The Drake Equation = Number of Detectable Civilisations in Our Galaxy

Work out for yourself aided by the brilliant BBC 'Horizon' programme videos on the Drake Equation the number of Detectable Intelligent Civilisations (N) in the Milky Way Galaxy.

Drake Equation: N = Rs x Fp x Ne x Fl x Fi x Fc x L

I calculate (my maths is probably wrong!) that the possible range is huge: there may be many billions or only one Detectable Intelligent Civilisation (us!) in the known Universe. My calculations of the worst and best cases for Detectable Intelligent Civilisation are based on the estimates given in the BBC 'Horizon' video.

• Rs = rate of star formation (20 per year)
  
• 20 per year x 10 billion years = 200,000,000,000 = 200 billion stars in our Milky Way galaxy
  
• Fp = fraction of stars with planetary systems (0.1)
  
• 0.1 x 200 billion stars = 20 billion
  
• Ne = average number of planets which could support life per star with planets = 0.2 to 0.4
  
• 8 billion (20 billion x 0.4) best case
• 4 billion (20 billion x 0.2) worst case
• Fl = fraction of earth like planets that contain life = 0.9 to 1 x 10-6
  
• 7.2 billion (8 billion x 0.9) best case
• 4000 (4 billion/1 million) worst case
• Fi = fraction of planets that contain life that evolves into intelligent life = 0.5 to 1 x 10-6 (1 in a million)
• 3.6 billion (7.2 billion x 0.5) best case
• 0.004 (4000/1,000,000) worst case
• Fc = fraction of worlds that develop science and technology = 1 (best case) to 1 x 10-3 (worst case)
  
• 3.6 billion (3.6 billion x 1) best case
• 0.000004 (0.004/1000) worst case
• L = longevity of civilisation (years) / Age of Galaxy = 0.01 (100 million /10 billion) (best case) to (100/10 billion) 0.000000001 (100/10 billion) (worst case)
  
• N = 3.6 x 10 power 7 = 36 million (3.6 billion x 0.01) best case
• N = 4 x 10 power minus 13 (0.000004 x 0.000000001) worst case
• N (universe) best case = 3.6 x 10 power 17 (3.6 x 10 power 7 x 100 billion (10 power 11)
  
• N (universe) worst case = 1 civilisation (4 x 10 power minus 13 x 100 billion (1 x 10 power 11 galaxies)

Other information: wikipedia Drake Equation

Solar System's 'look-alike' found

By Paul Rincon Science reporter, BBC News, Belfast

Almost 300 planets have now been found outside our Solar System

Astronomers have discovered a planetary system orbiting a distant star which looks much like our own.

They found

two planets that were close matches for Jupiter and Saturn orbiting a star about half the size of our Sun.

Martin Dominik, from St Andrews University in the UK, said the finding suggested systems like our own could be much more common than we thought.

And he told a major meeting that astronomers were on the brink of finding many more of them.

The St Andrews researcher said

this planetary system, and others like it, could host terrestrial planets like Earth.

It was just a matter of time before such worlds were detected, he explained.

Dr Dominik told BBC News: "We found a system with two planets that take the roles of Jupiter and Saturn in our Solar System. These two planets have a similar mass ratio and similar orbital radius and a similar orbital period.

"It looks like this may have formed in a similar way to our Solar System. And if this is the case, it looks like [our] Solar System cannot be unique in the Universe. There should be other similar systems out there which could host terrestrial planets."

Dr Dominik presented his work at the Royal Astronomical Society's National Astronomy Meeting in Belfast.

Ultimate goal

The newfound planetary system, which orbits the star OGLE-2006-BLG-109L, is more compact than our own and is about five thousand light-years away. (Astrophysics - Science journal - Feb 2008 - Full Article pdf)

Although nearly 300 extrasolar planets have been identified, astronomers have consistently failed to find planetary systems which resemble our own. Dr Dominik said only 10% of systems discovered so far are known to host more than one planet.

But he explained that all the

techniques currently used to find exoplanets were strongly biased towards detecting gas giant planets orbiting at short distances from their parent stars.

The OGLE (Optical Gravitational Lensing Experiments) planets were found using a technique called Gravitational Microlensing, in which light from the faraway planets is bent and magnified by the gravity of a foreground object, in this case a another star.

"It's a kind of scaled-down version of our Solar System. The star the planets are orbiting is half as massive as the Sun and they orbit half as distant to their host star as Jupiter and Saturn orbit around the Sun," said Dr Dominik.

He said that

the ultimate goal for exoplanet researchers was to find habitable Earth-like and Mars-like planets. This aim was achievable, he said, because technology was improving all the time.

"I think it will happen quite soon," he said, adding: "Micro-lensing can already go below Earth mass and it has detected more massive planets in the habitable zone. So in the next few years, we will see something really exciting."

Dr Dominik said there was competition between teams of astronomers using micro-lensing and those who favoured the transit technique, which seeks to detect new planets when, from our point of view, they pass directly in front of the parent star they are orbiting. The planet blocks a tiny fraction of the star's light, causing the star to periodically dim.

But he added that there was little chance to detect Earth-like worlds in OGLE-2006-BLG-109L because the system was too distant for current techniques to resolve planets the size of our own.