The search for Earth 2.0 is an exciting endeavor, and a new study has shed light on a crucial factor in determining a planet's habitability: its size. The research, conducted by scientists at the University of California Riverside, introduces the Smaller Than Earth Habitability Model (STEHM), which identifies a critical threshold for atmospheric retention and, by extension, the potential for life. This model suggests that planets slightly smaller than Earth may be the key to finding extraterrestrial life.
The study reveals that the magic number is 0.8 Earth radii. Planets of this size can maintain an atmosphere for billions of years, a crucial factor in supporting life. This finding is based on two significant challenges that smaller planets face: gravity and internal cooling.
Smaller planets have lower gravity and escape velocity, making it easier for high-energy atmospheric particles to escape into space. This process, known as Jeans escape, is a significant hurdle for maintaining an atmosphere. Additionally, the high surface area-to-volume ratio of smaller planets causes their interiors to cool down rapidly, leading to a thickening of the lithosphere and reduced volcanic activity. Volcanic outgassing is essential for sustaining an atmosphere over the long term, so less volcanic activity means shorter atmosphere lifetimes.
The STEHM model, while simplified, demonstrates a clear cutoff between 0.7 and 0.8 Earth radii. Planets larger than 0.8 Earth radii can retain an atmosphere for billions of years, while smaller planets, especially those below 0.7 Earth radii, are likely to lose their atmosphere due to extreme ultraviolet (XUV) radiation from their host stars. For instance, a 0.6 Earth-radius planet might retain an atmosphere for 400 million years, while a 0.5 Earth-radius planet would be stripped of its atmosphere in just 30 million years.
However, there are exceptions to this rule. Smaller planets can cheat their atmospheric fate if they possess rare features. A large carbon budget can delay the stripping of the atmosphere for billions of years, while a small planet with a low core radius fraction retains a larger mantle volume and volatile inventory, enabling continuous outgassing of atmosphere-giving gases. Additionally, a 'cold start' where the mantle takes time to heat up can result in reduced XUV radiation from the star, allowing the atmosphere to persist for longer.
These rare features are not common, so the study's implication is clear: astronomers should focus on exoplanets larger than 0.8 Earth radii in the search for extraterrestrial life. Smaller planets, unless they have an unusual composition, are likely to be airless rocks drifting through space. This finding highlights the importance of considering planetary size in the quest to discover life beyond Earth.
