Earth could retain oceans and comfortable surface temperatures even if life never appeared, according to a new computer simulation spanning five billion years. The modeled planet changed relatively little without biology and remained comfortably habitable. Its biggest difference was an atmosphere without oxygen.
That result draws a useful line for astronomers searching for life elsewhere: a planet with conditions suitable for life isn’t necessarily a planet where anything lives. The study, published in The Planetary Science Journal and reported by ScienceAlert, could help distinguish atmospheric signatures that arise without biology from those that can only be explained by its presence.
Earth remains the only planet in the universe currently known to harbor life. To understand what other habitable worlds might look like, the researchers modeled an alternative version of our own: one where life never developed. A central question was whether Earth could maintain habitable conditions without organisms ever intervening.
The team built a computer model of Earth’s geochemical systems, incorporating 19 non-biological properties of a rocky planet. These included the properties of its core, the movement of its crust, and its oceans, along with how those factors affect the exchange of chemicals from the core to the surface and atmosphere.
Across the simulated five billion years, the lifeless Earth’s oceans retained a similar amount of water, while surface temperatures stayed well within a comfortable range. In the model, the planet’s non-biological systems were enough to sustain those conditions without help from life.
The missing atmospheric oxygen was less surprising. Around three billion years ago, cyanobacteria, also known as blue-green algae, helped fill Earth’s atmosphere with oxygen. An oxygen-rich atmosphere may therefore be an example of a feature that requires life to emerge. For astronomical assessments of habitability, though, oxygen is a secondary concern compared with the presence of liquid water.
“When we interpret observations of an exoplanet’s atmosphere, this tool will help us determine the probability that what we observe indicates life on the surface,” said lead author Samantha Gilbert-Janizek, an astronomer at UC Riverside, in a statement about the work.
The distinction is between identifying a place where life could survive and finding evidence that life actually exists there. This simulation could give astronomers a lifeless baseline against which to test the atmospheric clues they observe on other worlds.
