Picture this: an exoplanet could sit in the habitable zone, where liquid water is theoretically possible. But if it is too small, it may never hang onto an atmosphere long enough for life to get a foothold. That is the core takeaway from new simulations by University of California Riverside planetary scientist Michelle Hill and her colleagues, published in June in The Planetary Science Journal.

Their modeling suggests the atmosphere problem has a surprisingly strict cutoff. For a rocky, Earth-like world in the habitable zone of a sun-like star to maintain an atmosphere for a few billion years, it needs to be at least as big as Mars. More specifically, the team found an atmosphere-sustaining planet needs to be about 80% as wide as Earth (0.8 Earth radii), though it could technically be as small as 60% (0.6 Earth radii) under the right internal conditions.

Why this matters is not just academic. The habitable zone is prime real estate in the hunt for alien life, but it is also a tough neighborhood for atmospheres. The closer a planet orbits to its star, the more opportunities there are for radiation and stellar wind to strip away its gas. And because modern telescopes are discovering exoplanets with staggering frequency, astrobiologists face the classic problem of abundance with constraints: too many targets, not enough telescope time to follow up on all of them for biosignatures.

Hill’s team tackled that bottleneck by using what they call the “Smaller Than Earth Habitability Model.” The simulated planets are rocky worlds similar to Earth, orbiting in habitable zones around sun-like stars. Some start as near carbon copies of Earth in chemical makeup and internal structure, while others vary key parameters: carbon abundance, core size, starting temperatures, and more. Then the model tracks what happens over a few billion years by balancing two opposing processes. One side is atmospheric escape, driven by stellar wind and radiation. The other side is atmospheric replenishment, powered by volcanoes that pump out gas, mostly carbon dioxide.

This is where the Mars comparison turns into a rule of thumb executives and strategists should understand, even if your job is not exoplanets. Smaller planets tend to lose gas faster than volcanic activity can replace it. The reason is physical, not philosophical: smaller worlds have less gravity, weaker magnetic fields to help hold onto their thinner envelopes of gas, and often a mantle that releases less volcanic material over time. They also cool and harden faster at the top, which means volcanic eruptions taper earlier in the planet’s lifespan. In the model, you do not just need “some” volcanism. You need volcanism that keeps pace with atmospheric loss for billions of years.

The simulations also highlight an important lever beyond size: carbon. When Hill and her colleagues changed parameters, carbon became the biggest factor other than size in whether a planet keeps its atmosphere. They focused on a pure carbon dioxide atmosphere as a best-case scenario for retention, because carbon dioxide is a heavy molecule that is harder to lose. Their results imply that planets with higher carbon in their mantles can make a meaningful difference, even though it takes more carbon than Earth contains to noticeably change the outcome. In other words, the model is telling you where “maybe” lives: not everywhere, but in specific internal chemistries and structures.

Even with a strict retention threshold, the story does not end with “airless and done.” Hill and her colleagues suggest smaller worlds could get a second chance to build an atmosphere if they lose their initial one. One route is later comet and asteroid impacts delivering volatile elements like hydrogen, oxygen, and carbon, which could combine to form atmospheric gases. And timing matters: these impacts could be especially helpful if they happen after the star passes its youthful phase of intense flares, when harsh radiation bursts are less likely to instantly erase newly delivered gas.

For next steps, the research points toward where future observational priorities could shift. The team plans simulations for worlds around smaller, cooler stars called red dwarfs, which make up about 75% of the stars in our galaxy and include systems like TRAPPIST-1. TRAPPIST-1 has at least seven rocky planets, with three in the habitable zone, and its dim light makes it easier for telescopes like the James Webb Space Telescope (JWST) to capture images of starlight passing through planetary atmospheres during transits. Hill and her colleagues also hope to explore tidally locked planets, where tidal forces keep interiors hot and seismically active, similar to Jupiter’s moon Io.

So what should decision-makers take from this? The exoplanet pipeline is moving fast, but this work adds a practical filter: if you are trying to chase biosignatures, “habitable zone” alone may not be enough. The atmosphere retention hurdle looks size-driven, with a rough lower bound near 0.8 Earth radii and a theoretical edge case down to 0.6, depending on carbon, core and mantle properties, and eruption timing. In a world where teams are already sorting targets for follow-up with limited telescope time, this is the kind of constraint that can materially change which planets rise to the top of the list.