by Scott Reddiex MRSV

Regardless of any surprises that tomorrow may hold, one thing is certain: life currently exists on Earth. In a universe ~13.77 billion years old, on a planet formed ~4.54 billion years ago, the first signs of something resembling life can be traced back ~4.1 billion years¹ – meaning that not only does life currently exist here, but it has done so for quite a while.

Earth as a Model Planet

Earth is just one planet, orbiting a single star. In turn, our sun is one of 100-400 billion stars in the Milky Way Galaxy. With our galaxy being one of an estimated 2 trillion galaxies², this takes the total number of stars in the universe up to somewhere between 2-8 billion trillion (2-8 x 10²¹).

We know that our solar system has eight planets (sorry Pluto!), with more than 200 moons shared between them, and many more (~1 million) minor planets (which includes asteroids and dwarf planets like Pluto). However, regardless of the many different objects orbiting our star, the only evidence of life we have comes from here on Earth. Using this as our starting point, we can define a range of potential locations in the universe that life could similarly emerge: a planet with all of Earth’s characteristics, orbiting a star with all of the Sun’s characteristics…while also acknowledging that life could also exist elsewhere.

To make some rough calculations, we can call a Sun-like star one that falls into the F, G, or K-class of stars, which makes up around 20% of stars in the galaxy³. Each of these stars is estimated to have 0.4-0.9 rocky planets in its habitable zone³, bringing our range down to only 0.26-1.04 billion trillion ‘Earth-like’ planets in the universe. For comparison, this number (1.04×10²¹) is greater than the number of milliseconds that have passed since the beginning of the universe (4.3×10²⁰).

The Drake Equation

The task of combining the various mathematical factors into a logical formula was undertaken in 1961 by the astronomer/astrophysicist Dr Frank Drake (who passed away just last month, on 2 September). Focusing only on the Milky Way Galaxy, his work resulted in the eponymous Drake Equation: N = R_∗ x f_p x n_e x f_l x f_i x f_c x L.

This equation states that:
N (the number of civilisations in our galaxy emitting a detectable electromagnetic signal)
= R_∗ (the average rate of star formation in our galaxy)
x f_p (the fraction of those stars that have planets)
x n_e (the number of planets per star that can support life)
x f_l (the fraction of planets on which life emerges)
x f_i (the fraction of planets on which intelligent life evolves)
x f_c (the fraction of civilisations that develop technology that produces a signal we can detect)
x L (the length of time that a civilisation produces the detectable signal).

The value of the Drake Equation lies less in producing a magic number, and more in the breakdown of the individual contributors to the chance of detecting life. As our technological ability to detect any signals advances, the value for f_c will increase, and it has done so over the past 150 years of human history. The value of f_l varies greatly depending on how the question is approached – Earth is our only ‘model planet’ for Earth-like conditions, and life definitely emerged here. Does this mean that abiogenesis (life originating from non-life) is something that is very likely to occur on an identical planet? Life seems to have emerged here only once (i.e., all life on Earth shares a common ancestor, and there has been no evidence of any extinct alternate lineages), however systems like the Miller–Urey experiment have shown that amino and nucleic acids will form under conditions similar to early Earth^4,5. Should we ever discover evidence of life (either extinct or extant) on a planet like Mars or a moon like Titan, it would understandably increase the values of f_l, and n_e.

The Fermi Paradox

Now that we’ve established 1. that there are a lot of places in the universe where life could possibly start like it did on Earth, and 2. that life on Earth started ~440 million years after it was formed, this raises a question: where is everybody?

This particular question is called the Fermi Paradox. Named after physicist Professor Enrico Fermi, the story goes that he exclaimed it after discussing faster-than-light travel and reports of UFO sightings with fellow physicists. Put simply, the nature of the paradox is: if the chances of life starting somewhere are relatively high, then why haven’t we seen any signs of anyone else in the universe?

By this point, you have probably had some ideas coming to mind as to why we haven’t heard from any potential cosmic compatriots. We can make different groups from some of the many hypothetical reasons:

(Intelligent) Life is Rare

As we saw from the breakdown of the Drake Equation, it seems abiogenesis has a good chance of occurring on a planet like Earth, even though we only have Earth as our model for this. However, something that is significantly rarer than abiogenesis is the subsequent emergence of intelligent life. While it only took ~440 million years for life to get started on early Earth, it took another ~4.1 billion years for the only intelligent life form to emerge (and even then, we all have our slow days). There are many hypotheses for why this is the case, with one of them being the number of mass extinction events throughout the planet’s history, culling the more complex forms of life and resulting in a delay in progression towards intelligent life. Although…

Evolutionary Pressure

…intelligent life isn’t really ‘progress’ in an evolutionary sense. While humans did arise from non-intelligent ancestors, evolution by process of natural selection is about survival and advantage in a niche, and not about reaching any preordained target. This means that ‘life’ + ‘time’ isn’t guaranteed to equal ‘intelligent life that builds a spaceship’. An example of this is the coelacanth: a fish that has remained relatively unchanged in its underwater niche for ~400 million years and is very unlikely to build a spaceship anytime soon. Therefore, while there may well be life on other planets, there is no guarantee that there was any evolutionary pressure on it to become intelligent.

However, if intelligent life did emerge elsewhere, human history shows us that, despite surviving mass extinction events, there has always been a great threat from other members of the same species. The Doomsday Clock was devised in 1947, following the use of atomic bombs in World War II, as a representation of how close we are to human-made global catastrophe (signified by the clock hands reaching midnight). With present threats from nuclear war and climate change, the time on the Doomsday Clock is currently set to 100 seconds to midnight, however it is possible that other intelligent life in the universe has already been the cause of its own demise.

Finding Life is Too Hard for Us

As we worked out earlier, you could say that the universe is pretty big and has been around for a while. In contrast, humans have only been actively searching for signs of life elsewhere in the universe since the invention of the radio telescope – around 85 years. The vast scale of time and space in our universe means that intelligent life may have come and gone in multiple places, and might even be sending signals of their own, but nothing has yet reached us in the relatively short window of time we have been listening.

If we couple this with the evolutionary pressures discussed above, even if signals travelled close to the speed of light directly from an intelligent civilisation to Earth, by the time it reached us, their Doomsday Clock may have struck midnight tens of thousands of years ago.

Willingness to Communicate

A.k.a., we’ve been ‘left on read’. As part of actively searching for extra-terrestrial life, humanity has been transmitting different messages (such as the Arecibo Message sent in 1974) intermittently into space for around 60 years. These messages typically provide basic information about the planet and humanity, with some (including Prof Stephen Hawking) criticising the practice, saying that it has the potential to put Earth at risk.

These messages from Earth may indeed be reaching intelligent life in our galaxy, informing them of our existence, but they might be reluctant to respond out of fear for their safety. Other reasons we haven’t heard back could be that they might simply see no benefit to responding to us, they are indeed planning to attack us, or, like David Attenborough observing lions hunting gazelles, they have a ‘no intervention’ policy and have opted to leave us blissfully unaware of their existence.

What’s Next?

For now, at least, all is quiet. Working through the different components of the Fermi Paradox and the Drake Equation, it does seem logical that life on many other planets would be quite common, and equally logical as to why might never detect signs of anyone else. Humanity is actively searching for any signs of life beyond Earth, supported by ever-advancing technological capabilities. NASA’s Perseverance rover is currently exploring Mars for any indication that microbial life has ever existed on the Red Planet⁶, and data gathered by NASA’s Cassini spacecraft has this month revealed that the six elements required for all life on Earth (carbon, nitrogen, hydrogen, oxygen, sulphur, and phosphorus) are all present on Saturn’s moon Enceladus, in addition to liquid water below its surface^7,8.

The only thing we do know with 100% certainty is that intelligent life exists here, now, for the only time in Earth’s history. We must hope that the hands of humanity’s Doomsday Clock retreat from midnight, so that we have the chance to continue exploring this amazing universe.

Do you have a particular theory about where everyone is hiding? Should we be sending a signal out into space advertising our location? Have you ever visited the Planetarium at Scienceworks, read up on abiogenesis and the Miller-Urey experiment, or seen a coelacanth build a spaceship? We’d love to hear from you! Send your letters, news, and feature articles to [email protected] for inclusion in future editions of Science Victoria.

References

1. E. A. Bell, et al. Potentially biogenic carbon preserved in a 4.1-billion-year-old zircon. Proc. Natl. Acad. Sci. 112, 14518–14521 (2015) (https://doi.org/10.1073/pnas.1517557112)
2. C. J. Conselice et al. The Evolution of Galaxy Number Density at z < 8 and its Implications. ApJ 830, 83 (2016) (https://doi.org/10.3847/0004-637X/830/2/83)
3. S Bryson et al. The Occurrence of Rocky Habitable-zone Planets around Solar-like Stars from Kepler Data. AJ 161, 36 (2021) (https://doi.org/10.3847/1538-3881/abc418)
4. S. L. Miller, A Production of Amino Acids Under Possible Primitive Earth Conditions. Science 117, 3046 (1953) (https://doi.org/10.1126/science.117.3046.528)
5. J. Oró, Stages and Mechanisms of Prebiological Organic Synthesis. From Origins of Prebiological Systems and of Their Molecular Matrices. New York Academic Press. p. 137-171. (https://doi.org/10.1016/B978-1-4832-2861-7.50020-X)
6. Witze, A. NASA’s Perseverance rover begins key search for life on Mars. Nature 606, 441-442 (2022) (https://doi.org/10.1038/d41586-022-01543-z)
7. Hao, J. et al. Abundant Phosphorus Expected for Possible Life in Enceladus’s Ocean. Proc. Natl. Acad. Sci 119, 39 (2022) (https://doi.org/10.1073/pnas.2201388119)
8. Le Gall, A., Leyrat, C., Janssen, M. et al. Thermally anomalous features in the subsurface of Enceladus’s south polar terrain. Nat Astron 1, 0063 (2017). https://doi.org/10.1038/s41550-017-0063