A recent study by Researcher GUO Jianheng from the Yunnan Observatories of the Chinese Academy of Sciences offers a perspective on the violent atmospheric escape processes of low-mass exoplanets, specifically a process known as hydrodynamic escape. This research reveals various driving mechanisms affecting the hydrodynamic escapes and proposes a new classification method to understand these escape processes. The study, "Characterization of the regimes of hydrodynamic escape from low-mass exoplanets," was published in Nature Astronomy .

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

Hydrodynamic atmospheric escape might have happened in the early stages of the solar system's planets. If Earth had lost its entire atmosphere via hydrodynamic escape at that time, it might have become as desolate as Mars. Now, this intense escape no longer happens on planets like Earth. However, 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.

This study found that the hydrodynamic atmospheric escape of 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 was driving the fluid escape on a planet, and the conclusions were often obscure. The authors of the study propose that just using the basic physical parameters of the star and planet, such as mass, radius, and orbital distance, can classify 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. The study found 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, the author defines an upgraded Jeans parameter by introducing tidal forces from stars. With the upgraded Jeans parameter, researchers can easily and accurately distinguish the roles of the star's tidal forces and extreme ultraviolet radiation in driving atmospheric escape.

Additionally, the study found 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 fluid escape.

The results of this study not only help us understand how a planet's atmosphere evolves over time but also have potential applications in exploring the evolution and origins of low-mass planets. As we continue to explore other potentially habitable worlds in the universe these findings will help us better understand the habitability and evolutionary histories of these distant worlds.

Figure 1, various driving mechanism affecting the hydrodynamic escapes in low-mass exoplanets. Image by GUO.

Contact:
GUO Jianheng
Yunnan Observatories, CAS
E-mail:guojh@ynao.ac.cn