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The Drake equation is a probability law which estimates the abundance of intelligent life in our Galaxy, the Milky Way. It is quite simple in appearance, and anyone can play with the variables in order to make their own personal estimate.
The variables encountered in the equation include the proportion of intelligent to non-intelligent life; the proportion of stars which would be capable of sustaining life in their environment to those who cannot; the number of planets a star is probable to have existing in this habitable zone, if the star were to harbour planets.
Although the scientific results of this equation are in great debate, it was developed by Prof. Frank Drake in order to open discussion on the topic for the famous meeting at the Green Bank radio observatory in 1961.
Current estimates ranging from the opinions of pessimists to optimists, are of the order of it being next to impossible to communicate with other lifeforms in our Galaxy, to a possible ten different alien civilisations who are currently in the same positions as us with appropiate technology who could be trying to communicate with us and others like us.
Hence the popularity of the SETI project (the Search for Extraterrestrial Intelligence). The equation brings many interesting topics to light such as how long intelligent civilisations may continue on living, with most estimates being of short duration. One hypothesise is that once nuclear power is developed by a civilisation, they will quickly destroy themselves through their new technology.
To date there have been 684 confirmed planets discovered orbiting a total number of 474 stars other than our Sun. With thousands more proposed from the Kepler mission awaiting comfirmation. More recently the Kepler mission has discovered the first planet known to be orbiting two stars.
The techniques involved in detecting these extra-solar (i.e. orbiting other stars than our Sun) objects favor the discovery of larger, more massive planets which have a more visible influence on their parent star. The techniques follow principles as simple as; does the parent star wobble?
If so, by how much and then knowing the distance to the parent star we can calculate the mass of the orbiting planet and orbital period, which in turn would give us the distance between the parent star and planet, using Kepler’s third law. This technique follows the principles of astrometry (basically astronomical geometry).
Then from analyzing the richness of the chemical environment of such systems through spectroscopy it is possible to say if at least one component of this system would be capable of sustaining life. Unfortunately, due to the large number of complexities which arise in the observational and analysis stages no one can say for sure if these planets are currently harbours of life.
However, all is not lost as we know already that our solar system contains life on a small out of the way planet amicably called Earth. So would it be possible for other lumps of rock in our Galaxy to host complex biological species? It is of popular opinion that yes, it is possible but due to the harsh environments in which they may exist they may not of had the possibility to evolve beyond microbial stages of evolution.
For example, if we ignore Mars for a minute and concentrate on the more probable hosts, the Gaililean satelites orbiting our local failed star Jupiter or Saturn’s Titan are good bets. It was initially thought that light was a neccessary ingredient for life to come into being. That was until the discovery of strange looking creatures living in the depths of our deepest darkest oceans close to hydrothermal vents.
This would lead a reasonable mind to believe that Europa, Ganymede or Titan may be hosts to such creatures thanks to their water ice crusts encasing their volcanic prone H2O oceans. How could we detect such life? Well as we know from studies of biological creatures back home we create a chemical diversity in our atmosphere which wouldn’t exist if we did not.
So we could look for gases such as methane trapped in ice crystals such as Clathrate Hydrates on the surface of these near by objects, with techniques such as infra-red reflection spectroscopy.
An exo-planet nicknamed ‘Snow White’ has been found to have a partially water ice surface with a possible light methane atmosphere. So in conclusion, no life has been currently detected aside from on planet Earth and it will prove difficult to find, but, we are off to a good start.