Scientists searching for life beyond Earth have developed a new way to identify which distant planets might be able to support life.
Their new tool, called the Smaller Than Earth Habitability Model (STEHM), focuses on a simple but important question: Can a planet hold onto an atmosphere long enough for life to develop?
Since the first exoplanet was discovered in 1992, astronomers have confirmed more than 6,000 planets outside our solar system.
Thousands more are waiting to be verified. With so many worlds to investigate, researchers need better ways to narrow down the list of promising candidates.
An atmosphere is essential for life as we know it. It protects a planet’s surface from harmful radiation and helps maintain temperatures suitable for living things.
Scientists can also study a planet’s atmosphere from afar, making it one of the best places to search for signs of life.
Michelle Hill, a researcher at Stanford University, developed STEHM to examine how a planet’s size affects its ability to create and keep an atmosphere over billions of years.
Using computer simulations, Hill modeled rocky planets ranging from half the size of Earth to the same size as Earth. The study focused on planets with carbon dioxide-rich atmospheres and rigid surfaces, unlike Earth’s moving tectonic plates.
The results suggest that planets need to be at least about 80% the size of Earth to reliably maintain an atmosphere for 10 billion years or more, provided they orbit at a suitable distance from a Sun-like star.
Smaller planets generally lose their atmospheres much sooner. Planets around 70% of Earth’s size may still keep an atmosphere under especially favorable conditions, but their chances are lower.
The study found that a planet’s initial supply of carbon is one of the most important factors. Carbon dioxide helps trap heat and keep a planet warm. Volcanic activity continually releases carbon dioxide into the atmosphere, helping replace gases that are lost to space.
Heat-producing elements inside a planet, such as uranium, thorium, and potassium, also play a key role. As these elements slowly decay, they generate heat that keeps the planet’s interior active. This helps drive volcanism and maintain the atmosphere over long periods. Once these heat sources run low, volcanic activity declines and atmospheric replenishment slows.
The model also showed that planets that start out extremely hot may actually struggle to keep their atmospheres. Intense early heat can expose the atmosphere to strong stellar radiation, allowing gases to escape more easily.
To test the model, Hill applied it to Venus and Mars. STEHM successfully predicted Venus’s thick atmosphere and Mars’s thin, depleted one. The findings suggest that Mars was always at a disadvantage because of its small size and lack of plate tectonics.
The research provides a useful filter for future planet-hunting missions. By focusing on planets large enough to retain atmospheres, scientists can better target their search for life in the vast number of worlds orbiting distant stars.
The study also highlights how unusual Earth may be. Among the thousands of planets discovered so far, our world appears to have a rare combination of size, geology, atmosphere, and location that has allowed life to thrive for billions of years.
Source: KSR.
