Part III How Rare is life?

Now that we’ve considered the goals that are possible and how many possible places we might look let’s look at the critical unknowns and estimate the probability of finding something.

Let’s start with the variable Ne the first of the really unknown and highly variable quantities:

Ne = Fraction of planets that are suitable for life to develop

Ne 18.75%
50% being in galactic zones with suitably low radiation and collision with other
75% high star metallicity
50% protected from high bombardment of asteroids
50% %age of stars existence planet is viable ( or millions of years habitable)
other factors may impact this percentage including some people who believe high angular tilt of a planet is important or a moon that provides tides
2 Number of planets in a typical star system that are in Habitable zone

This value is a percentage that given that we have a planet it may be inhospitable for life to ever develop on it.   It may be so close to the sun the surface temperature is 2,000C.  It may be bombarded constantly by high Xrays or it may be so massive and gaseous that life as we can imagine is not possible.  However, if a star is in the habitable zone what are the chances it might support life.  Not that it actually has life but that if life were to show up, say we were to drop a care package that life might take root.

A typical solar system may have 1 to 10 planets.  In our solar system there are 2 planets in this position Mars and Earth or 2/9 = 22%.    However, there may be other moons or objects in our solar system suitable to life.  We don’t know.

The values for this parameter are all over the map.  Some estimate it at close to 100% pointing out that life emerged on Earth about as soon as it could and life exists in many different stressful environments but the fact is the universe is a hostile place to life.  Life needs to establish a toehold and the conditions for that are difficult no matter how you look at it.   I believe many scientists have not really considered how much may be needed to make a planet even suitable for life.

Consider that Mars has the right sun, the right heat approximately, it has water and many minerals.   At one point it had lots more heat and atmosphere.  We still have found no real signs of life.  Although some might say there is evidence there might be microbial life. The viking probes did an experiment that some scientists have concluded showed evidence of life-like activity.  If this could be confirmed it might change a lot of these probabilities because another positive example so close would be quite a proof point that life is pretty tenacious.   If life doesn’t exist on Mars it is a real negative because about 10lbs of material is exchanged between Mars and the Earth annually on average over millions of years a lot of life bearing material from the Earth must have impacted Mars over time.  If none of this life could attain even the smallest foothold it would be pretty devastating.   If we discovered the life on Mars if we find it had a completely separate starting point this would be massively positive again.  We should know the answers to these questions in the next 20-40 years I would guess.  Maybe sooner.

We know that many parts of the galaxy are extremely high radioactivity and are much “busier” than our world.   Such things cannot be good for life.  Catastrophic events may occur far too frequently.  Colliding star systems, radiation bursts, exploding stars means that a fraction, maybe 50% of the galaxy is a decent neighborhood to evolve life.    In order for the planets of a star system to be viable they need a lot of chemicals for nature to play with.   Many stars don’t have higher proton count elements in abundance.   It seems that you need a sun with a high metallicity.  About half of suns have this from what we’ve observed.  So, even if a planet is theoretically in a “habitable zone” as its distance and size might indicate it doesn’t mean it is in a star system located in a good neighborhood or has the right materials to possibly have more complicated chemical processes necessary for life.

The planetary system must evolve.  In the early parts of our solar system gases were congealing and asteroids were smashing into things constantly.  This means that during a significant part of the 5 billion years (possibly half) even the Earth was not in a position to be a host planet.     One of the advantages we have is a magnetic field.   No other planet we know in the solar system has such a high magnetic field as the Earth.   Our magnetic field protects us from harmful levels of solar and interstellar radiation.  We don’t know the reason for our magnetic field precisely and therefore how rare it is but I would say an optimistic assessment is that 75% of potential planets don’t have enough magnetic field.   Life has lived in hostile cosmic ray environments.   It has been shown that some forms of cells have evolved extremely powerful nucleus DNA protective measures that survived periods of extreme radiation on Earth.   So, it may be irrelevant.  Maybe we simply would have evolved with thicker skin but it certainly makes life a lot harder to have lots of stuff banging into your molecules from outside with high energy.

It is also very possible that for life to evolve beyond the most primitive other conditions need to prevail that “push” evolution along.   Plate tectonics is an outlier but the movement of the plates has encouraged spreading of biology from one area of the Earth to another and separated species at times to evolve differently.

Periodic catastrophic events are probably crucial to higher evolution.    As in the brain there is a 2 layer system at work here.   At first you need stable environment for plants and animals to adapt and grow.  However, things may stop evolving and settle in too stably.

It seems apparent that periodic mass extinctions or stresses are needed to mix things up, force evolution to be crafty to work its way around obstacles and invent new things.   In this way periodic stresses “filter out”  and force evolution to be creative.

A recent MIT article shows that after catastrophic extinction events evolution accelerates diversity.  This makes complete sense for multiple reasons.    Mutations that occur once a lot of competitive species have been eliminated are more likely to survive.  So, that the successful species that survive find they are able to have all kinds of successful progeny that might not have worked before.  Hence increasing diversity.  We have also found that some animals that are stressed are able to produce offspring with more copies of DNA resulting in greater mutations and variety.  This is an environmental effect on evolution we didn’t think was possible before.  It seems the animal once it is stressed remembers this and produces the variation even a long time after the stressful event.

The last catastrophic species wipe-out happened about 65 million years ago and an explosion of life that followed was very much more advanced leading to human beings.  It is quite unknown and possibly unlikely that humans would have evolved at all without that final shot.   It was important for this disaster to happen when the DNA evolution had reached the point that the jump to more advanced brainy creatures could evolve.

It is also thought that certain environments associated with the Earth like tidal pools are ideal locations for chemicals to mix it up and try different combinations.  Deep undifferentiated water may not provide the varied environments needed for life to emerge beyond the most primitive.

All in all, whether or not I have picked the right conditions needed for a “successful” planet for life the fact is we have examples here locally that show that even with a lot of conditions right life doesn’t just pop out.   The counterpoint to that which a lot of scientists use is that life emerged on the Earth within 50 million years of when it was remotely possible.   This says that given these “ideal” conditions life will emerge pretty reliably but I think many people have not considered how “ideal” the Earth’s condition was compared to other planets.


Kepler was a satellite that looked for planets around other suns.   It was an incredibly successful satellite.  Kepler detected 31 planets that are Earth-like.   Amazingly Kepler was able to detect these at a surprisingly far distance.    Kepler detected planets that are the right size, in the right zone of its star to be warm and similar to the Earth from 12 LY (Light Years) to 1250 LY from Earth.  Kepler has died but the success has spawned interest in following missions which could be vastly better and new techniques.

The Seager experiments coming up are an offshoot of this.   Seager proposes to detect not only planets but to measure the gases in the atmospheres of planets to detect those that may be suitable for life and even the possibility we will detect gases whose only possible origin we believe would be living things.   As we build the James Webb telescope and other satellites we will refine more and more our ability to peer out.   I expect that we will have at least 3 orders of magnitude and maybe much more ability to detect planets, gases and even eventually see worlds in other solar systems without having to travel there.

In addition, we must consider that the sun gets hotter every year.  Over the next 10 million years the sun will get hot enough that it will potentially trigger catastrophic global warming on the Earth. We may want to consider moving the Earth farther from the sun.   It would be a good idea to do this gradually.  🙂   Some have speculated we should migrate to Mars because it will be more suitable for life in some millions of years.   It’s not as far off as some imagine.

Other solar systems where life has not gotten to the point that it can consider moving the planets around or migrating populations will find that the window for life in that solar system has gone by.  So, again there is another condition on the planets viability.  Is it in the right time period in evolution of its star to support life.

Conclusion:  Ne = 18.75%   i.e. only 1 in 5 planets is truly suitable for life to evolve to beyond microbe level.   I worry that this is a terribly optimistic number.  I can imagine lots of these factors are worse than I suggest.  However, on the other side of the coin life is tenacious surviving numerous problems here on Earth.  So I believe the 18.75% number is balanced optimism on the whole.

Fl = Fraction of planets that develop multi-cellular life

Fl 9.75%
 50% single to multi-cellular jump
75.00% enough catastrophic events of right type
50.00% magnetic field for protection from cosmic rays
25.00% original conditions for life exist (possible panspermia)


I have slightly changed this parameters definition from simply anything that could be classified as life to a more substantial requirement that life get to multi-cellular development.   I do this because we may look for planets where there is life.  If so, we would be interested mainly in planets with something more than microbes.    Those may be common in the galaxy.  We are looking for planets where life can get beyond single cellular and eventually to mega-fauna.   So, I have made this parameter a little more demanding.

As I mentioned before there is some evidence that Mars may still harbor single cellular life.  The viking lander detected signs that when water was placed in a sample of Mars soil an exponential production of methane and other gases known to be biologic byproducts occurred.   A recent re-analysis of that data shows even more convincingly that the reaction of Mars soil to water  does contain life.   The Viking also took martian soil and sterilized it by exposing it to high temperatures.   After putting water into this soil the reaction was vastly different than the untouched soil.   A statistical analysis showed a very high probability that the behavior of the martian soil was consistent with biologic life.    Comparison with terrestrial soil showed a high correlation with the behaviors of the martian soil.

It is unknown how rare the initial conditions are for life to evolve.  Does there need to be a special set of chemicals in a special place?  Does there need to be a combination of events?  I have given the probability that 75% of the time whatever original conditions are needed don’t happen.   There is a theory that through the conscious effort of another intelligent species or through random luck DNA fragments or other primordial soup components are delivered to the Earth from outside.   This theory is called panspermia and is a leading theory for how life emerged on the Earth.   We know that something like 10 lbs of rocks are exchanged with Mars and vice versa annually on average.    So, there is a lot of interchange and if life exists on one planet in a solar system even for a brief time it will likely spread to other planets that are viable.  The fact that contamination of Mars hasn’t resulted in life on Mars to our knowledge is evidence that Fl isn’t as big as some scientists think it is.   However, if panspermia is needed it does reduce the probability of initial conditions being right because these “life bombs” probably happen rarely.  If they are indeed gifts from a far off civilization then the number would be infinitesimal because they cannot possibly populate many worlds with this seed soil.   Hopefully if it is another intelligent species they have carefully chosen the worlds they send the soup of life.  If it isn’t intelligent benefactor then the rudiments of life may evolve on interstellar objects and then fall on the Earth or similar with asteroids extra-solar objects from living planets could possibly find their way to Earth-like planets to fall on them.

In the Fl case I didn’t include the catastrophic events although I mention how they may be crucial.   Here I include that besides a planet needing to have a number of conditions to be viable it also needs to have the possibility of right sequence of catastrophic incidents, not too often, not too rarely.

We don’t know how long or difficult it is for single cell creatures to evolve to multi-cellular.  In principal this is a huge step.   It is not entirely obvious to me why single cellular creatures would figure out how to work together.   It may take far longer on some planets than others.  It took about a billion years here.  It could be 10 billion on other planets.   Since most suns will become inhospitable after some range of the suns existence the chance for life goes away if the evolutionary steps per chance take too long in some solar systems.

Fl  = 0.0125 only roughly one in 100 planets which are suitable for life actually evolve multi-cellular life forms or mega-fauna

Fi = Fraction of planets where life evolves to sentient intelligent species

Fi 4.5563%
50% sufficient resources or viability to get to agricultural
90% sufficient brain size ever obtained
90% Netwon happens
50% not too violent/unstable
50% do not destroy their environment / use resources
50% no catastrophic event
90% no strong memes prevent advancement

I have changed this parameter as well.   I believe that Drake intended this to be simply species that have some level of intelligence.  That could mean anywhere from monkey or elephant level to neanderthal.   I am putting a greater requirement.   I am saying what is the chance the particular species has achieved at least say 800 AD intelligence, has gotten an agricultural society.

There are many things that might stop life from evolving past multi-cellular / mega-fauna level to this intelligent level.  I have already mentioned there needs to be multiple catastrophic events most likely to kick start evolution but not too many or too difficult.   One thing that Jared Diamond who wrote the book Guns, Germs and Steel is that in order for humans to finally make the transition to agriculture and start to develop science and math required there be enough plants with high enough caloric content that could be easily farmed.  Also, the existence of domestic-able animals that could be employed to help was critical.   This is observed by looking at the places where civilization emerged separately on the Earth.   Jared points out that frequently societies don’t make it past a minimal level of intelligence before destroying their environment or using all the resources.  There has to be enough resources to allow a civilization to grow to develop the technology so that at each stage it doesn’t destroy the environment before it innovates out of the problem.  We also don’t know how special people like Newton are that may have that aha moment.  Possibly on some other planets they never emerge from primitive civilization to scientific civilization.

There is a question of does this parameter measure if a species emerges from a primitive agricultural society to a more scientific society.  If it makes it to that then presumably it will achieve creating signals to the extra solar environment so it is tricky where we draw the line from monkey level intelligence to pre-scientific to scientific civilization.

It is possible that a smart creature evolves but that its brain is 30% smaller than humans.  Would that creature ever make it to intelligence we call intelligence?  Is the size of our brain because of requirements in nature or pure luck?  Is it relevant?   Even if a species could make it to intelligent scientific culture does a catastrophic event happen that takes it out before it goes far.

Our brains are full of evolutionary memes.   These things control our behavior and make us believe in god or to belong to groups.  If the memes of the creature are too strong it may never be able to get beyond the most basic level of advancement.

I don’t know if these are all requirements or what other things might impede such development.  However, I believe there are things that would impede our development to an intelligent society.

Fi = 4.6% or 1 in 20 mega-fauna like planets eventually develop an agricultural semi-scientific society capable of rudimentary science.

Fc = Fraction of Civilizations that develop communication technology and actively send messages

Fc 6.25%
50% They aren’t hiding
50% They’re not communications are on frequencies we aren’t looking
50% They’re communications are likely to look like random noise
50% They use mechanisms we don’t search – quantum, highly focused light or fiber optics

Assuming a species on some planet has gotten intelligent and is at the 800 AD level it needs to develop really to the 2000 level of scientific advancement at least and beyond.  It may or may not need to actually transmit signals to us.  The energy output of our radio waves we’ve been putting out would not go very far in stellar terms.   If we hope to be found we would need to make an effort to send a cohesive signal to the rest of our neighboring planets.

Lots of civilizations may decide they never want to do that.   They may decide they are scared of meeting us or not interested.  They may hide.  I am going to assume that they aren’t but I believe this parameter could be a lot worse for us than the 95% I put in.   Essentially I am saying only 1 in 20 civilizations such as ours decides NOT to broadcast itself.

More significant there are problems with communication.   Unless they are sending signals to external places specifically it is likely they will maximally encrypt and compress any communications they do.   We are getting better and better at that.   What this means is that for someone peering at our signals the signals appear more and more to be essentially random data.  It becomes harder and harder to “discover” if a signal is random or real data from someone intelligent.

Another big issue I have is that intelligent species may learn new ways to communicate which don’t use the frequency spectrum we think is useful.  For instance, future civilizations may do all their communications on fiber optics type cables.  They may choose a completely different mode of signaling possibly using gravity waves or some form of quantum teleportation that is impossible to intercept or see and is instantaneous communication (theoretically impossible to do exactly like this.)    They may be using frequencies that are beyond what we are looking for.   Altogether I believe there is much higher probability that we simply are not looking in the right places or cannot look.

Fc = 6% or 1 in 16 civilizations are communicating in a way we can discover


If you compare my numbers above to numbers used by most people who have studied this you will see my numbers are MUCH LESS than theirs.  Most people are more optimistic.   I hesitate to say that because I am not being pessimistic at all.   I am simply expounding all the possible gotcha’s in finding an intelligent species.   It’s apparent to me after doing this that the probability of finding an intelligent species communicating is less than I’d assumed it might be.

When you look at each of these parameters by themselves and don’t think about all the things that could go wrong there is a tendency to extrapolate from our experience.  So, many astronomers or scientists will conclude that the possibility of life is very high because it evolved so robustly on Earth.  We tend to think if we are typical then maybe other solar systems have a similar situation.

The fact is when we look out we see there are MANY other possibilities.  There can be stars in extremely inhospitable regions of the galaxy where stupendous scale events are happening all the time.   I have documented some of the things that could go wrong.  Something as simple as having plants that are nutritious enough that an intelligent species can afford to give up hunter gathering to farming was pointed out by Jared Diamond as a precondition to agricultural society.  Barley was one of the only plants that grew that without modification produced enough caloric content for us to sit down, specialize and do more than hunt every day for the next meal.

The only thing working in our favor is the sheer numbers of stars.   They number in the billions thank god.


In this series

Part I – Goals for our desire to understand and find extraterrestrial friends

Part II – how big is our search area?

Part III – what are the problems in finding life

 Part IV the equations 

Part V  the conclusions