👽 A leap forward in the search for extraterrestrial life

The number of exoplanets already discovered numbers in the thousands, and the Milky Way may harbor billions. This represents a veritable ocean of worlds to explore, far beyond what our current technology allows: how do we identify those truly worthy of in-depth observation with next-generation telescopes?

A new computer tool, named STEHM (Smaller Than Earth Habitability Model), could make this task easier. Developed by researchers at Stanford University, this model quickly sorts through rocky planets that have little chance of retaining an atmosphere—an essential condition for life as we know it. The idea is to save observation time on large telescopes by immediately weeding out the least promising worlds.


Traditionally, scientists rely on the concept of the habitable zone, the region around a star where temperatures could allow liquid water to exist on the surface. But being in this zone does not guarantee everything: a planet without a significant atmosphere cannot regulate its temperature, protect itself from radiation, or maintain liquid water. STEHM therefore adds an extra layer of analysis by estimating whether a small rocky planet is capable of generating and retaining an atmosphere over geological timescales.

To build this model, Michelle Hill, lead author of the study, used the ExoPlex planetary simulation code. She modeled six rocky worlds, ranging from half the size of Earth to Earth-sized, testing how internal structure, volcanic activity, internal heat, and stellar radiation influence atmosphere retention. The model was validated with Venus and Mars, correctly reproducing Venus's thick carbon dioxide atmosphere and Mars's long-term atmospheric loss.

The results indicate that rocky planets at least 80% the size of Earth can retain an atmosphere for 10 billion years or more, provided they orbit within the habitable zone of Sun-like stars. Smaller planets generally lose their atmosphere more quickly, although those around 70% of Earth's size could remain habitable if other conditions are favorable. Atmospheric longevity also strongly depends on the initial carbon content and heat-producing elements that fuel volcanic activity.

This model could be particularly useful for future missions such as the European Space Agency's PLATO space telescope, which will greatly increase the catalog of rocky exoplanets around nearby stars. By narrowing down the field of candidates, STEHM allows astronomers to focus their efforts on the most promising worlds, without wasting precious resources on unlikely targets. In the words of Michelle Hill, the only way to detect signs of life is to observe the atmospheres of these planets from Earth, and this model provides a method for selecting the best targets.

STEHM addresses not only the question of "where" to look for life, but also "when": it models whether exoplanets can actually retain their atmospheres over geological timescales, a prerequisite for life to emerge and develop. Perhaps the current lack of life detections is because we are still very early in cosmic history, as Michelle Hill notes: "We may be among the first."