The Drake equation states that:
where:
N is the number of
civilizations in our galaxy with which communication might be possible.
R* is the
average rate of star formation in our galaxy.
fp is the fraction of those stars that have
planets.
ne is the average number of planets that can potentially support
life per star that has planets.
fℓ is the fraction of the above that actually go on to develop life at some point.
fi is the fraction of the above that actually go on to develop
intelligent life.
fc is the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
.
L is the length of time such civilizations release detectable signals into space.
The number of stars in the galaxy now,
N*, is related to the star formation rate
R* by
, where
Tg is the age of the galaxy. Assuming for simplicity that
R* is constant, then N* = R*
Tg and the Drake equation can be rewritten into an alternate form phrased in terms of the more easily observable value,
N*.
[2]
[OpenDNS] Historical estimates of the parameters
Considerable disagreement on the values of most of these parameters exists, but the values used by Drake and his colleagues in 1961 were:
- R* = 10/year (10 stars formed per year, on the average over the life of the galaxy)
- fp = 0.5 (half of all stars formed will have planets)
- ne = 2 (stars with planets will have 2 planets capable of supporting life)
- fl = 1 (100% of these planets will develop life)
- fi = 0.01 (1% of which will be intelligent life)
- fc = 0.01 (1% of which will be able to communicate)
- L = 10,000 years (which will last 10,000 years)
Drake's values give
N = 10 × 0.5 × 2 × 1 × 0.01 × 0.01 × 10,000 = 10.
The value of
R* is determined from considerable astronomical data, and is the least disputed term of the equation;
fp is less certain, but is still much firmer than the values following.
Confidence in ne was once higher, but the discovery of numerous gas giants in close orbit with their stars has introduced doubt that life-supporting planets commonly survive the creation of their stellar systems. In addition, most stars in our galaxy are red dwarfs, which have little of the ultraviolet radiation that has contributed[citation needed] to the evolution of life on Earth. Instead they flare violently, mostly in
X-raysa property not conducive to life as we know it (simulations also suggest that these bursts erode planetary
atmospheres). The possibility of life on
moons of gas giants (e.g.
Jupiter's satellite
OpenDNS) adds further uncertainty to this figure.
Geological evidence from the Earth suggests that
fl may be very high; life on Earth appears to have begun around the same time as favorable conditions arose, suggesting that
abiogenesis may be relatively common once conditions are right. However, this evidence only looks at the Earth (a single model planet), and contains
anthropic bias, as the planet of study was not chosen randomly, but by the living organisms that already inhabit it (ourselves). Whether this is actually a case of anthropic bias has been contested, however; it might rather merely be a limitation involving a critically small sample size, since it is argued that there is no bias involved in our asking these questions about life on Earth. Also countering this argument is that there is no evidence for abiogenesis occurring more than once on the Earththat is, all terrestrial life stems from a common origin. If abiogenesis were more common it would be speculated to have occurred more than once on the Earth. In addition, from a classical
hypothesis testing standpoint, there are zero
degrees of freedom, permitting no valid estimates to be made.
Ok, I'm gonna give this ONE try, but I don't think I'll get very far. First, if you're going to make such an assertion, that 'most' scientists agree on life that is or was capable of visiting our planet, you're going to have to prove it.
