Calculating the probability of life in outer space

Probability is a mathematical expression of the likelihood of a given scenario occurring. It is normally expressed as a number between zero and one. Zero, meaning there’s no probability of it occurring, and one, meaning it is an absolute certainty. The probability of a coin toss coming up heads, as an example, is 0.5.

So what is the probability of life existing in outer space? Well, you’re probably thinking we’re going to need something like the Drake equation and take into account the number of stars being formed within our galaxy, the fraction of those stars with planets and then the sub-category of those planets that are capable of supporting life (as we know it), etc.

But we need not go to all this effort because we already know the answer. The probability of life existing in outer space is one. It is beyond doubt. It is an absolute certainty. We know space can support life because we’re alive. We are in outer space (or, at least, our planet is).

Ok, there’s groans from the cheap seats, but this is a perfectly valid point. Without realising it, we tend to think of the Earth as the centre of the universe. We think of the Earth as being somehow special and unique, and from our perspective, it is. It’s the pride of ownership, the pride of tribalism applied to “our” planet. But Copernicus demonstrated long ago that the Earth has no special place of importance in the universe. It’s special to us, but both the Earth and the Sun are rather ordinary and average in a dull kind of way. That life exists here, in an unassuming, less-than-remarkable neighbourhood, is strong grounds for considering that it could occur elsewhere.

Ok, so what about any other life in outer space?

This is where things get particularly interesting. NASA has observed Glycolaldehyde and over 120 other simple molecules in vast gas clouds in space spanning three light years in size (which is huge. Voyager is currently on the edge of our solar system and it’s only 13 light hours away, less than one light day as compared to three light years).

Glycolaldehyde is a basic sugar that is found in both Ribose and Glucose sugars. Ribose is the building block of DNA, the basis of all life of Earth, while Glucose provides the metabolic energy for cellular life. So space is brimming with the basic building blocks for creating and sustaining life.

Another recent discovery has found 140 trillion times as much water in one small region of space than in all the oceans on Earth!

Numbers like this are meaningless until you put them in context. You probably read that last sentence in roughly a second. So if we think of seconds as a point of comparison, being the equivalent of all the water on Earth for the purpose of our analogy, then how long is a trillion seconds? Well, you’ll hit a million seconds after 11 days, but it will take you 32 years to pass a billion, while a trillion will take a whopping 32,000 years. So when we say there is an abundance of stuff out there essential for life, it’s a gross understatement. The components for life exist in such overwhelming quantities it is unfathomable.

And this brings up an interesting point. When scientists talk about the prospect of life in outer space, they talk almost exclusively in terms of probabilities. Creationists, determined to downplay any rational thinking about the universe, will often liken these probabilities to things like a 747 being randomly constructed as the result of a tornado hitting a junk yard. But this misses a crucial point, and that is the context in which things occur changes the on-going probability.

The probability of tossing five heads in a row might be slim, but after four heads, the probability of making it five in a row is actually quite good, at 0.5. In the same way, the probability of life accumulates and increases as each component required for life compounds and builds upon the previous.

150 years ago, Charles Darwin demonstrated that evolution works through Natural Selection, the gradual accumulation of inherited traits according to natural laws. Natural selection applies to living, organic creatures because of the compounding effect of numerous small changes adding up over time.

Could a similar principle apply to inorganic substances?

At first, it’s tempting to say, no. But this is not necessarily the case. Remember, the only reason Natural Selection works is because there is a mechanism in place for traits to accumulate and pile up from one generation to the next (which we call DNA). Inorganic selection is possible if there is a similar mechanism causing a compound effect.

Stay with me here.

There’s no cosmic DNA but there is the accumulation of compounds, the slow clumping of “stuff” that makes climbing mount improbable no arduous task, and that is a galaxy, accumulating into solar systems, accumulating into stars and planets. Together, they have a compounding effect, like interest in your bank.

With such an abundance of mater (metals, gases, water and pre-organic molecules like glycolaldehyde) being drawn together by gravity to form planets and moons around stars, we have, in effect, the same accumulation of the fundamental components required for life occurring all throughout the universe.

So why don’t we see ET staring back at us?

SETI are struggling with that question right now, and it seems there are a variety of possibilities. Firstly, there are a phenomenal number of stars within the Milky Way, some 200 to 400 billion of them, and it takes considerable time and effort to examine each of them. Also, we cannot see all of them. Radio astronomy can see through dust clouds, but if life existed in abundance, directly opposite us, on the other side of the galactic core at roughly the same point on a similar spiral arm, we would never be able to see it directly with a regular telescope, determine what planets there were in each system, etc. So there are limits to how well we can search.

Also, it may be that not every solar system is conducive for life. It seems our Sun is a third generation Population I heavy-element rich star. So our solar system may well be one of the early occurrences of compounding accumulation that allows life to thrive.

And the sheer distances involved mean the other several hundred billion galaxies (minimum) in our universe are just too distant for us to observe life. It would be like being stranded on a island off the coast of Africa and doubting there was any life on the distant desert shores when, in fact, the continent is teeming with the most remarkable diversity of life.

So… is there life in outer space?


At the very least there is us, but, more than likely, the universe is teeming with life because it is teeming with the building blocks for life.

2 thoughts on “Calculating the probability of life in outer space

  1. This comment was originally posted by Forrest Darby on the About page. I’ve moved it here as this is the article it relates to:

    Our Earth has been called the “Miracle Planet.” How many similar planets are there in the Universe? A sub-question: How many planets have been populated by intelligent life that transmitted radio signals, or other more advanced signals?


    Over the past month I have spent a lot of time and energy trying to answer these questions. This five page report is the result of that research, and it is to my scientifically minded friends. This is a work in progress; I plan to keep adding material to these pages as scientists find, and pass on new information. This data was gleaned from about 15 scientific articles and from shows on the Science Channel. Everything in this paper, except my conclusion, is generally agreed upon by the portion of the scientific community that works on this subject matter.

    One huge advantage that astronomers and other scientists have in puzzling out our universe is how physics work. The physics in our solar system are exactly the same as the physics in a different solar system on the other side of the universe. Physics is a constant! Therefore, the discoveries we make in the Milky Way apply everywhere.

    Within the last few years supercomputers have greatly improved and made life much easier for scientists. It is remarkable how these computers have been able fine tune things – like how Earth began.

    I have included information about our Sun and Earth that may seem unnecessary; but my thought was: If we want to know how many Earth like planets there are, first we need to fully understand our own planet and solar system.

    In the early 1970’s the United States Government began building large antenna and/or dishes to listen for messages from outer space. They called this the SETI Project (Search for Extraterrestrial Intelligence). At the time I was living in Alaska and had recently graduated from college. Some of my college friends and I were interested in SETI and talked a lot about it. We argued/speculated about how many signals they would find.

    Despite Carl Sagan’s optimism, and high guess, I thought there would be very few detectable incoming signals. As time passed, we learned that we really do live on “A Miracle Planet.” Our group of college friends realized that creatures on another planet would have to be very advanced in order to send radio signals, or signals of any kind, toward earth. Signals that could be picked up by our SEDI receivers.

    The Facts:

    The Big Bang occurred about 13.82 billion years ago, and the first stars “blink on” about 300 million years after the Big Bang. The first stars were immense, superhot giants that lived briefly and then exploded as brilliant supernovae, but they seeded the universe with basic elements that were the building blocks for the Sun and the Earth, and for life itself. The first stars were also fast-spinning; these massive stars spun 250 times faster than our Sun’s current rotation.

    The first stars were extremely hot and very short-lived. After just a few million years, they collapsed and exploded as supernovae. That violence created the heavier elements that completely changed the universe. Elements from oxygen to carbon to iron were blasted into space where they eventually became part of a new generation of stars.

    Supernovae continued to explode, seeding the universe with more and more heavy metals. Eventually, there were enough of these metals to create long-lived stars and for planets to accrete into their orbits. On at least one planet, the Earth, all the ingredients came together in the right place and time for life to evolve.

    Our Sun is “a third generation star.” Material from two previous star explosions formed our Sun, our solar system, and the Earth. This material may have come from the explosion of more than two stars billions of years apart. For example, two second generation stars that were close to each other may have exploded at about the same time, providing the material to form our Sun and solar system. 5% of the stars in the universe are larger than our Sun, and our Sun is brighter than 85% of the stars in the Milky Way Galaxy.

    Our Sun is 4.57 billion years old, and is hotter now by 148 degrees Celsius than when it was formed. The Earth was formed 4.54 billion years ago.
    Recent Cosmological models show that new star formation has nearly come to a halt. Star formation is 1/30th of what it was at its peek 11 billion years ago. Less than 5% of the universes total stars will form in the future.

    All rocky planets near the Goldilocks Zone in the universe are nothing but molten spheres for at least the first 300 million years of their existence. After that early stage, a planet like Earth needs to be hit by icy asteroids and/or comets. The right amount of water needs to arrive from outer space to cool the planet and create oceans and a protective atmosphere. For this bombardment to occur something radical inside the solar system must take place. In the case of Earth, many scientists believe this bombardment occurred when Saturn and Jupiter changed their obits; this disturbed and dislodged asteroids from the asteroid belt, the Kuiper belt, and comets from the Oort cloud. Many of these water carrying objects hit earth. This shower of space rocks is called the “Late Heavy Bombardment” (commonly referred to as the lunar cataclysm, or LHB), it started about 400 million years after the Earth formed and lasted 300 million years, or from 4.1 to 3.8 billion years ago.

    All intelligent life planets must have a large moon. Without our stabilizing moon Earth would radically oscillate and spin much faster, preventing anything other than microbial life.

    Very early in the Earth’s life we were hit by another planet. Astronomers and Astrophysicists have named this deceased planet Theia, and computer models claim it was the size of Mars. Theia hit earth at an “oblique angle,” of about 45 degrees, 4.533 billion years ago; or 7 million years after the earth began to come together. At the time of the collision Earth was spinning very fast, a day was only 2.3 hours long; when Theia hit Earth, Theia was traveling at 44,740 miles per hour, and the collision slowed Earth down by about 5 hours a day, our new Moon continued to slow the speed of Earth’s rotation. Because the collision was so overwhelming and violent it turned the Earth into something like molten jello; this viscosity, combined with our rotation, size and gravity, allowed the Earth to round itself out in less than 24 hours.

    Over the next 100 years some of the debris from the collision fell back to Earth. Due to the effects of gravity, the debris that did not return to Earth coalesced into two moons; this took less than 200 years. About 10 million years later the smaller moon crashed into the larger moon and formed what is now our only moon. At the time of this final Moon collision/consolidation, our Moon was only 14,000 miles from the Earth; it is now 238,900 miles from the Earth. The Theia collision, and the Moon that evolved from it, gave Earth many of the things we needed for life to have a chance. For example, the collision helped Earth sustain a liquid iron Outer Core. The heat, pressure and liquid nature of this Outer Core creates conditions that melt part of our Mantle producing magma chambers in the Outer Mantle; some of this magma on a regular basis breaks through the Crust as seepages or volcanoes. Earth continues to get bigger all the time, each day over 10 tons of meteoric rocks and dust plough into our atmosphere.

    Beginning from the outside, the Earth is composed of a Crust, a Mantle, an Outer Core and a solid Inter Core. The entire Earth has a radius of just less than 4,000 miles, and from the outside to the center the individual thicknesses are as follows: Our Crust averages 25 miles, our Mantle 1,800 miles, our Outer Core 1,408 miles, and our Inter Core 760 miles. This comes to 3,993 miles, but the number is slightly larger under our mountain ranges. The entire Crust occupies just 1% of the Earth’s volume.

    Our solid Core rotates in the opposite direction as the Outer Core. The Mantle and the Inter Core make up over 80% of the Earth’s volume. The Earth’s Outer Core is composed of molten iron and nickel; its temperature ranges from 4400 °C (8000 °F) in the outer regions to 6100 °C (11000 °F) near the inner core.

    None of the other three rocky planets in our solar system have compositions anything like ours. And none of the thousands of planets that the Kepler telescope has examined bear any resemblance to Earth.

    If fact, within the last year, all astronomers who will go on record call our solar system a startling anomaly within the Milky Way.


    Like many others, I constantly upgrade my cosmological knowledge. Below are things that another planet would need to possess in order for intelligent life to evolve and build radio transmitters. I’m sure this list will get longer as scientists learn more about the universe, and record their findings:

    1. It must be rocky and located within the Goldilocks Zone; it is better if it is close to the center of that zone. Approximately 1% of all the rocky planets in the universe are in the Goldilocks Zone. The argument for a lot of intelligent life outside the Earth is that there are over a hundred trillion (not billion) rocky planets in the universe.

    2. A large moon.

    3. A molten Outer Core and an adequate protective atmosphere. Our liquid Outer Core is necessary for many reasons; for one it creates a blanket of magnetic protection around our planet. The Earth’s magnetic field is generated by a self-sustaining dynamo caused by the fluid motions of the conductive liquid metal in the outer core. Without enough magnetic protection, and a good atmosphere, life on any Goldilocks planet could not exist. The Sun’s solar winds and ultraviolet radiation would sterilize the planet.
    4. Moving tectonic plates. In 2009 the idea was raised by a planetary scientist that tectonic plates were essential for the development of complex life. (This is a very complex subject and should be Googled.) However, by September of 2013 this idea has reached broad acceptance from astrobiologists and others, including the Science Channel.
    5. A slow rotation speed; close to one rotation every 24 hours would be best.

    6. An internal composition very similar to Earth’s.

    7. Something close to a round orbit around its Sun; an orbit like Mercury’s will not allow intelligent life to evolve. This would be true even if a perfect planet started its orbit in the middle of the Goldilocks Zone, but followed the elliptical path of Mercury.

    8. It cannot be completely covered with water (Dolphins cannot send radio signals), but will need to have enough water, very similar to Earth.

    9. The right amount of carbon; if the land masses are covered with an oily film the planet cannot support intelligent life.

    10. The right age. A rocky planet under perfect conditions in the Goldilocks Zone will need to be at least 4.3 billion years old for intelligent life to emerge (intelligent live cannot evolve quicker than that), but a planet older than 4.8 billion years cannot sustain intelligent life, due to dramatic climate changes that will take place at that time. (This timeframe could be much shorter due to the environmental effects intelligent life has on a planet.) This means when a Sun shines for over 10 billion years, as our Sun will, less than 5% of this time is available for intelligent life on that Sun’s Goldilocks Planet.

    11. Number 11 and 12 are add-ons that I forgot to mention in my original paper. Number 11 concerns the planet Jupiter. I added this paragraph a few days after the main body of this piece was written. There have been a number of scientific articles published in the last two years about Jupiter. Does Jupiter help or hurt Earth? I think this is an easy call; without Jupiter, I may not have been able to start this paper! The segmented comet Shoemaker-Levy 9 collided with Jupiter in July of 1994. These collisions lasted for nearly a week; they began on July 16th and ended on July 22nd. If Jupiter was not our big protective brother, and if it was not located where it is in our solar system, Shoemaker-Levy 9 may have hit us and destroyed everything except Earth’s microbial life. Also, computer models show this has happened many times in the past. Another intelligent life planet in the universe will need its own Jupiter.
    12. Our solar system is in the Goldilocks Zone of the Milky Way Galaxy. If our solar system was located inside the Goldilocks Zone, radiation would kill everything on Earth. If we lived outside this zone we would have other problems. So, intelligent life planets in any solar system in the Milky Way, or in any other Galaxy will need to be located in the Goldilocks Zone of their Galaxy. This alone will eliminate over 50% of the planets in the universe.

    Carl Sagan once said that there were “billions and billions” of Earth like planets in the universe – now, 17 years after Carl’s death, with new information from our earthbound and space telescopes, and with help from our latest supercomputers, I believe this number is far less than Carl imagined. In fact, getting back to my original question, I believe that the odds against another earth/moon forming somewhere in the universe are well over a trillion to one. Therefore, I believe that less than 100 extraterrestrial civilizations have ever existed; and even fewer have built anything that could transmit a radio signal, or a more advanced signal, from their planet.

    • Forrest, thank you for your very detailed reply. You’ve clearly spent considerable time thinking about this and collating a lot of information on the subject, and it is certainly fascinating to consider.

      I’ve moved your comment to the page Calculating Probabilities so it occurs against the article you refer to.

      On the face of it, The Rare Earth Hypothesis or “Miracle Earth” concept is compelling, but there are other possibilities to consider as well. As we only have one sample of life in space, ours, it is easy to think ours is the only type of life that can exist. There could be types of life we can’t even imagine. As an example, there are vast molecular clouds containing precursor chemicals for DNA that dwarf not only our planet but our solar system by thousands of times. Hypothetically, these could contain life, but in the words of McCoy from Star Trek, “It’s life, but not as we know it.”

      Another point to consider is that even though Kepler has found thousands of planets, they represent an insignificant fraction of the sheer number of planets in our galaxy (estimated at 60 billion+). If our galaxy was a twenty foot high haystack we would have effectively searched through a teaspoon’s worth of hay looking for a needle. If our solar system were viewed from a far using something similar to Kepler, Earth might not even register as it is so small and so distant from the Sun, so this is a field of science we’re still refining.

      The cosmological principle suggests that as remarkable as Earth is, it is not unique in being the only place where life exists. As you note, there could be hundreds of other extraterrestrial civilisations out there, but for a variety of reasons we haven’t found them yet.

      If you’re interested in learning more about this, I highly recommend Seth Shostak’s book Confessions of an Alien Hunter, where he recounts several decades spent working for SETI. It’s a great book to read, and really well written.

      All the best,

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