More than 6,000 exoplanets have been confirmed, but only some are candidates for life. The search has focused on planets in the habitable zone, a range of distances from a star where liquid surface water can exist. But water alone does not guarantee habitability.
"When you are searching for life in the broad landscape of the universe with limited resources, you have to filter out some planets," said lead author Haskelle White-Gianella, a UW doctoral student of Earth and space sciences.
The new study, published in the Planetary Science Journal, investigates planets with very limited surface water - far less than one Earth ocean - to determine whether such worlds could sustain life even when positioned in a star's habitable zone. The answer, the researchers found, hinges on the geologic carbon cycle.
The geologic carbon cycle is a water-driven process that exchanges carbon between the atmosphere and a planet's interior over millions of years, stabilizing surface temperatures. Carbon dioxide emitted by volcanoes accumulates in the atmosphere, then falls back dissolved in rainwater. Rain erodes rocks, and runoff transports carbon to the ocean, where it sinks to the seafloor and is driven below the crust by plate tectonics. Millions of years later, carbon resurfaces as mountains form.
If water levels fall too low for rainfall, carbon removal through weathering cannot keep pace with volcanic emissions. Carbon dioxide levels in the atmosphere spike, trapping heat, evaporating remaining surface water, and triggering runaway warming that renders the planet uninhabitable.
"So that unfortunately makes these arid planets within habitable zones unlikely to be good candidates for life," White-Gianella said.
Rocky exoplanets are difficult to observe directly, so the researchers ran a series of complex simulations to model how water behaves on desert worlds. Previous carbon cycle models focused on cooler, wetter planets and factored in evaporation from sunlight but did not include other drivers such as wind. White-Gianella adapted existing models to drier planets by refining evaporation and precipitation estimates.
"These sophisticated, mechanistic models of the carbon cycle have emerged from people trying to understand how Earth's thermostat has worked - or hasn't - to regulate temperature through time," said senior author Joshua Krissansen-Totton, a UW assistant professor of Earth and space sciences.
The results show that even planets that form with surface water could lose it over time, transitioning from potentially habitable to uninhabitable through carbon cycle disruption. One such example exists in our own solar system: Venus. Roughly the same size as Earth and likely formed around the same time with a similar water budget, Venus today has a surface temperature hot enough to melt lead and atmospheric pressure roughly equivalent to being crushed under 10 blue whales.
White-Gianella and Krissansen-Totton propose that Venus, being closer to the sun, may have formed with slightly less water than Earth, which imbalanced the geologic carbon cycle. As surface temperatures rose with atmospheric carbon dioxide levels, Venus lost its water - and any life it may have hosted.
Upcoming missions to Venus will attempt to understand what happened to the planet and whether it ever hosted life. The findings could also offer insight into planets much farther away.
"It's very unlikely that we will land something on the surface of an exoplanet in our lifetime, but Venus - our nextdoor neighbor - is arguably the best exoplanet analog," White-Gianella said.
"This has implications for a lot of the potentially habitable real estate out there," Krissansen-Totton added.
Research Report:Carbon Cycle Imbalances on Arid Terrestrial Planets with Implications for Venus
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