Astronomy
The Influence of Stellar Winds on the Atmospheres of Exoplanets
Quick fact
Some hot Jupiters lose up to 100,000 tonnes of gas per second due to intense stellar winds and high-energy radiation from their host stars.
Why this is interesting
You have probably seen images of comet tails streaming away from the Sun, but imagine a planet’s entire atmosphere being blown away like that. Which exoplanets are most at risk, and could Earth ever face such a fate?
Read the full explanation
Understanding The Influence of Stellar Winds on the Atmospheres of Exoplanets
Think of a star as a giant fan blowing a constant breeze of charged particles, called a stellar wind. When a planet orbits close to its star, this wind constantly bombards the planet’s outer atmosphere. Particles in the wind can transfer energy and momentum to gas molecules, helping them reach escape velocity and drift off into space. This is like gusts of wind eroding a sand dune, but on a planetary scale. The planet’s magnetic field acts as an umbrella, deflecting many incoming particles, while gravity holds the atmosphere down. Over time, if the wind is strong enough, the atmosphere thins and may even disappear. The key factors are the star’s wind intensity, the planet’s distance from the star, and the strength of its magnetic field.
A deeper explanation
Stellar winds are streams of ionized plasma accelerated by the star’s corona and magnetic activity. When these fast-moving particles hit an exoplanet, they can collide with neutral atoms and molecules in the upper atmosphere, energizing them and causing them to escape. This process, known as atmospheric escape, includes mechanisms like Jeans escape and hydrodynamic escape. Hydrodynamic escape is particularly effective: high-energy stellar radiation heats the upper atmosphere, causing it to expand and blow off gas like a weak tornado. Additionally, the stellar wind can strip away ions directly, a process called ion pick-up. The efficiency of erosion depends on the balance between the wind’s pressure and the planet’s magnetosphere, which forms a protective cavity. If the magnetosphere is weak, the wind can reach the atmosphere and erode it faster. For exoplanets, especially those close to red dwarf stars, which are very active, atmospheric loss can be so severe that they become barren rocks. In contrast, planets with thick atmospheres and strong magnetic fields, like Earth, are shielded and can retain their atmospheres for billions of years. Observing atmospheric escape in exoplanets helps scientists understand planetary evolution and assess which planets might be habitable.