The atmosphere of a planet may leave the planet and enter space for various reasons. Among them, the upper atmosphere violently leaves the planet in a holistic manner, known as planetary fluid atmospheric escape. Compared to the atmospheric escape of other individual particles, the fluid escape process is much more violent. Fluid atmospheric escape may have occurred in the early stages of planets in the solar system, but it no longer exists. However, researchers have found through observations from space and ground-based telescopes that fluid escape exists on some exoplanets that are very close to the host star. Fluid atmospheric escape not only changes the mass of planets, but also affects their climate and livable environment.

Recently, Guo Jianheng, a researcher at the Yunnan Observatory of the Chinese Academy of Sciences, published a research paper entitled "Characterization of the hydrodynamic escape of exoplanets in low mass systems" on Nature Astronomy, which provides a new perspective for exploring the atmospheric escape of fluids occurring on exoplanets in low mass systems. This study explores how internal and external energy sources such as planetary energy, stellar extreme ultraviolet radiation, and tidal forces affect planetary fluid atmospheric escape through simulation and theoretical analysis of hydrogen rich atmospheres. This study reveals different driving mechanisms that affect the atmospheric escape of these fluids and proposes a new and more accurate classification method to analyze these escape processes.

Research has found that even in the absence of other external energy sources, high planetary temperatures can drive atmospheric escape on low mass and large radius planets. The heat released from the planet's core or the optical and near-infrared radiation of stars deposited in the lower atmosphere largely determines the equilibrium temperature of the planet. Therefore, this process requires the planet's Kings parameter (a dimensionless parameter of the ratio of gravitational potential energy to thermal energy) to be less than 3.5.

However, when considering external energy driven processes, such as the heating of the atmosphere by extreme ultraviolet radiation from stars and fluid escape driven by tidal forces, the Kings parameter is powerless to handle this. In view of this, the study proposes an improved Kings parameter that takes into account the influence of tidal forces. The new parameters allow researchers to more accurately distinguish the different physical processes caused by external energy sources leading to atmospheric escape. Research has shown that when a planet is very close to the main star and the improved Kings parameter is below 3, the planet is subjected to strong stellar tidal forces, and atmospheric escape is mainly driven by tidal forces; When the improved Kings parameter is in the range of 3 to 6, both the extreme ultraviolet radiation and tidal force of the star may trigger atmospheric escape; When the improved Kings parameter exceeds 6, the tidal force of the star is no longer important, and atmospheric escape is mainly driven by the extreme ultraviolet radiation heating of the star.

In addition, research has found that planets with high gravitational potential may experience slow, subsonic fluid atmospheric escape if they receive weaker stellar extreme ultraviolet radiation, while planets may primarily experience fast, transonic fluid escape. The study also found that the gravitational potential of a planet and the received main star's XUV radiation largely determine the ionization degree of the planet's atmosphere.

This study proposes a method that can succinctly classify the driving mechanisms and ionization structures of fluid atmospheric escape using only the basic parameters of star planet systems. Meanwhile, this method can be applied to planetary evolution calculations. The above achievements have enriched scientists' understanding of the escape mechanism of exoplanet atmospheric fluids and provided reference for future planetary atmospheric exploration and theoretical research.



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