Drake equation shows a high probability of alien life

The argument

The Drake equation is an attempt to replace intuition with arithmetic. Rather than asking whether alien life feels likely, Frank Drake set out the factors that would have to hold for a communicating civilisation to exist elsewhere and expressed the expected number as the product of those factors. Each one is an estimate, and some are poorly constrained, but the structure of the calculation is what matters: the result is a probability derived from stated assumptions that others can inspect and argue with. What the equation returns, on Drake's own account, is a figure that makes Earth's uniqueness very hard to sustain. The galaxy is large enough that even cautious values for the uncertain terms, multiplied across it, leave many opportunities for civilisation to arise. To conclude that it has arisen exactly once requires every uncertain factor to be not merely small but extraordinarily small, and nothing that is known justifies setting them that low. The weight of the reasoning therefore falls against Earth being the only place in the galaxy where civilisation has developed. The argument has also strengthened over time rather than weakening. When the equation was first written, several of its parameters were little more than guesses; the subsequent detection of exoplanets has allowed some of them to be refined against observation rather than assumption, narrowing the range of plausible values. What the equation cannot do is supply proof, and proponents of this position are explicit that it does not. It is a probability estimate, not a detection: it says what to expect, not what has been found. That is precisely why it supports the position that alien life probably exists while we still lack evidence of it. The reasoning points strongly in one direction, and the absence of confirmation so far is a fact about how little of the galaxy has been searched rather than a result against the expectation.

Premises

Counter-arguments

Some of the parameter values of Drake equation are simply guesses, or can have a very high variance. Therefore the range of values of its solution are so large that can not be of practical usage

Rejecting the premises

[Rejecting P1] The equation does not compute a probability. It is a way of decomposing a question into factors so they can be discussed separately — Drake presented it as an agenda for research rather than a calculation — and it produces a number only once every factor is supplied. [Rejecting P2] The counter-argument on record identifies the fatal difficulty: several terms are unconstrained by any evidence. The fraction of habitable worlds on which life arises, the fraction of those on which intelligence appears, the fraction that develop detectable communication, and above all the average lifetime of such a civilisation are estimated from a single instance, so the output can be varied across many orders of magnitude by choosing plausible-looking inputs. A result that can be made to say anything says nothing. [Rejecting P3] Exoplanet data has constrained only the astronomical terms — how many stars have planets, and how many of those lie in habitable zones. Those were always the least contentious factors, and tightening them does not narrow the range the biological and sociological terms produce. [Rejecting C] The argument also ignores the observation that motivates the opposing view: no signal, artefact or trace has been detected despite decades of searching, which is exactly what the equation's optimistic readings would not predict. (The body is a typo-affected stub and is a candidate for replacement.)