Life on Earth is plentiful and nearly all-encompassing to the point where it is hard to truly imagine what a ‘lifeless’ area would look like. Nevertheless, we have only to look away from the Earth to find such places. Our Moon and other planets in our solar system are all utterly devoid of life. Only Mars is thought to have had microbes at some point long ago, but no plants, animals, people, etc.
This stark contrast is daunting and has puzzled people for as long as we were aware of the fact that there was in fact no life on these planets. One potential answer for this problem is the so-called ‘Rare Earth theory‘, presented by paleontologist Peter Ward and astronomer Donald Brownlee in 2000.
The Rare Earth Theory
The duo combined their expertise in an attempt to find a reason for why Earth was so unique in our solar system. In a book titled ‘Rare Earth: Why Complex Life is Uncommon in the Universe’ (Copernicus Books, 2000) they explain how many seemingly small details about our planet help in making it habitable. Some obvious requirements are the planet that the planet has to be inside of the ‘Goldilocks Zone’, have a mass large enough to retain an atmosphere, a magnetic field to protect life from radiation, a small tilt for mild seasonal changes, a Moon that causes tides and stabilizes its axis, plate tectonics and much more.
Most of the other rocky planets in our solar system, and even some of the moons around Saturn and Jupiter, fulfill some of these requirements but never all. This could be the reason for why life does not form there. For example, Mars once had liquid water and even an atmosphere but was not heavy enough to keep them from evaporating into space. On the other hand, Europa has liquid water and warmth caused by the tidal forces of its host planet and is in fact the strongest candidate for extra-terrestrial life in our solar system.
The duo does not dispute that simple life can and does exist out there, simply because of how hardy and adaptable bacteria are on Earth. However, they do think that more complex multi-cellular life is much rarer due to the fact that it needs much more favorable and stable environments to flourish. They give Earth as an example, where we can see that complex life has only been possible for about 10-20% of its lifetime. The rest of the time the planet was either barren or only populated by microbes.
Not Everyone Convinced
On the other hand, it is important to mention that there are many detractors to this theory, especially to the specificity of it. If we go back to the case of Europa, it is clear that life would be possible there despite the distance from the Sun, lack of an atmosphere, lack of a magnetic field, and plate tectonics. Instead, the moon has a hard shell of ice that insulates the liquid water inside, as well as strong tidal forces that move its insides and warm it up. Life in such a place would not be like we know it and certainly not utilize chlorophylls to convert light into energy. Instead, it would likely be closer to the thermophilic life that can be found at the bottom of our oceans, huddled around thermal vents. On top of that, their hypothesis assumes that life as we know it is the only type of life that is possible, which is a bold claim in the eyes of many astronomers.
Regardless of who we side with, it is clear that more research is needed. Rovers on Mars will keep looking for signs of past life, and telescopes including the newly-launched James-Webb Space telescope will study the atmospheres of extra-solar planets in our galaxy for signs of (complex) life. All in all, it is important to remember that absence of evidence is not evidence of absence.