It's a dizzyingly busy time for scientists searching for life on distant planets. To date, astronomers have cataloged more than 6,000 planets beyond our solar system and are searching the atmospheres of the most promising worlds for chemicals that might be byproducts of life. Astrobiologists are particularly interested in atmospheres like Earth's that trap enough heat to support liquid water on the planet's surface, which scientists consider the key factor in determining whether a planet can support life as we know it.
But scientists also want to distinguish between habitable and inhabited worlds. To do that, they need to know what sorts of atmospheric conditions on a planet can be explained without life, so that they can identify planets with atmospheres that can only be explained by life's influence.
To provide that planetary baseline, University of Washington researchers built a complex virtual Earth with one crucial tweak: no life.
In a paper published Sept. 18 in The Planetary Science Journal, the researchers simulated the Earth's geochemical systems for 5 billion years - from its formation 4.5 billion years ago through roughly 500 million years in the future. The simulation modeled the exchange of chemicals between the core, mantle, crust, surface and atmosphere over time. It accurately predicted 19 planetary properties like heat flow through the mantle, but left out the influence of life on those systems. The simulation can be generalized to predict how an atmosphere might evolve on any rocky planet, and can help pinpoint evidence of life in atmospheres that don't fit the model.
"This model can help us see how habitable but non-living planets might mimic living planets," said lead author Samantha Gilbert-Janizek, who completed this research as a UW doctoral student of astronomy and is now a postdoctoral fellow at UC Riverside. "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."
Gilbert-Janizek's work builds on a model developed in the lab of senior author Rory Barnes, a UW associate professor of astronomy. Earlier versions of the model simulated the core, mantle, crust and stellar radiation, but did not include the surface or atmosphere.
A planet's atmosphere is shaped by many nonbiological factors, including heat and gases from the interior and from the planet's star. But the atmosphere of a distant planet is also where astronomers hope to spot explicit evidence of life.
There are many ways that life might change an atmosphere: On Earth, plants and animals pump steady streams of oxygen and methane into the sky. Those two elements destroy each other when they mingle. Thus, an alien astronomer studying our planet might suspect that something alive is replenishing the supply of both gases.
That logic led human astronomer Gilbert-Janizek to add surface and atmosphere modeling to the existing Earth simulation, completing the picture of how chemicals move through all of the Earth's systems.
The result is a model that can be tuned to study any rocky exoplanet that astrobiologists find interesting. Researchers can plug in basic details about that planet and run the simulation to see whether the model can reproduce the observed atmosphere without the influence of life. If they can't get a match, the planet in question gets even more interesting.
"Dr. Gilbert-Janizek's work represents an exciting step toward understanding where life can exist in the universe," Barnes said. "After more than a decade of work by many scientists, Dr. Gilbert-Janizek has finally completed a model of the Earth that simultaneously reproduces properties of Earth's core, mantle, surface and atmosphere. That calibration gives us confidence that future simulations of exoplanets will be accurate."
Gilbert-Janizek says the new and improved model can also help inform the design of future telescopes built to study distant planets, like the proposed Habitable Worlds Observatory.
"Realizing the ambitious goals of a telescope like the Habitable Worlds Observatory requires decades of work across many generations of scientists," Gilbert-Janizek said. "I'm so grateful to join this lineage and one day pass the torch to the next generation so that we can continue chipping away at this question of whether or not we are alone in the universe."
Other UW co-authors include former astronomy doctoral students Jessica Birky, now a postdoctoral researcher at the University of Oklahoma and Rodolfo Garcia, now a high school teacher in the Seattle area.
A complete list of co-authors is included with the study.
This research was funded by NASA.
