Scientists develop model to determine minimum size of rocky planets with atmospheres

Researchers at the University of California have created a computational model to estimate the minimum size a rocky planet needs to possess in order to retain an atmosphere over billions of years.

Planetary Habitability Limits

The findings, recently published in The Planetary Science Journal, indicate that celestial bodies smaller than Mars have great difficulty maintaining this gaseous layer for long enough to establish conditions favorable for the emergence of life. The absence of a stable atmosphere compromises a planet's ability to maintain adequate temperatures and liquid water on its surface, in addition to increasing its susceptibility to space radiation, which makes the environment less favorable to life.

The STEHM Model and Simulations

To investigate this dimensional limit, scientists developed the Smaller than Earth Habitability Model (STEHM), a model that simulates the evolution of Earth-like rocky planets orbiting Sun-like stars. The simulations show that, under conditions analogous to Earth's, a planet must have approximately 80% of Earth's radius to preserve its atmosphere for billions of years. However, in more advantageous scenarios, such as a carbon-rich interior or a higher concentration of radioactive elements, this requirement may decrease to about 60% of the planetary radius.

This necessity is linked to the balance between the loss and replenishment of atmospheric gases. Smaller planets have weaker gravity, which facilitates atmospheric loss. Simultaneously, their geological activity tends to decline more rapidly, resulting in less release of volcanic gases, a process essential for atmospheric renewal over time.

New Criterion in the Search for Life

The authors suggest that these results could serve as a new filtering criterion in the selection of exoplanets for future observations. Currently, many targets are selected primarily because they are in the habitable zone of their stars. However, the study emphasizes that the planet's size must also be considered, since very small worlds may lose their atmosphere before reaching viable conditions for the development of life.

Despite this, the research points out that certain planets can acquire atmospheres later, for example, through asteroid collisions. Thus, even those that initially lose their atmospheres should not be immediately discarded as potential habitats. Michelle Hill, the lead scientist on the study, reinforces this perspective. The team's next steps include adapting the model to analyze worlds orbiting red dwarfs and tidally locked planets, aiming to expand knowledge about which exoplanets deserve priority in future space exploration missions.