Michelle Hill's team at Stanford has built a computational model that could help astronomers stop searching for habitable planets blindfolded. Over the past three decades, NASA has confirmed the existence of more than 6,000 exoplanets orbiting distant stars, with another 7,000 suspected planets awaiting verification. In humanity's rush to find worlds where life might exist, we've lacked a simple way to ask a fundamental question: can this planet even hold onto an atmosphere?
The atmosphere is everything for habitability. It shields a planet's surface from the radiation and particle bombardment of space, regulates temperature through greenhouse effects, and harbors the chemistry that sustains life as we understand it. But not every world can keep one. The Smaller Than Earth Habitability Model (STEHM), which Hill developed with her research group in the Stanford Doerr School of Sustainability, offers astronomers a tool to identify which planets meet the bare minimum for atmospheric retention before they waste expensive telescope time.
The research, published in The Planetary Science Journal, tested six different planet profiles ranging from 0.5 to 1.0 Earth radii—in other words, rocky worlds ranging from half Earth's size to Earth's size exactly. The results were clear: planets with a radius of at least 0.8 Earth radii can maintain their atmospheres for 10 billion years or more if they orbit a sun-like star at a comfortable distance. Anything smaller tends to lose its protective blanket within 1 billion years, though worlds at 0.7 Earth radii might hold on if other factors align favorably.
But size alone doesn't determine fate. Hill's model revealed that a planet's carbon content matters enormously. Because planets form from the collision of dust and gas particles circling a star, each world inherits whatever elemental mix chance provided. Higher concentrations of carbon dioxide, a greenhouse gas that traps heat, mean longer atmospheric survival. The problem is that this CO2 must be constantly replenished to prevent atmospheric loss, and that replenishment depends on volcanic activity—which in turn depends on heat.
Heat-producing elements in a planet's mantle—thorium, uranium, and potassium—release energy as they decay, fueling volcanic cycles that resurrect atmospheric gases. Yet there's a cruel paradox: too much heat too early can be fatal. "Hot-start" planets, which form with high internal temperatures that melt the mantle and disable its heat-regulating features early on, expose their atmospheres to loss before they've had a chance to stabilize.
The architecture of a planet also matters. Those with smaller cores and thicker mantles can pack in higher concentrations of carbon and heat-producing elements, allowing them to maintain volcanic activity—and atmospheric replenishment—across billions of years.
"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. The STEHM model helps focus that search. Rather than studying every candidate in the expanding catalog of exoplanets, astronomers can now use this framework to filter for worlds with the right dimensions and probable composition before committing resources to detailed atmospheric observations. As missions like the European Space Agency's Plato prepare to hunt for Earth-like planets orbiting sun-like stars, this model offers a more efficient roadmap—narrowing the cosmic haystack so that the needle of habitable worlds becomes easier to find.