Study defines minimum planet size to sustain life and atmosphere

A new study developed by researchers at the University of California has established a fundamental criterion for the search for life beyond Earth: the size of rocky planets. According to the research, celestial bodies smaller than Mars may be unable to maintain a stable atmosphere long enough to allow life to emerge.

Published in The Planetary Science Journal, the results utilize the computational model Smaller than Earth Habitability Model (STEHM) to simulate the evolution of rocky planets. The model analyzes how the balance between gas loss and geological replenishment determines a world's habitability over billions of years.

Simulations indicate that, under conditions similar to Earth's, a planet must have at least 80% of Earth's radius to conserve its atmosphere for prolonged periods. In more favorable scenarios, such as those with a higher concentration of radioactive elements or carbon in the interior, this limit may drop to 60% of our planet's radius.

The difficulty for smaller planets lies in reduced gravity, which facilitates the loss of gases into space, combined with the rapid decline of geological activity. Without volcanism to replenish atmospheric gases, the planet becomes vulnerable to space radiation and loses the ability to maintain adequate temperatures and liquid water on its surface.

The discovery offers a new filter for astronomers. Currently, the search for life focuses primarily on exoplanets located in the habitable zone of their stars, but the study suggests that a planet's size should be a priority selection criterion for identifying promising targets.

Despite the conclusions, scientists emphasize that small planets should not be immediately discarded, as they may acquire new atmospheres through asteroid impacts. The team's next step is to adapt the model to study worlds orbiting red dwarf stars and tidally locked planets.