Life in the Universe part 8: What are the chances of finding life in our galaxy?

This is the final part of the Life in the Universe series of posts. Go here for the first one.
Frank Drake

Part 7 asked how long will a civilisation remain willing and able to communicate, and was the last piece of the puzzle needed to be able to answer this final question.

In 1961 American astronomer, astrophysicist and founder of SETI*, Frank Drake, devised the Drake Equation. It was used by Drake as an agenda for discussion at the three day Green Bank meeting, a conference for scientists and industry leaders to discuss the possibility of detecting life beyond the planet Earth.

The Drake Equation breaks down the question of the possibility of finding life in our galaxy into seven smaller questions which can be discussed independently. Each of these questions can be 'answered' with a value, which can then be multiplied together to give a value for the number of communicative civilisations within our own galaxy. In practice, the seven discussion points are often difficult or impossible to answer definitively, so the Drake Equation serves more as a thought experiment or a framework for discussion than something which may actually yield a valid scientific result.

The Drake Equation
The Drake Equation looks like this:

N = R* x fp x ne x fl x fi x fc x L

Each of these terms has been discussed in a previous life in the universe post, but here's a summary of what each means:

N: This is the 'answer' that we're looking for; the number of intelligent civilisations with a desire and ability to communicate within our own galaxy. The rest of the terms are seen to be
R*: The rate at which stars are born in our galaxy, i.e. how many suitable stars are born each year?
fp: The fraction of those stars that have planets orbiting them.
ne: The (average) number of planets per star that can support life.
fl: The fraction of those planets that actually go on to produce life.
fi: The fraction of these that will actually go on to develop intelligent life.
fc: The fraction of these that will develop the ability and desire to send out signals across space.
L: How long, in years, such civilisations will continue to send out signals across space.

The Drake Equation gives an idea about how many intelligent civilisations are in our galaxy
So how many other civilisations are there out there?
To find that out we need to know a value for each of the points listed above. As it is difficult (sometimes impossible) to know an exact value for each one, a certain amount of guesswork is involved: as stated before, this is a thought experiment more than exact science!

The Green Bank values
Back in 1961, Drake and his colleagues used the following values for each parameter:
  • R* = 10 stars formed per year, on average
  • fp = half of all stars formed will have planets
  • ne = stars with planets will have 2 planets capable of supporting life
  • fl = 100% of these planets will develop life
  • fi = 1% of these will develop intelligence
  • fc = 1% of these will be willing and able to communicate...
  • L = ... for 10,000 years.
Putting these values into Drake's equation gives: 10 x 0.5 x 2 x 1 x 0.01 x 0.01 x 10,000 = 10. So, with Drake's values we could expect to find 10 other communicating civilisations within our own galaxy at any particular time, if only we know where to look.

Current estimates
Since 1961 much thought and debate has gone into these seven discussion points, and our scientific understanding of many related issues has improved. With this in mind, the following are perhaps slightly more realistic estimates:
  • R* = 7 stars formed per year, on average (using calculations by NASA and the ESA)
  • fp = half of all stars formed will have planets (better equipment and techniques mean that new planets are being found all the time. Current data suggests that somewhere between 40-60% of sun-like stars form planetary systems)
  • ne = stars with planets will have 2 planets capable of supporting life (Earth-like planets do seem to be fairly rare, but then again they are more difficult to detect than larger planets).
  • fl = 33% of these planets will develop life (based on a statistical analysis of the time it took for life to appear on Earth)
  • fi = 1% of these will develop intelligence (we don't really have a much better clue about this one than Drake did: some argue that once life takes hold, the development of intelligence is inevitable given enough time; others point out that of the billions of life forms on Earth, only one has developed a high level of intelligence: the jury's out on this one!)
  • fc = 1% of these will be willing and able to communicate... (again, there has been no real development on this front, and no reason to deviate from Drake's original estimate)
  • L = ... for 10,000 years. (again, there's no reason to deviate from Drake's original value as there are equally convincing arguments that take the value both considerably higher and much lower).
So our new estimates give: 7 x 0.5 x 2 x 0.33 x 0.01 x 0.01 x 10,000 = 2.1. So our revised estimate shows that we could expect to find about two communicating civilisations somewhere in our galaxy.

Your estimates
Use the widget below to enter your own guesses for each parameter, then click the 'how many' button to see what your thoughts about the universe tell you about the chances for life:

But...
The answers to all seven of these questions are highly contended and far from being known conclusively. They are based largely on conjecture rather than evidence, and as such cannot be used to draw firm conclusions. I would, however, encourage anybody to think about each of the seven questions, come up with their own values for each one and then see what they tell you about the possibilities of finding life in our universe.

Perhaps the biggest thing that can be taken from the very existence of the Drake Equation and it's continuing place in scientific discussion is that scientists are serious about searching for life outside our own small area of the universe.

Life in the Universe
This is the final post in a series of eight about the possibility of life being found elsewhere in the universe. Here is a list of all of the posts in this series:







* Search for ExtraTerrestrial Intelligence

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