The latest research from the University of Washington in the United States points out that many exoplanets previously regarded as "habitable candidates", even if they are located in the habitable zone of their stars and have surface temperatures suitable for the existence of liquid water, are still very likely to be completely unsuitable for life if they are too arid.
The research team found that for a rocky planet similar in size to the Earth, in order to maintain a stable and habitable surface environment over a long geological time scale, its surface water volume needs to reach at least about 20% to 50% of the total volume of the Earth's oceans. This means that a large number of so-called "desert planets" - even if their orbits are in the "just right" position - are likely to be far from suitable for supporting life in terms of water resources.
To date, astronomers have confirmed more than 6,000 exoplanets, and billions of similar objects are expected to exist throughout the Milky Way. A significant portion of it falls within the star's habitable zone, where temperatures theoretically allow liquid water to exist. However, the University of Washington team emphasizes that being "in the right place" is only part of the equation; the planet still needs to have a long-term stable climate regulation mechanism, and this largely depends on how water interacts with the lithosphere and atmosphere.
Haskell White-Giannella, the first author of the paper and a doctoral student in earth and space sciences, said that when searching for life in the vast universe and limited observation resources, we must learn to "screen out" some planetary targets in a targeted manner. This study is focusing on arid planets with extremely low surface water reserves, far less than an entire Earth's ocean, to assess whether they may actually be habitable.
The research results were published in the Planetary Science Journal. The core lies in the key process of planetary geological carbon cycle. On Earth, this water-driven cycle moves carbon between the atmosphere and the planet's interior over millions of years, helping to regulate global surface temperatures.
On Earth, volcanoes release carbon dioxide into the atmosphere, which then dissolves in rainwater. Rainwater reacts chemically with surface rocks, and rivers carry carbonaceous materials into the ocean, where they are deposited on the seafloor. Along with plate tectonic movements, carbon-rich ocean crust was subducted beneath continents, and during processes such as mountain building, carbon was brought back to the surface over a long period of time.
However, if a planet lacks enough water to sustain steady and widespread rainfall, this carbon cycle "thermostat" breaks down. As precipitation and weathering weaken, the efficiency at which carbon dioxide is "pulled" out of the atmosphere decreases significantly, while volcanic release continues. The result is that carbon dioxide in the atmosphere continues to accumulate, the greenhouse effect is enhanced, the temperature further rises, and the remaining water evaporates at an accelerated rate, ultimately forming a vicious cycle that makes the planet's surface too hot and uninhabitable.
White-Giannella pointed out that this means that even dry Earth-like planets located in the habitable zone are most likely not ideal targets for searching for life. The study also reminds that in previous theoretical work, the carbon cycle mechanism on arid planets has been relatively lacking in systematic examination, which may make people overly optimistic about the habitable potential of "desert exoplanets".
Since direct observation of rocky exoplanets is still extremely difficult, scientists often rely on numerical simulations to explore their long-term climate evolution and water cycle characteristics. In this work, the research team improved the existing carbon cycle model, recharacterized key processes such as evaporation and precipitation especially for arid environments, and introduced factors that were often ignored in the past, such as the impact of wind fields on water vapor distribution and evaporation efficiency.
Joshua Krissanson-Totten, co-author of the paper and assistant professor in the Department of Earth and Space Sciences at the University of Washington, said that this type of refined "mechanism-based" carbon cycle model was originally used to understand the climate evolution and temperature regulation of the Earth in its long geological history, and is now being extended to the study of exoplanets. The new results show that even if an arid planet has a certain amount of surface water in the early stages, it will have a high probability of losing water due to an imbalance in the carbon cycle in the later stages, evolving from a potentially habitable world to a hot and uninhabitable "imbalanced planet."
The research also turned its attention to a nearby "natural experiment": Venus. Venus is similar in size to Earth and formed around the same time, and some models even suggest that it may have had as much water as Earth in its early days. However, today the surface temperature of Venus is comparable to that of a wood-fired pizza oven, and the surface pressure is so high that "it feels like ten blue whales are pressing on it at the same time."
The scientific community has long debated why Earth and Venus took such different evolutionary paths. White-Giannella and Crisanson-Totten proposed that Venus may have triggered a carbon cycle imbalance and a runaway greenhouse process early because it was closer to the sun and had a slightly lower initial amount of water. As carbon dioxide continues to accumulate in the atmosphere and the temperature gradually rises, a large amount of water is eventually lost, and life, if it once existed, loses its habitat.
In the coming years, multiple upcoming missions to Venus are expected to answer this "sister planet mystery" and test key inferences of the carbon cycle model mentioned above. White-Giannella believes that although it is almost impossible for humans to land on the surface of any real exoplanet in the foreseeable time, Venus - "the closest analogue of Earth-like exoplanets" - provides a unique window.
The research team expects that the data from these missions will help verify the theoretical framework of carbon cycle imbalance on arid planets, and be used to interpret the atmospheric characteristics and evolutionary states of distant exoplanets. Krissanson-Totten noted that this research has important implications for how we assess the "real inventory" of potentially habitable planets in the universe. Many targets that were once roughly classified as "habitable candidates" are likely to be reclassified under more stringent water content and carbon cycle criteria.