refers to the distance from the star at the centre of a
system whilst the value of L is the luminosity of that star (once again a
constant for shortish time periods). Whilst the equation itself is not that
important, what it does show is that there is a relationship between
temperature and the distance from the star. As a result we can calculate a
region of distance from the star where the temperature will be of the correct
level to support the presence of liquid water.
This zone or region which we are describing is what some of
you might know as the Goldilocks zone or the habitable zone. It is the region
in which we believe a body within a certain star system must be located in
order for there to be a possibility of liquid water to exist and hence a
possibility to find life. This is part of the process that is used by
scientists who search for potential life bearing planets.
However despite the apparent brilliance of this process,
distance is not the only factor that will affect the temperature of a planet or
body within a star system. One factor that can have a huge impact is the albedo
of a body. For those that aren’t GCSE geographers, the albedo of an object is a
measure of how much incident radiation it reflects. As a result, the
temperature of a planet could fall outside of the required range even when
inside of the habitable zone if it has a particularly high or low albedo. More
importantly however, there is the possibility of the converse being true, where
we miss potentially life bearing planets because they fall outside of our magic
‘habitable’ zone yet due to a particularly high or low albedo, could
potentially support the presence of liquid water.
The same could also be said for the greenhouse effect, which
can raise the temperature of a planet as the greenhouse gasses trap re-emitted
infrared radiation and cause adjacent molecules to vibrate. How can we be sure
that we have not missed some of the viable planets in our reduction of the
sample.
As we search for life on other planets, it is all too easy to
try to ‘narrow the field’. But, should we be removing candidates or should we
keep our minds totally open for the off chance that we find something equally
amazing, yet totally different to life as we know it.
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