The rare-Earth hypothesis was proposed in 2000 by geologist Peter D. Ward and astronomer Donald Brownlee, and it is a shot at the waterline of the most optimistic astrobiology. It assumes that life is common in the universe, but only in its simplest forms , which have demonstrated their ability to survive in the most extreme environments. But we cannot say the same about animal life, which is much more sensitive than microbial life to environmental conditions. Moreover, the processes that have allowed the appearance of complex, animal life are not as common as we are led to believe .
These are their reasons:
1. Situation, situation, situation
For example, those in the vicinity of the galactic center are subjected to deadly high levels of radiation ; areas with a high density of stars are also prohibited by the threat of supernovae, which when exploding are capable of seriously affecting life on planets located less than 30 light-years away. This creates a barren sphere around the galactic center 10,000 light-years in radius. The outermost regions are also not appropriate because there are few elements such as iron, carbon, nitrogen, nickel, magnesium…, which are essential for life. The same thing happens in elliptical galaxies: life can only arise in spiral galaxies .
2. Not just any star
Evolution takes its time so the star must live long enough . That means it shouldn’t have a lot of mass because it burns its nuclear fuel very quickly and burns out sooner. As if that were not enough, they also produce a large amount of deadly ultraviolet radiation, sterilizing the planet since its inception. Nor can they be stars that have too low a mass because they emit a small amount of energy. If we take into account that 95% of the stars in the Galaxy have less mass than our Sun, to say that ours is a typical star is a bit of an exaggeration. The same is true of all solar-type stars with low amounts of heavy elements: current observations have found no planetary systems in stars containing less than 40% of our Sun.
3. The planet must be in the right place
Life needs water and therefore the planet must orbit at a distance from the star that makes its surface temperature allow the presence of liquid water: it is the Habitable Zone of a star . Now, as stars change brightness throughout their lives, the Habitable Zone changes over time. This imposes a new constraint: that the planet is throughout its entire history in the Continuously Habitable Zone . Calculations suggest that in our Solar System it is between 0.95 and 1.15 times the Earth-Sun distance. What planet is right there? Only ours. His situation does not obey any law; it’s a matter of fluke.
4. The importance of having a Jupiter
The orbital location of a Jupiter-like planet is a tricky question: if ours had been born a little closer or had a little more mass, its gravitational pull would have prevented the formation of the Earth . We might think that for that it is better not to have any gas giant nearby. Big mistake. The existence of our gas giant has been fundamental in “sweeping” dangerous asteroids and comets out of the Solar System : without Jupiter, the number of 10-kilometer objects -Doomsday asteroids- that would collide with Earth would be 10,000 times greater.
5. Size and geology matter too
The planet cannot be too small, as Mars is, because it would not exert enough gravitational pull to prevent the atmosphere from escaping into space . On the other hand, plate tectonics plays a fundamental role in the natural greenhouse effect caused by gases such as carbon dioxide, which means that the average temperature of our planet is 18º C and not -25º C. Now, the CO2 is removed from the atmosphere by reacting with rainwater to form carbonic acid, which ends up on the seabed. If that CO2 did not return to the atmosphere, the temperature would fall globally. Thanks to plate tectonics, CO2 returns to it through volcanoes, creating an average temperature that has allowed the development of animal life. On the other hand, the planet must be geologically active for it to have a strong enough magnetic field to protect life from cosmic rays.
6. The need for a big moon
Our planet is unique because it has an exceptionally large satellite. If it did not exist, the orientation of the Earth’s axis would not be stable and would experience chaotic variations over time: that we enjoy a regular succession of seasons for millions of years is thanks to the Moon . Without it, the climatological consequences of a chaotic variation of the axis of rotation would be catastrophic for any complex life. Is it rare for a planet to have a large Moon? Certainly yes. Ours has it because when it was in its infancy it was hit by an object larger than Mars and the Moon arose from the debris of that collision. Chance has again played in our favor.
7. A planet is a dangerous place
On 5 occasions life on Earth has been about to disappear. The most dangerous moment happened 250 million years ago, when more than 90% of marine species were lost . Nobody knows what happened, but the threat to life is always present. To this we must add the snowball Earth phenomena that our planet has gone through. It seems that the Earth has completely frozen over in four situations, with global temperatures of -50º C. The last one took place about 700 million years ago, before the famous Cambrian explosion , an unprecedented burst of biological diversity that gave rise to to all forms of life that we know today. To what extent does the existence of animal life depend critically on this peculiar and unique event? And is it something common or is it a special fact?
Referencia:
Ward, P. D. y y Brownlee, D. (2000) Rare Earth: Why Complex Life is Uncommon in the Universe, Copernicus Books
Gribbin, J. (2011). Alone in the Universe: Why our planet is unique, Wiley
Conway Morris, S. (2003) Life’s Solution: Inevitable Humans in a Lonely Universe, Cambridge University Press
