Scientists exploring possibilities of life beyond Earth have introduced a new method to identify distant planets that could potentially support life. Their innovative approach, called the Smaller Than Earth Habitability Model (STEHM), addresses a key question: Can a planet retain its atmosphere long enough for life to develop?
Since the discovery of the first exoplanet in 1992, astronomers have confirmed over 6,000 planets outside our solar system, with many more awaiting verification. With such a vast number of worlds to investigate, researchers need more efficient ways to narrow down promising candidates.
An atmosphere is critical for supporting life as we know it. It shields a planet’s surface from harmful radiation and helps regulate temperatures suitable for living organisms. Additionally, scientists can analyze a planet’s atmosphere remotely, making it an essential aspect of the search for extraterrestrial life.
Michelle Hill, a researcher at Stanford University, developed STEHM to study how a planet’s size influences its ability to sustain an atmosphere over billions of years. Using computer simulations, Hill examined rocky planets ranging from half to the full size of Earth. The study focused on planets with carbon dioxide-rich atmospheres and solid surfaces, unlike Earth’s tectonic plate movements.
Findings suggest that planets need to be at least approximately 80% the size of Earth to reliably maintain an atmosphere for 10 billion years or longer, assuming they orbit a star similar to the Sun at an appropriate distance. Smaller planets tend to lose their atmospheres much sooner. Those around 70% of Earth’s size might still retain an atmosphere under ideal conditions, but their likelihood is significantly reduced.
The research emphasizes that a planet’s initial carbon content is crucial. Carbon dioxide plays an essential role in trapping heat and keeping the planet warm. Volcanic activity continually replenishes carbon dioxide, counteracting gases lost to space. Moreover, internal heat sources, such as uranium, thorium, and potassium, decay slowly over time, generating heat that sustains geological activity and helps maintain the atmosphere. As these heat sources diminish, volcanic activity decreases, and atmospheric replenishment slows down.
Interestingly, the model indicates that planets starting out extremely hot might struggle to hold onto their atmospheres. Excessive early heat can expose the atmosphere to intense stellar radiation, causing gases to escape more readily.
To validate STEHM, Hill applied it to Venus and Mars. The model accurately predicted Venus’s thick atmosphere and Mars’s thin, depleted one. The results suggest that Mars’s small size and lack of tectonic activity have historically hindered its ability to retain an atmosphere.
This research offers a valuable tool for upcoming planet-hunting missions. By focusing on planets large enough to preserve atmospheres, scientists can more effectively target worlds where life might exist, amid the countless planets orbiting distant stars.
Moreover, the study underscores how unique Earth might be. Out of the thousands of exoplanets discovered to date, Earth’s particular combination of size, geological activity, atmosphere, and orbital position appears to be exceptionally favorable for life to flourish over billions of years.