2026-04-15 ワシントン大学（UW）
This image of Venus taken by NASA’s Mariner 10 spacecraft (left) is paired with an artist’s depiction of three possible atmospheres on a recently discovered exoplanet, Gliese 12b. This new University of Washington study explores how much surface water a planet needs to support life. Photo: NASA/JPL-Caltech/R. Hurt (Caltech-IPAC)
＜関連情報＞
- https://www.washington.edu/news/2026/04/15/planets-need-more-water-to-support-life/
- https://iopscience.iop.org/article/10.3847/PSJ/ae4faa
乾燥した地球型惑星における炭素循環の不均衡と金星への影響 Carbon Cycle Imbalances on Arid Terrestrial Planets with Implications for Venus
Haskelle T. White-Gianella and Joshua Krissansen-Totton
The Planetary Science Journal Published: 2026 April 15
DOI:10.3847/PSJ/ae4faa
Abstract
Arid terrestrial exoplanets are potentially abundant and are thus interesting targets in the search for life. In particular, M-dwarf planets such as those in the TRAPPIST-1 system may possess limited surface water, whereas early solar system terrestrials may have had small surface water inventories postmagma ocean solidification. On modern Earth, there is enough surface water for a balanced geologic carbon cycle, meaning silicate weathering balances the volcanic outgassing of CO2. However, on arid planets, there may not be enough surface water for this silicate weathering thermostat to maintain habitable conditions. Here, we show that arid planets enter a regime where weathering cannot keep up with volcanic degassing of CO2. Using a coupled model of the geologic carbon cycle, we find that terrestrial Earth-like planets require an initial surface water inventory of at least ∼20%–50% of Earth’s ocean mass to maintain a balanced geologic carbon cycle and temperate surface temperature over 4.5 Gyr of evolution. Arid planets with less than ∼20%–50% of Earth’s oceans cannot maintain high silicate weathering fluxes, potentially causing a runaway increase in atmospheric CO2. In addition, we explore Venus-like instellations and find that limited surface water could have destabilized Venus’s carbon cycle, triggering a transition from temperate to uninhabitable. Even if a planet resides in the habitable zone of its star, if arid, it may transition to an uninhabitable state due to an unbalanced carbon cycle. More broadly, arid terrestrial exoplanets are less likely to remain habitable on long timescales, and may thus be poor candidates for biosignature searches.
