The Drake equation (1961) - is an equation which tries to estimate the number of alien species who live in our galaxy Milky Way
For 2020 there is an updated version of this equation (the ACP) with a more accurate estimate of the number of planets that could support life.
The study: The Astrobiological Copernican principle →
A review of the study in Forbes magazine →
The Astrobiological Copernican principle
(Drake equation 2020)
Number of stars (N*) ⌄
Total number of stars in our galaxy - the Milky Way
Mature stars (fL) ⌄
The percentage of stars in our galaxy, which are older than 5 billion years
Habitable zone (fHZ) ⌄
The percentage of stars in our galaxy, which have a planet that is old enough in the habitable zone
Metallicity (fM) ⌄
The percentage of stars in our galaxy, which have a high enough metal content for life and technology to exist
Available time (τ′) ⌄
The average amount of time available for life to evolve on a planet
1 bn yrs
2 bn yrs
4.8 bn yrs
Lifetime of signals (L) ⌄
How long has the advanced civilisation being transmitting into space
Number of Alien civilizations (N) ⌄
Estimated number of advanced civilisations in the Milky Way
|N = N* * fL * fHZ * fM * (L/τ')|
|Estimated number of Earth-like planets in Milky Way||
|Odds of intelligent life / planet||
1: 10 million
1: 490 million
1: 200 million
How do we apply the revisited Drake equation (2020) to the whole Universe?
We multiply the results from our galaxy Milky Way to 2 trillion galaxies (our latest estimate for the number of galaxies in the Universe as of 2020).
Even if this approach might seem simplistic, we are just trying to make an estimation of scale.
To apply the Drake equation to the whole Universe, could be a much more complicated affair with many other variables and estimations - and it is definitely something we hope scientists will look into someday.
|Drake Model Optimistic||Drake Model Moderate||Drake Model Pessimistic|
|Aliens in Our Galaxy||928||60||36|
|Aliens in Universe||
1.8 quadrillion ⌄
928 * 2 trillion galaxies
120 trillion ⌄
60 * 2 trillion galaxies
72 trillion ⌄
30 * 2 trillion galaxies
1.8 quadrillion alien civilizations in Universe?
Isn't that a bit too much?
Those huge numbers could be because the scientists in the study assumed that life always appears on Earth-like planets within a few billion years as a natural part of evolution (mathematically, chances are assumed to be 1:1 - life always appears).
What if life is rarer than they believe?
Another study → that is using bayesian statistics found that chances of life appearing could be 9:1 for primordial life and 3:2 for intelligent life.
|Chances for Intelligent Life||Drake Model Optimistic||Drake Model Moderate||Drake Model Pessimistic|
|1:1 (Drake 2020)||1.8 quadrillion||120 trillion||72 trillion|
|3:2 (bayesian study *)||1 quadrillion||64 trillion||38 trillion|
|1:million||1.8 billion||120 millions||72 millions|
|1:billion||1.8 million||120 thousands||72 thousand|
However, what if we assume some aliens actually did not die as assumed in the latest 2020 Drake equation?
After all, why assume they died after 100 years? Maybe they just evolved away from using the radio.
How do we calculate how many aliens ever appeared?
For the 1:1 chances - that means that life always appears on a planet like Earth. So basically we only need to find the number of planets in the Universe. To find this, we have the stars in Universe (1×1024) and then use the latest Drake equation (ACP) and take the worst scenario from there which gives us:
1×1024 (stars in Universe)
* 0.19 (% Stars with Earth-like planets - this is constant in ACP)
* 0.015 (Matured stars at least 5bn y old - this is the fL from the pessimistic scenario from ACP)
* 0.4983 (Metalicity - fM from the pessimistic scenario from ACP)
= 1.4E21 (1.4 sextillion Earth-like planets in Universe which could support life according with the most pessimistic scenario of the latest Drake equation)
|Chances for Intelligent Life||Aliens who now have the same technological level as us||Some Survived (1%)||All aliens who ever appeared|
|1:1 (Drake 2020)||72 trillion||14 quintillion (1.4E19)||1.4 sextillion (1.4E21)|
|3:2 (bayesian study *)||38 trillion||75 quadrillion (756E16)||756 quintillion (756E18)|
|1:million||72 millions||14 trillions||1.4 quadrillion (1.4E15)|
|1:billion||72 thousands||14 billion||1.4 trillion (1.4E12)|
|1:trillion||72||14 million||1.4 billion (1.4E9)|
The Fermi paradox, is the apparent contradiction between the lack of evidence for extraterrestrial civilizations and the high estimates for their probability
There are a lot of explanations for this apparent paradox among which:
Civilizations broadcast detectable radio signals only for a brief period of time or Earth is deliberately not contacted, which look reasonable.
In a 2006 Sky & Telescope article, Seth Shostak wrote, "Moreover, radio leakage from a planet is only likely to get weaker as a civilization advances and its communications technology gets better. Earth itself is increasingly switching from broadcasts to leakage-free cables and fiber optics, and from primitive but obvious carrier-wave broadcasts to subtler, hard-to-recognize spread-spectrum transmissions."
Our galaxy - 250 billion stars
Universe - 2 trillion galaxies (2×1012)
Universe stars - 1×1024 - more stars than all the grains of sand on planet Earth