Unraveling the hydrodynamic mysteries of exoplanets

New research advances understanding of atmospheric escape on exoplanets, focusing on classification and habitability implications.

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 different mechanisms of hydrodynamic escape and proposes a new classification system to understand these escape processes. The study was published on May 9 Natural Astronomy and conducted by Dr. Jianheng Guo from the Yunnan Laboratories 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 atmospheres of these planets can leave the planet and enter space for various reasons. One such cause is hydrodynamic escape, which refers to the upper atmosphere leaving the planet entirely. This process is much more intense than the predicted ejection of particles from the planets of our solar system.

Scientists theorize that hydrodynamic atmospheric escape occurred in the early stages of some planets in our solar system. Venus and earth. If the Earth had lost its entire atmosphere through this process, it might have become desolate tuesday. However, this extreme escape no longer occurs on Earth-like planets.

In contrast, space and ground-based telescopes have found that hydrodynamic escape still occurs in 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.

Different driving mechanisms of hydrodynamic escape in low-mass exoplanets Credit: Jianheng Guo

Atmospheric escape mechanisms

In this study, Dr. Guo found that hydrodynamic atmospheric escape from hydrogen-rich low-mass exoplanets can only be driven by the planet’s internal energy, 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 led to hydrodynamic escape on a planet, and the results were often ambiguous. This study proposes that basic physical parameters of the star and planet—such as mass, radius, and orbital distance—are sufficient to characterize hydrodynamic escape mechanisms from low-mass planets.

New Insights into Atmospheric Escape Dynamics

In low-mass and large-radius planets, not enough internal energy or high temperature escapes the atmosphere. This study shows that by using the classic Jeans parameter, the ratio between the planet’s internal energy and potential energy can be used to determine whether the aforementioned escape occurs.

For planets that cannot escape an internal energetic atmosphere, Dr. Guo defined an improved Jeans parameter by introducing tidal forces from the stars. With the improved Jeans parameter, the roles of the star’s tidal forces and the extreme ultraviolet radiation emitted from the atmosphere 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 escape into slow hydrodynamic atmospheres; Otherwise, the planet is primarily a rapid hydrodynamic escape.

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

Reference: JH Guo, 9 May 2024, „Characterization of Hydrodynamic Escape Regimes from Low-Mass Exoplanets” Natural Astronomy.
DOI: 10.1038/s41550-024-02269-w