Why it's so unlikely there's alien worlds in space
Thursday, 21 June 2018
On the 18th April a new space telescope was launched by NASA - the Transiting Exoplanet Survey Satellite (TESS).
An exoplanet is a planet outside our solar system that orbits a star.
The objective of this mission is to look for exoplanets over a much greater area than its predecessor, Kepler.
It is estimated that our galaxy the Milky Way, has 200 billion stars and at least as many planets, possibly as many as 500 billion.
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Up to 10 per cent of these planets could be earth-like and as many as 11 billion of these orbit sun-like stars.
The key is to find planets that potentially are in the 'habitable zone' that is, they can support liquid water.
That dramatically reduces the number of potential candidates.
Liquid water is a mandatory requirement for life, at least as we know it, DNA based life.
Examples of criteria for a planet to be in the habitable zone are: it must be the right distance from its star, be massive enough to retain an atmosphere, have a stable orbit and not be subjected to frequent asteroid impacts.
The host star is required to have the right mass - too big and it won't be around for the billions of years required for life to evolve, too small and it won't provide enough heat to drive the chemistry of life.
This list comprises scores of other criteria if the planet is to support complex multi-celled life and the list extends into hundreds of items, in the right sequence, for the presence of intelligent complex life.
Many scientists think that the requirements and chain of events required for the evolution of intelligent life are so numerous that it is highly likely we are the only intelligent life in our galaxy.
They point out that the Search for Extra-Terrestrial Intelligence (SETI) has been running for 54 years with nothing detected.
SETI is monitoring electromagnetic transmissions from other planets.
This means SETI is restricted to hunting for intelligent life capable of sending signals.
Professor of geochemistry, Marc Defant highlights how special the events leading to intelligent life on Earth are.
He draws attention to just one example that has been crucial to our evolutionary history – the geology of East Africa.
Our primate ancestors and many of their characteristics represent adaptations to life in a challenging environment.
These include significant brain power, 3D vision, colour vision and digits.
We have these because we evolved from creatures that once lived in trees.
Evidence suggests that all the major events in the evolution of our ancestors only took place in East Africa and the key driver for rapid evolutionary change was the increase in open grassland in this region.
About 7 million years ago the deforestation of huge areas in East Africa drove the primates out of the trees and onto the grassland.
Our subsequent evolutionary history - bipedalism, increased intelligence and rapid speciation, are a result of the dramatic change in this landscape which occurred because of the formation of the East African Rift – a zone in which the Earth's lithosphere (its crust and upper mantle) are being pulled apart.
This rift changed the landscape from being a relatively flat region covered with tropical forest to a much more varied environment comprising mountains and vegetation from desert to substantial grasslands.
Computer simulations show that the pre-rift flatland permitted moisture transport and substantial rainfall in this region of Africa, sustaining large forests.
But the rift produced a dramatic reorganisation of atmospheric circulation leading to more arid conditions conducive to grasslands.
Marc Defant makes the point that the sequence of events eventually leading to Homo sapiens, included a geological rifting event that had to occur exactly in a region where the Great Apes were already present – that's a pretty improbable set of conditions to happen elsewhere in our galaxy.
It is expected that the TESS mission will find more than 20,000 exoplanets (3,758 are currently known).
The chances of simple single-celled microbial life existing on some of these planets may be significant but if Marc Defant is right then the chances of any of the half billion or so exoplanets in our galaxy being home to complex intelligent life is – almost zero.