Unfortunately for science fiction fans, desert worlds outside our solar system are unlikely to host life, according to new research from University of Washington. Scientists show that an Earth-sized planet needs at least 20 to 50% of the water in Earth's oceans to maintain a critical natural cycle that keeps water on the surface.
Scientists believe that there are billions of planets outside our solar system. More than 6,000 of these exoplanets are confirmed, but only some of them are candidates for life. The search for life has focused on planets in the "habitable zone," a sweet spot that is neither too close nor too far from a central star. Planets in this zone are considered viable because they can maintain liquid surface water.
"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.
Water, although essential, does not guarantee the existence of life. With this study, researchers worked to further narrow the search by investigating planets with just a small amount of water.
"We were interested in arid planets with very limited surface water inventory - far less than one Earth ocean. Many of these planets are in the habitable zone of their star, but we weren't sure if they could actually be habitable," White-Gianella said.
The team's results, published April 13 in Planetary Science Journal, show that habitability hinges on the geologic carbon cycle - a water-driven process that exchanges carbon between the atmosphere and interior over millions of years, stabilizing surface temperatures.
Carbon dioxide, which comes from volcanoes in a natural system, accumulates in the atmosphere before falling back to Earth dissolved in rainwater. Rain erodes and chemically reacts with rocks on the Earth's surface and runoff transports carbon to the ocean, where it sinks to the seafloor. Plate tectonics drives carbon-rich oceanic plates below continental land. Millions of years later, carbon resurfaces as mountains form.
If water levels drop too low for rainfall, carbon removal - from weathering - can't keep up with emissions from volcanic eruptions and carbon dioxide levels in the atmosphere spike, trapping water. Rising temperatures evaporate the remaining surface water, initiating runaway warming that makes the planet too hot to support life.
"So that unfortunately makes these arid planets within habitable zones unlikely to be good candidates for life," White-Gianella said.
Although scientists have instruments that can measure surface water, rocky exoplanets are difficult to observe directly. In this study, the researchers ran a series of complex simulations to better understand how water might behave in these desert worlds.
Previous efforts to model the carbon cycle focused on cooler, perhaps wetter planets. The models factored in evaporation from sunlight, but didn't 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 Krissanen-Totton, a UW assistant professor of Earth and space sciences.
However, the function of the geologic carbon cycle on arid planets was largely unexplored. The results show that even planets that form with surface water could lose it, transitioning from potentially habitable to uninhabitable due to carbon cycle disruption.
One such planet exists far closer to home: Venus. The planet of love is roughly the same size as Earth, likely formed around the same time and may have started with a similar amount of water.
Yet today, the surface of Venus rivals the temperature of a wood-fired pizza oven. Standing on the surface would feel like being crushed by 10 blue whales, White-Gianella said.
Many theories attempt to explain why Earth and Venus are so different. White-Gianella and Krissanen-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.
The researchers hope that results from future missions will help validate the results of their modeling.
"This has implications for a lot of the potentially habitable real estate out there," Krissanen-Totton said.
This study was funded by the National Science Foundation, the NASA Astrobiology Program and the Alfred P. Sloan Foundation.
