Drake Equation - determining how many intelligent civilizations are in the Milky Way (N):
where, R* is the rate at which life-supporting stars (not too big and not too small, but just right) form;
fp is the fraction of those life-supporting stars that has planets;
ne is the number of those planets, per star system, that is habitable;
fl is the fraction of the habitable planets where life develops;
fi is the fraction of that life that develops intelligence;
fc is the fraction of the intelligent life that uses electromagnetic communication;
and, L is the length of time that those intelligent creatures actually send electromagnetic waves into space.
Estimates of The Drake Equation by Kevin A Barnes
The Drake equation can result in a very wide range of values, depending on the assumptions. Thanks to the existence of humans (an intelligent form of life capable of transmitting signals into space), however, we know that at least one civilization exists in the Milky Way galaxy that is capable of transmitting radio signals. That single fact establishes a lower limit for many of the factors in the Drake equation.
|Factor||Original Estimate (1961)||Current Estimate|
|R*||1 (one new star forms per year)||7 new stars forms per year — Source: NASA and ESA, Milky Way Churns Out Seven New Stars Per Year, Scientists Say|
|fp||0.2–0.5 (one-fifth to one-half of all stars formed will have planets)||Value approaches one; nearly all new stars have one or more planets — Source: Nature, One or more bound planets per Milky Way star from microlensing observations|
|ne||1–5 (each star with planets will have between one and five planets capable of developing life)||0.4 (40%) of stars with planets have a planet capable of supporting life — Source: NASA Kepler space mission data, Far-Off Planets Like the Earth Dot the Galaxy|
|fl||1 (100% of these planets will develop life)||Unknown/Controversial: Some scientists assert this value should be close to 100% based on evidence that life on Earth began almost as soon as conditions were right; others counter that the value is likely tiny given we have no evidence of life developing beyond Earth.|
|fi||1 (100% of these planets will develop intelligent life)||Unknown/Controversial: Like fl, we only have the single case of Earth from which to infer values, leading to disagreement among scientists.|
|fc||0.1–0.2 (10–20% will be able to communicate)||Unknown|
Ultimately, we do not have reliable estimates for several of the factors of the Drake Equation, meaning the results for N can vary wildly. Proponents of the Rare Earth hypothesis conclude that there is only one planet with intelligent life in the galaxy (and perhaps in the entire universe) — Earth.4 On the opposite end of the spectrum, scientists using larger values for the unknown factors in the Drake Equation have estimated there could be as many as 36.4 million radio-transmitting civilization in our Milky Way galaxy.