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Thursday, 9 May 2013

The Search for another Earth

"Two possibilities exist: either we're alone in the Universe, or we're not.
Both are equally terrifying."

- Arthur C. Clarke.
This guest post is by Andrew Rushby, currently undertaking a PhD in earth systems modelling at the University of East Anglia. Andrew usually blogs at the II-I- blog, the Pale Blue Blog or the European Association of Geochemistry blog. He can also be found launching high altitude balloons into (near) space, and tweeting as @andrewrushby. This is the third in our series of posts about the many unknowns involved in the study of planets orbiting other stars across the galaxy.

In my last post I broadly covered the techniques for finding planets around other stars in the galaxy, as well as the role this technology plays in defining the current limits on our knowledge. We have discovered 885 other planets to date, but how many of them are like the Earth and why is this important?

As we live on a rather lovely watery planet ourselves, we seem to have a natural inclination to seek out others just like it because we consider them to be the most likely for hosting life. Why? Well, because our current sample of ‘inhabited planets’ stands at just one, we have a very limited understanding of where the boundaries for life lie as well as the important factors that affect habitability when considering the broad characteristics of life-bearing worlds. If other inhabited planets exist, is the Earth typical within the sample or an outlier? Are the furnaces of close-in gas giants the cradle of most flavours of life in the universe, or maybe the frigid surfaces of icy worlds in the far-flung outer regions of their star system?

Waterbear, taken by scanning electron micrograph
Some lifeforms live in extremely tough environments, and have even survived space vacuum conditions - like this water bear. Image credit Bob Goldstein and Vicky Madden (Creative Commons)
It might be fun to speculate about all the various forms and shapes that other life might take, but this lies outside the remit of science. It seems obvious to us that only on a planet able to support life would organisms (like intelligent Homo Sapiens) eventually evolve, but this instils in us a fundamental bias towards planets like Earth: it remains beyond our perspective to consider the possibility that can life operate outside of the physical and biological boundaries that we are familiar with. It therefore seems unsurprising that the limits of life lie so perfectly within those experienced on Earth, and why we seek out other Earth-like planets as possible oases of biology. This bias is known as the anthropic principle and is an important philosophical consideration to bear in mind when considering the search for ‘habitable’ planets.