In 2019 I wrote a blog post A Pale Blue Dot with these two pictures in it. Left the iconic picture of Earth, taken  in 1972 by the crew of the Apollo 17 spacecraft, on their way to the Moon. Right a picture taken in 1990 by the Voyager 1, leaving the solar system and looking back to Earth At 6 billion km only a pale blue dot.

Earth is our beautiful world, one of the eight planets in the Solar System and the only one where life has developed, as far as we know.

Are there other worlds in the Universe? Our Sun is one of about 100 billion stars in the Milky Way and the MIlky Way is one of an estimated 200 billion galaxies in the (observable) universe.

The left picture shows the spiral structure of the Milky Way, with the location of our Sun marked. The right picture is the famous Ultra-Deep Field image taken in 2003 by the Hubble telescope. The image shows an estimated 10.000 galaxies in a part of the sky with a diameter 1/10th of the moon.

In 1992 the first extrasolar planet (exoplanet) was detected, at the time of writing this blog more than 5000 have been found and it is now assumed that most stars will have at least one planet orbiting it. That means that in the Milky Way alone there are already billions of planets.

Wouldn’t it be strange if Earth is the only planet where (intelligent) life has developed? There could be numerous planets in the Milky Way and Universe where life has developed. Michio Kaku an American ‘science communicator’, who always enjoys being in the limelight, goes even further: “The Laws of Probability Tell Us That the Universe Should Be Teeming With Intelligent Life Forms” 

The Laws of Probability ? As usual I am a sceptic. In 2010 I wrote two posts about “Are we alone in the Universe“. My personal opinion at that time was: “Yes, we might well be alone“. Now, thirteen years later, my opinion is still the same, maybe even stronger.

After this lengthy introduction, time to go back to the topic of this post, the Drake Equation.

Speculation about extraterrestrial life dates back to antiquity. Around 1900 it was thought by many that the planet Mars had irrigation canals, built by intelligent beings. Development of more powerful telescopes showed that those canals were an illusion. But maybe there were intelligent beings outside our solar system? This led in the 1960’s to the SETI program. the Seach for Extra-Terrestrial Intelligence. Involved in this program was Frank Drake, a young American astronomer. To have discussion points for the first scientific SETI meeting, he came up with what is now called the Drake equation.

Actually it is NOT an equation, it is an estimate for the number of intelligent civilisations in our Milky Way. The idea is simple, you start with how many stars are born yearly in the Milky Way. How many of these stars will have planets, how many of these planets will be suitable for life, how many of these suitable planets will actually develop life. How many planets with life will develop civilistations (intelligent life), and how many of these civilisations will be able/willing to communicate with us. And finally, how many years will such a civilisation survive.

Here the Drake equation is visualised: The estimated number of intelligent civilisations in the Milky way who can communicate with us is given by N as the product of a number of factors.

R* = how many stars are born every year in our galaxy. (R_∗ = 1 yr⁻¹)

f_p = the fraction of these stars that have planets. (f_p = 0.2 to 0.5)

n_e = the average number of planets in the habitable zone of such a star (n_e = 1 to 5)

f_l = the fraction of habitable planets that actually develop life. (f_l = 1)

f_i = the fraction of those planets, where evolution leads to civilisations with intelligent life (f_i = 1)

f_c = the fraction of these civilisations that develop a technology capable of releasing detectable signs of their existence into space. (f_c = 0.1 to 0.2)

L = the length of time that such a civilisation will exist.(L = 1000 -100,000,000 years)

In the 1961 discussion the various factors were discussed. I have given these estimates above. Using the lower limits, it gives a minimum of N ≈ 20 technological;y advanced civilisations, who could send signals to us. If those civilisations have not self-destructed, L could be many millions of years resulting in maximum of N ≈ 50.000.000 !

With this evaluation you will understand that it made sense to start the SETI program. After a few years a distributed computing project SETI@Home was started, where volunteers could use the idle time of their PC’s to analyse data from radio telescopes, searching for signals. of intelligent life . Many years I have taken part in this program. My PC during idle time was doing this.

After about 20 years the program was stopped, without any results. But in 2016 a follow-up project started Breakthrough Listen. Basically the same as SETI, but much more powerful, it will generate as much data in one day as previous SETI projects generated in one year. Until now no positive results.

I ended my 2019 post with :”As soon as evidence of life will be found, on Mars or deep under the frozen oceans of Jupiter’s moon Europa, I will celebrate and be convinced that life indeed is teeming in the Universe. Until then, I believe in the Rare Earth Hypothesis , that we might well be alone.“

What is this Rare Earth hypothesis? In 2000 Ward and Brownlee published a book Rare Earth: Why Complex Life Is Uncommon in the Universe in which they argue that primitive (microbial) life may be common in the Universe, but that complex (intelligent) life is probably very rare.

I agree with them that complex life will be rare in the Universe, but I no longer think that primitive life will be common.

A few months ago I published a post Perseverance perseveres about the Mars rover who is looking for traces of past life on the planet Mars. The scientists were expecting/hoping to find stromatolites, fossils of microbial life formed in the time that Mars had water. Something similar to this, found in Australia, 3.4 billion year old.

Until now no sign of fossil microbial life has been found. So it could be that the chance that primitive life develops on a planet in the habitable zone is also small!

As long as no sign of (fossil) microbial life is found in our solar system or elsewhere, I think that even primitive life may be rare in the universe. I used to say, We may be alone in the Universe. I now go one step further:

We are probably alone in the Universe

But of course I hope that I am wrong