When will we find life? That question has been in the mind of quite a lot of people at one point or another… Well at least those of us who are interested in space travel and exploration. It is however, becoming an ever more common question for the general public, especially with the advent of human spaceflight to Mars looming right on the horizon. That question has been the subject of many conferences, debates, books and even television shows. The truth is, we don’t know. However, what I am going to try to do, using some back of the envelope calculations and approximations is show you why we cannot be hasty but we can be optimistic.
Let me quickly introduce myself. I am currently a 22-year-old Northern Irish student with the Open University studying a BSc in Natural Sciences (Astronomy and Planetary Science). I love everything space, from rockets to black holes. But one of the things that captivates me most is the vastness of the cosmos and trying to comprehend what could be out there. So, let’s get started…
We’ve all been to the beach, scooped up a handful of sand and let it run through our fingers. If we let all but one tiny little grain drop back to the beach, we’ll be left with a single grain that we can use to represent a star. We then pick up another grain, this represents a second star, then pick up a third, fourth and so on. When we eventually pick up the 32nd grain, our little pile of stars would fill a volume on our palm of about 1 mm3 when packed in the densest pattern possible. This is about half the size of the tip on a ball-point pen.
The first question is, how many grains of sand would it take to represent all the stars in the galaxy? Conservative estimates would suggest that there are around 100,000,000,000 (100 Billion) stars in the Milky Way (about one star for every human that has ever lived), although some estimates suggest 4 times this. The Milky Way is larger than average for a Galaxy so to keep things even, let’s take an average between the two estimates, 250,000,000,000. That would be how many of grains of sand we need to represent all these stars. If we take 32 of these grains out at a time and place them in little packages using the pattern we used above, then repeat this, placing each package beside one another to form a bigger cube, we can show that we would need just over 7.8 m3 in order to fit all the grains, approximately the size of large gazebo.
So far, we have worked out how many grains of sand we would need to represent the number of stars in the Milky Way galaxy, but what about other galaxies? Well first of all we need to find out how many galaxies there are in the observable Universe. Recent estimates from 2016 using the Hubble space telescope put the number around 2 Trillion. That’s 2,000,000,000,000! If we say for sake of argument that each of these galaxies is somewhat similar in size to the Milky Way with a similar number of stars,we would estimate there to be 5×1023 stars. That’s 500,000,000,000,000,000,000,000 stars. Incredible!
To work out how big this would be with our grains of sand analogy, we can use the same method as we did when working out the volume occupied by the stars in the Milky Way. The number? About 1.56×1013 or 15625000000000 m3. It is actually easier to work with this in km3 and it works out to be 15625 km3. Remember, this is the volume that little tiny grains of sand would fill if one grain represented a single star, not even a planet! This is roughly about 2/3 the volume of Mount Everest, filled with sand. I must emphasise that all these numbers are by no means exact, however it certainly gives you an idea of what we are dealing with.
On a side note, given that our nearest star, Proxima Centauri, is about 9.4 trillion km from the sun and the sun is only about 1.4 million km in diameter, a rough estimate would mean that instead of being compacted tightly together, each grain of sand should be separated from one another by about 6.7 metres. (However, this estimate is very loose because some stars are much closer to one another whilst others are much further away – but it gives you an idea.)
So that should show you just how immense the observable Universe is and just how many places we have to look. Despite this, not everywhere can harbour life. It is estimated that every star has at least 1 planet orbiting it. One of the jobs of the phenomenal Kepler space telescope alongside ESA’s (European Space Agency) Gaia and CHEOPS spacecraft, as well as NASA’s TESS spacecraft, is to search for “exoplanets”, which are planets not in our solar system. At the time of writing, Kepler and Gaia alongside some ground-based telescopes have found 4,380 exoplanets. Of these 4,380, about 350 of them are in the so-called “habitable zone”. This is the region around a star that allows conditions on the planet to be at levels that we think life could survive. As a percentage, that is not too bad at just under 1 in 10 exoplanets being discovered lying in the habitable zone of its star.
We’re finally getting close to answering our question. With around 10% of all exoplanets discovered lying in the habitable zone, it would be reasonable to assume that perhaps 10% of all planets orbit around a star in this zone. This gives us a measly 50,000,000,000,000,000,000,000 planets to search.
So, you may be wondering then, are we searching and if so, when could we find life? Well, we are searching and despite Kepler’s fuel depleting in 2018 (as planned), Gaia continues to be operational with the shiny new James Webb Space Telescope to be launched later this year. ESA’s future Plato and Ariel space telescopes are planned for launch in 2026 and 2029 respectively. With brand new instrumentation and increased resolution, the searching power will be greater than ever.
As for when will we find life? That question is much harder to answer. It’s actually impossible to answerright now. It could be weeks, months or years… but the delightful thing about being the curious specieswe are is that with every telescope we send up to space or with every new scientist that joins thesearch for life, our chances get better every day. If we take our simple back of the envelopeapproximation, only 0.00000000000000000876% of all exoplanets we estimate to be out there havebeen discovered. All we need is to find just that single planet other than earth that has life on it.
I’d like to end with one of my favourite quotes from the great Arthur C. Clarke.
“Two possibilities exist: either we are alone in the Universe or we are not. Both are equally terrifying.”
What do you think?