A new planetary modeling study suggests rocky exoplanets must be at least 80% the width of Earth to retain atmospheres for billions of years, narrowing the search for potentially habitable worlds around sun-like stars
Not every rocky planet orbiting in a star's habitable zone can hold onto an atmosphere long enough for life to emerge. New research indicates that exoplanets smaller than Mars are unlikely to maintain the thick, stable atmospheres needed to support biology as we know it, even if they orbit at the right distance for liquid water.
Atmospheric Loss and Planet Size
Researchers at the University of California Riverside used computer simulations to investigate how rocky planets of different sizes fare in retaining their atmospheres over billions of years. Their models focused on planets similar in composition to Earth, orbiting within the habitable zones of sun-like stars. The results show that a planet must be at least about 80% the width of Earth-roughly the size of Mars or larger-to keep an atmosphere for several billion years. Planets smaller than this threshold tend to lose atmospheric gases faster than volcanic activity can replenish them, primarily due to weaker gravity and magnetic fields.
For context, Mars has a diameter of about 6,800 kilometers, or just over half that of Earth. The study found that planets as small as 60% of Earth's radius could, in rare cases, retain an atmosphere if they possess unusually high carbon content in their mantles, but such conditions are considered unlikely for most worlds. This size constraint provides a practical filter for astrobiologists seeking to prioritize exoplanet targets for atmospheric characterization and biosignature searches.
Modeling Volcanism and Atmospheric Retention
The team's "Smaller Than Earth Habitability Model" simulated the interplay between atmospheric loss from stellar wind and radiation, and the replenishment of gases from volcanic eruptions. The models included variations in core size, mantle composition, initial temperature, and carbon abundance. Planets with more carbon in their interiors were better able to sustain thick carbon dioxide atmospheres, as CO2 is relatively heavy and less easily stripped away by stellar activity. However, even with favorable conditions, smaller planets generally cooled and solidified more quickly, shutting down volcanic outgassing and accelerating atmospheric loss.
Interestingly, the simulations showed that a cooler initial mantle temperature could help a planet retain its atmosphere. Delayed volcanic activity means that outgassed material is released after the host star's most active and destructive phase, increasing the odds that some atmosphere survives. The study also found that planets with relatively larger mantles and higher concentrations of radioactive elements-which keep the interior hot-were more likely to maintain volcanic activity over geological timescales.
Implications for Exoplanet Habitability
These findings have immediate consequences for the search for life beyond the Solar System. With thousands of rocky exoplanets now identified, astronomers must decide which worlds to study in detail with limited telescope time. The new size threshold helps narrow the field, focusing attention on planets large enough to plausibly retain atmospheres for billions of years. This is especially relevant for upcoming observations with the James Webb Space Telescope and other next-generation instruments, which can probe the atmospheres of transiting exoplanets for potential biosignatures.
Even so, the study leaves room for exceptions. Planets that lose their original atmospheres might regain them through late impacts by comets or asteroids, which can deliver volatile elements. Such secondary atmospheres could, under the right circumstances, support habitability. The researchers caution that while size is a key factor, composition, volcanic history, and stellar environment all play important roles in determining a planet's long-term atmospheric fate.
Expanding the Search and Remaining Questions
The modeling approach used in this study could be extended to planets orbiting red dwarfs, which make up the majority of stars in the Milky Way. Systems like TRAPPIST-1, with multiple rocky planets in the habitable zone, are prime candidates for future atmospheric studies. However, the intense stellar activity of red dwarfs may pose additional challenges for atmospheric retention, even for planets above the size threshold identified here.
Recent direct detections of exoplanet atmospheres, such as the helium signature found on LHS 1140 b, highlight both the promise and the complexity of characterizing distant worlds. As discussed in our coverage of atmospheric detection on a rocky exoplanet, the survival of an atmosphere depends on a delicate balance of planetary and stellar factors. The new modeling work adds a crucial piece to this puzzle, but many uncertainties remain about the diversity of planetary interiors and the frequency of secondary atmospheres.
Future research will need to combine improved models with direct observations to refine our understanding of which exoplanets are most likely to be truly habitable. The study was published in June in The Planetary Science Journal.
To understand why planetary size is so critical for atmospheric survival, it helps to consider the role of gravity and magnetic fields. Larger planets exert stronger gravitational pull, making it harder for atmospheric molecules to escape into space. They are also more likely to sustain a molten core, which can generate a magnetic field that shields the atmosphere from stellar wind and radiation. When a planet is too small, its gravity and magnetic protection are weaker, and its interior cools more rapidly, shutting down volcanic outgassing and magnetic field generation. These physical limits set a lower bound on the size of rocky worlds that can maintain atmospheres over billions of years-a key requirement for life as we know it.
