A study published in Nature Astronomy and conducted by Dr. GUO jianheng from the Yunnan Observatories of the Chinese Academy of Sciences increases our understanding of the violent atmospheric escape processes of low-mass exoplanets, specifically a process known as hydrodynamic escape. It reveals various mechanisms driving hydrodynamic escape and proposes a new classification method for understanding these escape processes. 

Exoplanets, i.e., planets outside our solar system, are a popular subject in astronomical research. The atmosphere of these planets can leave the planet and enter space for various reasons. One such reason is hydrodynamic escape, which refers to the upper atmosphere leaving the planet as a whole. This process is much more intense than the process of particle escape predicted in our solar system’s planets. 

Scientists theorize that hydrodynamic atmospheric escape occurred in the early stages of some of our solar system’s planets such as Venus and Earth. If Earth had lost its entire atmosphere via this process, it might have become as desolate as Mars. However, this intense escape no longer occurs on planets like Earth. In contrast, space and ground telescopes have observed that hydrodynamic escape still occurs on some exoplanets that are very close to their host stars. This process not only changes the planet’s mass but also affects the planet’s climate and habitability. 

In this study, Dr. GUO found that the hydrodynamic atmospheric escape from hydrogen-rich low-mass exoplanets could be driven either solely or jointly by the planet’s internal energy, the work done by the star’s tidal forces, or heating by the star’s extreme ultraviolet radiation. 

Before this study, researchers had to rely on complex models to figure out which physical mechanism drove hydrodynamic escape on a planet, and the conclusions were often obscure. This study proposes that the basic physical parameters of the star and planet—such as mass, radius, and orbital distance—are sufficient for classifying the mechanisms of hydrodynamic escape from low-mass planets. 

On planets with low mass and large radius, sufficient internal energy or high temperature can drive atmospheric escape. This study shows that using the classic Jeans parameter—a ratio of the planet's internal energy to potential energy—can determine whether the aforementioned escape occurs. For planets where internal energy cannot drive atmospheric escape, Dr. GUO defined an upgraded Jeans parameter by introducing tidal forces from stars. With the upgraded Jeans parameter, the roles of the star’s tidal forces and extreme ultraviolet radiation in driving atmospheric escape can be easily and accurately distinguished. 

In addition, this study reveals that planets with high gravitational potential and low stellar radiation are more likely to experience a slow hydrodynamic atmospheric escape; otherwise, the planet will primarily undergo rapid hydrodynamic escape. 

This study helps scientists understand how a planet’s atmosphere evolves over time, which is important for exploring the evolution and origin of low-mass planets. In this way, we can better understand the habitability and evolutionary histories of these distant worlds. 

Various driving mechanism affecting the hydrodynamic escapes in low-mass exoplanets. (Image by GUO Jianheng)