Simulated Lifeless Earth Stayed Habitable for Billions of Years, Challenging Gaia Hypothesis Assumptions

A team of researchers has run the clock on an Earth that never had life, and the planet they built in the computer stayed livable for billions of years. The work, published in The Planetary Science Journal, was led by astrobiologist Samantha Gilbert-Janizek at the University of Washington. It is a thought experiment, but also a direct test of ideas behind the Gaia hypothesis.

That hypothesis holds that life interacts with Earth's physical environment in ways that help keep the world habitable, a kind of planetary homeostasis. The researchers framed their question plainly: whether an Earth-like rocky planet could stay habitable over billion-year timescales without life's influence, or whether life is required to maintain that long-term habitability.

To get at it, Gilbert-Janizek and colleagues built a computer model of Earth with everything except biology. It included the core, mantle, crust, atmosphere, oceans, and the flux of energy arriving from the Sun, and it calculated how chemicals cycle between those parts of the planet. In total the model tracked 19 key planetary properties, among them heat flow, ocean chemistry, and surface temperature. The simulation ran for 5 billion years, starting around the time Earth formed and extending to roughly 500 million years in the future.

The result was that the absence of life made less difference than might be expected. The most striking exception was oxygen: with no photosynthesis, the lifeless Earth had no abundant atmospheric oxygen. That matters for complex life, but astronomers usually define habitability by a planet's ability to hold liquid water on its surface, and on that measure the abiotic Earth did well, showing water levels similar to those of the real planet.

The team attributes the stability to the carbonate-silicate cycle, which is driven by the interplay of silicate weathering, volcanic outgassing, and ocean chemistry. In their words, it supplies a robust negative feedback that keeps atmospheric carbon dioxide and surface temperature within habitable bounds, even as the Sun's luminosity rises by about 30 percent over 4.5 billion years. They conclude that an Earth-like planet can keep habitable surface temperatures over that span, and longer, through abiotic processes alone.

The findings do not disprove the Gaia hypothesis. The researchers say they cannot rule out that life on Earth contributes to processes that enhance climate stability. They also acknowledge limitations and simplifications in the model, including assumptions about plate tectonics, atmospheric chemistry, and the processes that regulate climate. Still, they write that their results suggest life and habitability may be somewhat decoupled, and that a lifeless Earth could reach a state similar to today's in global geophysics and climate, minus the oxygen.

There is a practical payoff beyond the philosophical one. Gilbert-Janizek says the model can show how habitable but non-living planets might mimic living ones, and that it should help estimate the probability that an exoplanet's atmospheric observations point to life on its surface. The next step is testing the model's simplifying assumptions, since the usefulness of that tool depends on them.