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An introduction to exoplanets

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# 6  Where are the aliens? The Drake equation

Kepler’s results about the numbers of planets, especially small ones, are important for assessing the likelihood of life elsewhere in our Milky Way Galaxy. The Drake equation gives this likelihood in mathematical terms. This equation, invented by astronomer Frank Drake, is a way of estimating the number of active, communicative civilisations in the Milky Way. Basically, it’s an estimate of how likely it is that there is an advanced alien race somewhere out there that we might be able to detect signals from. The number of these civilisations is worked out by a long multiplication of lots of different factors, illustrated in Figure 8:

Number of civilisations =

Rate of star formation

× fraction of stars with planets

× average number of planets per star that can support life

× fraction of those planets that develop life

× the fraction of planets bearing life on which intelligent, civilised life evolves

× the fraction of these civilisations that have developed communications technologies that release detectable signs into space

× the length of time over which such civilisations release detectable signals.

A lot of the latter factors are things we don’t really know the answers to, especially things like the fraction of planets that develop life, but the results from Kepler have helped us to get a handle on the second and third points – the average number of stars with planets, and the average number of planets per star that can support life.

Figure 8  The Drake equation. If you are struggling to see the labels even in the larger version, try viewing an even larger version here [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] .