China Focus: Researchers classify atmospheric escape mechanism of low-mass exoplanets

KUNMING, May 9 (Xinhua) -- A group of Chinese researchers has offered a perspective on the atmospheric escape processes of low-mass exoplanets, specifically a process known as hydrodynamic escape, according to a research article published in the journal Nature Astronomy on Thursday.

This research revealed various driving mechanisms affecting the hydrodynamic escapes and proposed a new classification method to understand the escape processes.

Exoplanets refer to planets outside the solar system and 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.

According to the researchers from the Chinese Academy of Sciences Yunnan Observatories, this process is much more intense than the particle behavior escape observed in the solar system's planets. They added that 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," explained Guo Jianheng, a researcher with the Yunnan Observatories.

Even though this intense escape no longer occurs on planets like Earth, 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 its climate and habitability.

Before this study, people relied on complex models to determine which physical mechanism was driving the hydrodynamic escape on a planet, and the conclusions were often obscure.

The researchers 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.

They proposed 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 the planet's internal and potential energy ratio can determine whether the aforementioned escape occurs.

For planets whose internal energy cannot drive atmospheric escape, researchers can accurately distinguish the roles of stellar tidal forces and extreme ultraviolet radiation by introducing tidal forces from stars to correct the ratio of internal energy to potential energy.

The results of this study are helpful in understanding how a planet's atmosphere evolves over time but also have potential applications in exploring the evolution and origins of low-mass planets.