The search for alien life just got a practical new tool. Researchers at Stanford University have developed a planetary habitability model that goes beyond the traditional habitable zone, allowing astronomers to quickly identify which rocky exoplanets are most likely to sustain the atmospheres needed for life as we know it.
The Smaller Than Earth Habitability Model (STEHM), developed by postdoctoral researcher Michelle L. Hill and colleagues at Stanford’s Doerr School of Sustainability, addresses a fundamental bottleneck in exoplanet science: telescope time is scarce, and not every rocky planet can hold onto an atmosphere over the billions of years required for life to emerge.
“The only way that we’re going to ever find out if there are signatures of life out there is by observing the atmosphere of these planets,” Hill said.
A size-based filter
Traditional habitability assessments focus primarily on whether a planet orbits within the habitable zone, the region around a star where liquid water could exist on the surface. STEHM adds a crucial additional constraint: it models whether a rocky planet can build and retain a substantial atmosphere over geologic timescales.
The model simulates six rocky planets ranging from half Earth’s radius up to Earth’s full size, using the ExoPlex planetary simulation code. It assumes CO2-based atmospheres (the most favorable case for retention, since carbon dioxide is heavy), stagnant lid tectonics (a rigid crust unlike Earth’s active plate tectonics), and volcanic outgassing driven by heat from radioactive elements such as thorium, uranium, and potassium.
The results, published June 4 in The Planetary Science Journal (DOI: 10.3847/PSJ/ae6804), establish clear size thresholds. Planets with a radius of at least 0.8 times Earth’s can retain an atmosphere for 10 billion years or more under favorable conditions, ample time for life to potentially develop. At 0.7 Earth radii, atmosphere retention is possible only under the most favorable circumstances: high initial carbon content, a cool mantle, and a low core-to-mantle ratio. Below 0.7 Earth radii, the atmosphere is lost within roughly 1 billion years, making these worlds effectively uninhabitable.
Tested on our neighbors
The model successfully reproduced the atmospheric histories of Earth’s nearest neighbors. Venus, roughly the same size as Earth, correctly produced its thick CO2 atmosphere. Mars, with just 53 percent of Earth’s radius, was correctly shown to be incapable of sustaining a substantial atmosphere over long timescales.
The most influential parameter for atmosphere retention turned out to be the planet’s initial carbon inventory. However, Hill noted that “orders of magnitude more carbon than Earth” would be needed to significantly shift the boundary, suggesting that size is the dominant factor.
A practical filter for the James Webb era
STEHM’s value lies in its efficiency. With limited observation time on telescopes such as the James Webb Space Telescope (JWST), which requires hundreds of hours for atmospheric characterization of a single exoplanet, and with future observatories like the Extremely Large Telescope (ELT) on the horizon, astronomers need a way to prioritize targets.
The model is designed to work in tandem with ESA’s PLATO mission, launching around 2026, which will dramatically expand the catalog of known rocky exoplanets around nearby Sun-like stars. STEHM can quickly screen PLATO’s discoveries, flagging planets above the 0.8 Earth-radius threshold as high-priority targets for follow-up atmospheric spectroscopy.
Co-author Stephen R. Kane of UC Riverside and Bradford J. Foley of Penn State contributed to the modeling framework. Laura K. Schaefer leads the Stanford Planetary Modeling Group where Hill conducted the research.
Maybe we’re one of the first
Hill said the model also raises a deeper question about humanity’s apparent solitude in the universe. “Maybe the answer to why we haven’t found any life yet is that we’re so early in the grand scheme of what has been created through the lives and deaths of stars,” she said. “Maybe we’re one of the first.”
The next phase of research will model planets with Earth-like plate tectonics, known as mobile lid planets, to see how active geology affects the threshold for atmosphere retention.
The study is available on arXiv (2605.00170) and was published in The Planetary Science Journal.