It is logical, almost obvious, to assume that the Earth is the habitable planet par excellence: at the perfect distance from its star (not too hot, not too cold), in a stable orbit, with liquid water, an atmosphere and a favorable chemistry… Everything comes together so that we take it as a model to look for other habitable planets, but… What does it really mean for a planet to be habitable? Is it enough to have liquid water and an atmosphere? Or is life itself integral to what makes a planet function as a stable system? In an unexpected twist it would be as if life made life possible. At least one that is increasingly diverse and evolved.
A team of scientists has boldly attempted to answer these questions using global models similar to those that NASA and other agencies use to understand Earth’s climate. The results have been published in arXivand the premise was very basic: evaluate what would happen if all life disappeared from Earth.
Before going into the details of the simulation, it is important to confirm that Life on Earth is an active part of the planetary system. Photosynthetic organisms (plants, algae, cyanobacteria) not only produce oxygen, but also absorb carbon dioxide (CO₂), modifying the composition of the atmosphere. Soil and ocean microbes participate in nitrogen, phosphorus, and sulfur cycles, which in turn influence soil fertility, ocean chemistry, and cloud formation.
Terrestrial and oceanic life modifies the albedo (the percentage of radiation reflected by the planet) and the distribution of humidity on the planet. In short, life not only inhabits the Earth: it transforms it and keeps it in balance. To imagine a lifeless Earth, the authors, led by Samantha Gilbert-Janizek, started from global climate models used to study both the current climate and future scenarios (in the style of those used to understand global warming). Instead of including actual biological processes (such as photosynthesis or microbial respiration), The model removed these effects and left only abiotic physical and chemical processes: exchange of solar radiation, atmospheric and oceanographic circulation, geological cycles, etc.
The surprising thing was to see how much the planet would change in the absence of life. The first detail resulted in a huge change in our atmosphere. Without life, oxygen levels would gradually disappear, because there would be no photosynthesis to regenerate it. Over time (geologically speaking) the remaining oxygen would combine with other elements and be lost from the atmosphere.
While this is happening, The levels of carbon dioxide (CO₂) and other greenhouse gases would change sharply because there would no longer be terrestrial or oceanic organisms to absorb and recycle these gases. This implies that the mechanisms that keep the average temperature of the planet relatively stable would be profoundly altered.
Without organisms that regulate albedo and surface metabolism (such as plants or phytoplankton), The simulation predicts that Earth’s average temperature would be more extreme: colder polar zones, hotter equatorial zones, and a different climate dynamic than the one we know today. Then come the oceans, which also depend on biological processes to maintain their chemistry. Marine organisms play a critical role in the carbon cycle, as phytoplankton remove CO₂ and transport it to the deep ocean.
In the absence of life, this cycle would stop, which would not only modify the levels of atmospheric CO₂, but also the acidity and chemical composition of the water, with effects on the density and global circulation of the oceans. But there is more. Even seemingly simple phenomena such as cloud formation or humidity distribution would be affected. Today, plants and oceans release water vapor into the atmosphere through processes such as evapotranspiration or ocean surface mixing. Without them, the availability of water vapor would be different, which would alter rainfall patterns and sequences of climate seasons on a global scale.
Finally comes the big question, without biology, would the Earth be habitable? Gilbert-Janizek’s team concludes that many of the mechanisms that we see as “balanced” or “stable” depend directly or indirectly on biological processes. Habitability, in this context, ceases to be a merely physical concept (water, atmosphere, distance from the Sun) and becomes an eco-bio-physical phenomenon, that is, something that cannot be separated from the presence and activity of life. Without living beings that regulate gases, recycle nutrients, influence the water cycle and the chemistry of the oceans and atmosphere, the Earth would be a very different world: more extreme, less self-regulated and with a chemistry less conducive to the spontaneous emergence of life (in the hypothetical case of re-emergence).
This simulation is much more than a curiosity or an imaginative exercise. NASA’s Habitable Worlds Observatory (HWO), currently under development, will be the first telescope capable of directly imaging rocky planets orbiting Sun-like stars. Its goal is to decipher atmospheres for signs of life. But, for the measurements to be reliable, Scientists need to know exactly what a habitable but lifeless planet looks like and be able to distinguish it from a truly inhabited one.