The search for life beyond Earth is one of the most challenging tasks in modern astronomy. Thousands of already discovered exoplanets and billions of potential candidates in the Milky Way require efficient ways to narrow down the search. A new model developed at Stanford University proposes filtering rocky worlds by their ability to retain an atmosphere for billions of years.

STEHM: A filter for potentially habitable planets

The model is called the Smaller Than Earth Habitability Model (STEHM). It assesses whether a small rocky planet can form and maintain a dense atmosphere over geological timescales. This is critically important, because without a stable atmosphere, a planet cannot sustain liquid water, protect its surface from harsh radiation, or maintain a relatively stable climate.

Lead author of the study Michelle Hill noted that the only way to find out if there are signs of life is to study the atmospheres of these planets.

Previously, the main criterion was whether a planet fell within the star's habitable zone — the region where temperatures theoretically allow liquid water to exist. However, location alone is not enough. STEHM adds a second important layer of analysis — the ability to retain an atmosphere.

How the model works

The researchers used the ExoPlex simulation code and modeled six rocky worlds ranging from half to the full size of Earth. They took into account the planet's structure, volcanic activity, internal heat, and the effects of stellar radiation.

The model successfully passed validation on real examples: it correctly reproduced Venus's thick carbon dioxide atmosphere and Mars's long-term atmospheric loss.

Main findings

According to the results, rocky planets at least 80% of Earth's size can retain an atmosphere for 10 billion years or more if they are in the habitable zone around Sun-like stars. Smaller planets lose their atmosphere significantly faster, although worlds around 70% of Earth's size could still be habitable under particularly favorable conditions.

Initial carbon content and heat-generating elements that sustain volcanism play an important role. These factors directly affect the long-term "survivability" of the atmosphere.

Why this matters for the search for alien life

The atmosphere is a key indicator of possible life. It is precisely through its chemical composition (using spectroscopy) that astronomers hope to detect biosignatures. The new model will allow more efficient use of expensive observation time on powerful telescopes by filtering out less promising targets.

STEHM will be particularly useful for future missions such as the European space telescope PLATO, which will search for rocky planets around nearby stars.

Michelle Hill emphasized that the search for life concerns not only "where" but also "when." It is possible that we simply appeared too early in cosmic history: many planets may not yet have developed stable conditions for life or may have already lost them.

In brief

The new STEHM model from Stanford helps select the most promising rocky exoplanets based on their ability to retain an atmosphere for billions of years. Planets at least 80% of Earth's size in the habitable zone around Sun-like stars are the best candidates. The model complements the classical habitable zone criterion and will allow astronomers to use telescope time more efficiently in the search for signs of life in the atmospheres of distant worlds. The study was published on June 4 in the Planetary Science Journal.