Rapid destruction of Earth-like atmospheres by young stars

The discoveries of thousands of planets orbiting our solar system have raised questions about the potential for life formation on these planets, which are of fundamental importance for modern science. The fundamental character of a planet's habitability is whether or not it can maintain an atmosphere, which implies that the atmosphere is not completely lost early in the life of the planet. A new study by researchers based at the University of Vienna and the Institute of Space Research of ÖAW in Graz has shown that young stars can quickly destroy the atmospheres of planets similar to the Potentially habitable land, which is an important additional difficulty for training. of life outside our solar system. The results will be published soon in the newspaper Letters of Astronomy and Astrophysics.

One of the most active and exciting questions in modern science is: what is the abundance of planets with Earth-like atmospheres and surface conditions, and thus a potential for life unleashed? Much recent research on this topic has focused on the stars in orbit around the planet, called M-dwarfs, which are smaller than our Sun and are the most numerous star type of our solar neighborhood.

The main factor of atmospheric losses in space is the central star orbiting the planet. The stars possess powerful magnetic fields that cause the emission of X-rays and high-energy ultraviolet rays. These phenomena are known collectively as the "activity" of the star. At a young age, stars have a high activity and therefore emit very large amounts of X-rays and ultraviolet rays. As the stars age, their activities diminish rapidly. Importantly for planets orbiting around M-dwarfs, while star-like activities such as the Sun diminish rapidly after a few hundred million years, M-dwarves often remain very active for billions of years.

High energy radiation is important because it is absorbed in the atmosphere of a planet, which causes the heating of the gas. For Earth, the gas is heated to a temperature above 1000 degrees Celsius in the upper region known as the thermosphere. This is the region in which space vehicles fly such as satellites and the International Space Station. In orbit around young high-activity stars, the planetary thermospheres are heated to much higher temperatures, which in extreme cases can cause gases to escape from the planet. The speed with which atmospheres are lost in these cases has not yet been explored in detail for Earth-like planets with atmospheres similar to Earth.

Researchers based at the University of Vienna and at the ÖAW Space Research Institute in Graz have calculated for the first time how quickly a planet resembling the Earth would be lost. a planet orbiting a very active young star. Their calculations showed that extreme hydrodynamic losses of the atmosphere would occur, which would lead to a total loss of Earth's atmosphere, in less than one million years, which for the evolution of a planet is almost instantaneous.

These findings have important implications for the early stages of Earth's evolution and for the possibility of formation of Earth-like atmospheres around the Dwarves. For the Earth, the most likely reason why the # 39; atmosphere was not lost is that the primitive atmosphere was dominated by carbon dioxide, which cools the upper atmosphere by emitting infrared radiation in the space, thus protecting it from the heating by the strong activity of the Sun at the beginning. . The atmosphere of the Earth could not have become dominated by nitrogen, as it is today, after several hundred million years, when The activity of the Sun has decreased to much lower levels.

More dramatically, the results of this study imply that, for planets orbiting M-dwarf stars, they can only form Earth-like atmospheres and surfaces that after the lowering of the level. of star activity, which can take several billion years. It is more likely that many planets orbiting the M-dwarf stars have a very thin or possibly zero atmosphere. In both cases, the formation of life in such systems appears less likely than previously thought.