How low-mass exoplanets lose their atmospheres

Various driving mechanism affecting the hydrodynamic escapes in low-mass exoplanets. Credit: Chinese Academy of Sciences / Guo Jianheng.

Researchers have made significant strides in understanding how some distant planets, known as low-mass exoplanets, lose their atmospheres into space through a process called hydrodynamic escape.

This study, led by Guo Jianheng from the Yunnan Observatories of the Chinese Academy of Sciences, was recently published in Nature Astronomy.

Exoplanets are planets outside our solar system, and their study provides crucial insights into the variety and nature of planetary systems in the universe.

One interesting phenomenon that occurs with these planets is the escape of their atmospheres, a process in which the upper layers of a planet’s atmosphere are lost to space.

This can have profound effects on the planet’s mass, climate, and potential to support life.

The process observed in these exoplanets, known as hydrodynamic escape, is much more dramatic than anything seen in our own solar system today. It involves the entire upper atmosphere being driven off by various mechanisms, rather than just individual particles escaping as is generally observed with planets like Earth.

Hydrodynamic escape could have been common in the early solar system, and if Earth had undergone such intense atmospheric loss, it might have ended up barren like Mars. Nowadays, this extreme atmospheric escape doesn’t occur on Earth, but it has been observed on some exoplanets, especially those that orbit very close to their stars.

Guo’s study focuses on identifying what drives this atmospheric escape in low-mass exoplanets. The research reveals that it could be triggered by several factors: the planet’s internal energy, gravitational tugging by the star (known as tidal forces), or heating by the star’s ultraviolet radiation.

Previously, scientists had to rely on complex models to determine which mechanism was responsible for the escape on any given planet, often with unclear results. However, Guo proposes a new, simpler classification method based on basic physical parameters like the planet’s mass, radius, and distance from its star. This approach allows researchers to better predict and understand the mechanisms driving hydrodynamic escape.

The study introduces the concept of an upgraded Jeans parameter, a scientific measure that uses the planet’s internal energy to determine if atmospheric escape is likely to occur. For planets where internal energy isn’t sufficient to cause escape, this parameter is adjusted to include the effects of tidal forces, helping to differentiate the influences of tidal interactions and ultraviolet radiation more clearly.

Overall, the research provides a clearer picture of how and why these planets lose their atmospheres.

Understanding these mechanisms is crucial for grasping how planets evolve over time and could offer insights into their ability to support life, painting a broader picture of planetary evolution in the cosmos.

Source: Chinese Academy of Sciences.