Recent findings suggest the potential number of planets capable of supporting alien life may be fewer than previously assumed, largely due to advances in understanding planetary climates. When carbon dioxide levels in an atmosphere surpass a critical threshold, conditions can become inhospitable.

Life as we know it requires liquid water, prompting astronomers to target “habitable zones” around stars—regions where temperatures allow for water to exist in liquid form. However, Haskelle White-Gianella from the University of Washington and her research team have revealed that having liquid water alone does not guarantee habitability.

The researchers conducted nearly 10,000 simulations to determine how CO₂ levels fluctuate based on surface water amounts on planets that are Earth-sized. Their results indicate that at least 20% of Earth’s total water must be present for a planet to be potentially habitable.

This is largely due to the role rainfall plays in carbon storage within the ground through chemical reactions in rocks; insufficient rainfall could lead to CO₂ accumulation in the atmosphere, trapping heat and rapidly increasing temperatures beyond 126°C (259°F).

“We discovered that there exists a water threshold essential for maintaining a stable climate,” White-Gianella stated during the Goldschmidt Geochemical Conference in Prague, Czech Republic, on July 10.

This indicates that simply being in a habitable zone does not guarantee that a planet can support life, according to White-Gianella, necessitating a deeper examination of geological histories.

A parallel situation may elucidate how Venus transformed into the inhospitable environment we observe today, White-Gianella shared at the conference. While the increase in the sun’s brightness since the solar system’s inception is believed to contribute to Venus’ atmospheric changes and temperature rise, it alone doesn’t account for all observed transformations. By re-running models with a Venus-like amount of starlight, the team found that even planets with water levels similar to Earth’s could lose too much CO₂, leading to uninhabitability.

This provides a compelling rationale for how planets similar to Venus can become excessively hot, as noted by Benjamin Tutoro from the University of Calgary in Canada. Over time, reductions in CO₂ emissions complicate planetary climates, as recorded in geological data from Mars.

In the case of Mars, liquid water attracted carbon dioxide and sequestered it as carbonate minerals, which ultimately thinned its atmosphere and cooled the planet, according to Tutoro. White-Gianella stated that the team’s simulations focused on Earth-like planets, agreeing that conditions on planets like Mars could differ significantly.