More than 6,000 planets have been found beyond our solar system, and University of Washington researchers have built a virtual Earth with one striking feature: it has no life.
The model gives scientists a way to ask a difficult question as they study distant worlds: could a planet’s atmosphere form without living things? A planet may have conditions that could support life, such as liquid water, without actually being inhabited. To spot possible signs of life, researchers first need to understand what a lifeless planet can do on its own.
In a paper published Sept. 18 in The Planetary Science Journal, lead author Samantha Gilbert-Janizek and her colleagues simulated Earth’s geochemical systems across five billion years. The run begins with Earth’s formation about 4.5 billion years ago and continues roughly 500 million years into the future. It tracks the movement of chemicals among the planet’s core, mantle, crust, surface and atmosphere, while leaving out life’s effects.
The model reproduced 19 properties of Earth, including heat flowing through the mantle. That matters because a planet’s air is shaped not only by biology, but also by its interior and the energy it receives from its star. Earlier versions of the model, developed in the lab of senior author Rory Barnes, a University of Washington associate professor of astronomy, included the core, mantle and crust, along with stellar radiation. Gilbert-Janizek added the surface and atmosphere to connect the full system.
The distinction could help astronomers interpret atmospheric observations. On Earth, plants and animals continually release oxygen and methane. The gases react with each other and are destroyed, so their continued presence together can suggest that something is replenishing them. But scientists need to know whether nonliving processes could create a similar signal elsewhere before treating it as evidence of life.
Researchers can adjust the model using basic information about a rocky exoplanet and see whether it can reproduce that planet’s observed atmosphere without life. If it can, the atmosphere may have a nonbiological explanation. If it cannot, the world becomes more compelling for closer study. The model is designed to be adapted to other rocky planets, not just Earth.
Gilbert-Janizek, who did the work as a University of Washington doctoral student and is now a postdoctoral fellow at the University of California, Riverside, said the model can show how “habitable but nonliving planets might mimic living planets.” Barnes said the completed simulation reproduces properties of Earth’s core, mantle, surface and atmosphere, building on more than a decade of work by many scientists.
The work may also help guide future telescopes designed to study distant planets, including the proposed Habitable Worlds Observatory. Those observations are still ahead, but a tested baseline gives scientists a clearer way to decide which atmospheric clues deserve attention. Each planet that cannot be explained without life would sharpen the search for an answer to a question that has followed humanity for generations: whether we are alone in the universe.