Whether Tatooine, Arrakis or Mars: In science fiction, there is usually extraterrestrial life even on desert planets. But how realistic are these scenarios? Planetary researchers have now examined this in more detail using models. The result: Even if planets orbit in the habitable zone of their star, they require a minimum amount of water to initially be present. This is around 20 to 50 percent of the amount of water in the earth’s oceans. If a planet has less water, this not only affects its water cycle, but also its carbon cycle. As a result, important climate buffers are missing and a runaway greenhouse climate develops, which ultimately causes all bodies of water and oceans to evaporate, as the team explains.
For a long time, the earth’s deserts were thought to be hostile, dead regions. But it is now clear: there are countless creatures that have adapted to the barren environment and the lack of water. This suggests that life could also exist on extrasolar desert planets in the habitable zones of their stars. At least in science fiction, extraterrestrial life forms are common on such exoplanets – whether the sandworms from “Dune”, Martian life forms or sand dwellers on Star Wars desert planets.
How water and climate are related
But what does it look like in reality? Could exoplanets in the habitable zone support life even if they lack large oceans and extensive bodies of water? “Many of these exoplanets are in the habitable zone, but we weren’t sure whether they could really be life-friendly,” says Haskelle White-Gianella from the University of Washington in Seattle. The reason for this is the close connection between the water cycle and water availability and important feedbacks in the climate system. The climate of a planet is also closely related to its hydrosphere. On Earth, for example, the weathering of silicate rocks and the formation of carbonates in the oceans act as a climate buffer. But this only works so well because the CO2 in the atmosphere can dissolve in the water that rises through evaporation and then returns to the earth’s surface as carbonic acid with the rain. The CO2-binding reactions then take place in the sea and on rocky surfaces. They compensate for the outgassing of CO2 by volcanoes, for example, and thus ensure a long-term stable climate.
However, on a desert planet with little surface water, these geological feedbacks only work to a limited extent. “The lack of water limits continental silicate weathering – and that in turn endangers the balance to volcanic outgassing of CO2,” explain White-Gianella and her colleague Joshua Krissansen-Totton. If not enough CO2 is bound, a greenhouse climate will develop that will continue to heat up the planet. Venus demonstrates what this can lead to. In its early days it could have been as life-friendly as Earth and probably also initially had a mild climate and liquid water. But as the sun grew stronger, Venus heated up and developed a runaway greenhouse effect, turning it into a hellishly hot desert planet.
20 to 50 percent of the earth’s quantity is the minimum
But what does this mean for extrasolar planets? White-Gianella and Krissansen-Totton have now used models to investigate how much initial water a planet in the habitable zone would need to avoid this fate. This showed that if a planet has as much water on its surface as the Earth or at least 20 to 50 percent of it, the climate feedbacks in the carbon cycle work: silicate weathering acts as a buffer and keeps the atmospheric CO2 content relatively stable in the long term. Thanks to these feedbacks, such a planet can even compensate for a slight increase in solar radiation, as the team explains.
But things are different for desert planets with only 0.1 to one percent of Earth’s ocean water: “They enter a regime in which silicate weathering cannot keep pace with CO2 outgassing,” the researchers explain. This creates a vicious circle: Rising CO2 levels heat up the climate, causing more and more water to evaporate from soil and water. The water vapor accumulates in the atmosphere and thereby increases the greenhouse effect. “At some point a critical threshold is reached, after which all surface water is present as vapor, precipitation stops and CO2 can no longer be bound,” write White-Gianella and Krissansen-Totton.
According to the researchers, desert planets are therefore not good candidates for extraterrestrial life – even if they orbit in the habitable zone. Because their climate will most likely not remain stable long enough to allow the evolution of life. These findings can help to further narrow down the search for extraterrestrial life – to only those exoplanets that have not only a mild climate but also enough water on their surface.
Source: Haskelle White-Gianella and Joshua Krissansen-Totton (University of Washington, Seattle), The Planetary Science Journal, doi: 10.3847/PSJ/ae4faa