Researchers report that dry, desert-like exoplanets are unlikely to support long-term surface water and thus are poor candidates for life. The study notes that, although many planets lie in the habitable zone where temperatures could allow liquid water, water alone does not guarantee habitability. Using adapted models and complex simulations, the team examined how reduced surface water affects the geologic carbon cycle on Earth-sized worlds.
The results indicate an Earth-sized planet needs at least 20 to 50% of the water in Earth’s oceans to maintain the carbon cycle that regulates climate. In this long-term process, carbon dioxide from volcanoes accumulates in the atmosphere, returns to the surface dissolved in rain, causes chemical weathering of rocks, sinks into the ocean and is carried into the deep interior by plate tectonics, and later returns via mountain building and volcanism. If rainfall is too limited, weathering cannot remove CO2 fast enough, atmospheric CO2 rises, heat is trapped, and remaining surface water can evaporate in a runaway warming.
The authors point to Venus as an example of a world that may once have had water but now has extreme temperatures and pressures. Upcoming missions to Venus could help test these ideas and validate the models. The study was published in Planetary Science Journal and was funded by the National Science Foundation, the NASA Astrobiology Program, and the Alfred P. Sloan Foundation.
Difficult words
- exoplanet — planet that orbits a star beyond the Solar Systemexoplanets
- habitable zone — region around a star where liquid water is possible
- carbon cycle — process moving carbon between atmosphere, rocks and oceansgeologic carbon cycle
- weathering — breakdown of rock by chemical action of water
- plate tectonics — movement of large plates of a planet's crust
- runaway warming — rapid uncontrollable increase in global surface temperature
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Discussion questions
- How would the requirement of 20–50% of Earth's ocean water change the priorities for searching for habitable exoplanets? Give reasons.
- What kinds of evidence could upcoming missions to Venus look for to test the idea that it once had water?
- How important do you think plate tectonics and the carbon cycle are for long-term climate stability on a planet? Explain with examples.
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