We live on a spaceship that travels around its host star at a speed of 30km per second. That star on the other hand, in fact, our whole solar system, orbits around the centre of our galaxy the Milky Way at a speed of 828, 000 km per hour. The Milky Way also travels through space with 2.1 million km per hour (or 1.3 million miles per hour). One of my favourite quotes about our pale blue dot comes from the great Carl Sagan and it goes like this:
Look again at that dot. That’s here. That’s home. That’s us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives. The aggregate of our joy and suffering, thousands of confident religions, ideologies, and economic doctrines, every hunter and forager, every hero and coward, every creator and destroyer of civilization, every king and peasant, every young couple in love, every mother and father, hopeful child, inventor and explorer, every teacher of morals, every corrupt politician, every “superstar,” every “supreme leader,” every saint and sinner in the history of our species lived there-on a mote of dust suspended in a sunbeam.
Astronomers estimate that there are 100 thousand million stars in the Milky Way alone. In the observable Universe, scientists believe that there are 200 billion galaxies. This means that the number of stars in the Universe is in the ballpark figure of a billion trillion. But if the Universe is so big, where is everybody? Many of the stars in space will host at least one planet, but would the conditions be just right for life to emerge and develop? It would be a cliche if I said that we are extremely lucky to be alive, to even exist and be able to ask the big questions. So let’s put things into perspective and look at what’s needed for life to appear, to flourish and develop to what we know and call home today.
What makes a planet habitable?
- For a planet to be habitable and support life, it needs to a star. The planet also needs to have a consistent orbit around its host star for billions of years so that the conditions remain stable for life to emerge.
- The planet also needs to be in the goldilocks zone, not too close to the star and not too far away. Planets in this zone are neither too hot nor too cold, conditions that could support liquid water.
- The planet also needs to have a protective layer that stops radiation killing off every little living thing. We call this atmosphere. But it can’t be just any atmosphere, it needs to be the right one. Earth’s atmosphere consists mainly of nitrogen and oxygen. We call this air. The atmosphere protects us from the radiation from our Sun and also helps reduce temperature extremes between day and night. For this perfect balance to exist, the planet needs to be the right size. If our planet was the size of Jupiter or Saturn it might also have retained hydrogen and helium, by far the most abundant elements in the Universe. If our planet was a similar size, similar to Mars for example, the gravity wouldn’t be able to keep the nitrogen and oxygen and only the heaviest gasses might be retained. Both these scenarios are not very favourable to us humans.
- For life to develop on our planet in question, the solar system must go uninterrupted for a very long time. This means steering clear from any collisions with other planets, stars, or asteroids that can potentially kill everything on the planet.
- We have our basics so what’s next? We need an energy source or at least enough light for organisms to support photosynthesis. Photosynthesis is responsible for every lungful of air we breathe and without it, the evolution on Earth would have followed a very different path.
- Although not compulsory, having big gas giants like Jupiter and Saturn as part of the solar system will help. Some astronomers believe that one of the reasons is habitable is because Jupiter’s gravitational pull protects us from comets and asteroids entering the solar our solar system.
Where is everybody?
This recipe for life may not seem like a lot but so far scientists have not been able to detect other intelligent life forms in our galaxy the Milky Way. Enter the Fermi Paradox. The famous Fermi Paradox goes like this – if there are millions of planets in our Universe that can support life, and millions of intelligent species out there, why haven’t they come to visit Earth?
This question leads to different conclusions. There might not be any other intelligent civilisations out there or they might have existed but a catastrophic event wiped them out before we could make contact. Another reason as to why we haven’t seen any signs of life beyond Earth is that we could be the first and in fact, life emerges and develops to our level at a chance, pure luck if you will. What if we, in fact, are completely alone in the vast cosmos that surrounds us? What if there are other intelligent beings out there and they have visited us but before life on Earth began? Or maybe the idea of colonisation is so barbaric to an advanced species that they wouldn’t have thought to visit us?
We don’t yet have answers to these and many other questions but contemplating exactly these ideas is what makes life so very special. Maybe we are completely alone or maybe we are travelling through the Universe completely unaware of what’s around the corner.