Escaping Atmospheres: Unraveling the Hydrodynamic Mysteries of Exoplanets

Research published in Nature Astronomy advances our understanding of atmospheric escape in low-mass exoplanets by detailing hydrodynamic escape processes and introducing a new classification method based on the planet’s mass, radius, and orbital distance. This study aids in predicting the impact of such escapes on the planet’s mass, climate, and habitability. Credit: SciTechDaily.com

New research enhances understanding of atmospheric escape in exoplanets, focusing on classification and impacts on habitability.

A new study 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. The research was published on May 9 in Nature Astronomy and conducted by Dr. Jianheng Guo from the Yunnan Observatories of the Chinese Academy of Sciences.

Hydrodynamic Escape in Exoplanets

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.

Various driving mechanisms affecting the hydrodynamic escapes in low-mass exoplanets Credit: Jianheng Guo

Mechanisms of Atmospheric Escape

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.

New Insights Into Atmospheric Escape Dynamics

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.

Conclusion and Implications

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.

Reference: “Characterization of the regimes of hydrodynamic escape from low-mass exoplanets” by J. H. Guo, 9 May 2024, Nature Astronomy.
DOI: 10.1038/s41550-024-02269-w