What would a planet require to be supportive of life as we know it? The answer seems simple: just look at the properties of the Earth. But scientists are still not sure what made it possess this set of characteristics. A new study has examined the role of the magnetic fields of stars in this process, and its results may not be very encouraging.
The atmosphere is one of the main elements that make a planet habitable. Through it the X-rays and ultraviolet radiation emitted by the host stars are absorbed, and in the process, the atmosphere itself is heated enough to remain “trapped” on the planet. This same radiation is produced by most stars, while generating magnetic activity that drives high-speed ionizing winds.
M dwarf stars, the most common type, can have very active magnetic fields, indicating that planets in their habitable zones (the range of distance from a star in which a planet’s surface water can remain liquid) can have favorable environments. It would be great to know that M dwarfs (usually a name for red dwarfs, although terminology varies) are good targets for searching for alien life, as they are relatively cold and therefore their habitable zones are closer to them.
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This means that planets orbiting M dwarfs in their habitable zones, the best candidates in this class of stars, will be closer to the star than the Earth is to the Sun. As a result, it is susceptible to the effects of photoevaporation – partial or total evaporation of the atmosphere by stellar radiation. But to be sure of this, simulations and studies that test all possible combinations and variants are necessary.
A team of astronomers has modeled the effects of stellar winds on extrasolar planet with a hydrogen-rich atmosphere orbiting near the M dwarf. This configuration was chosen because some theorists believe that with large hydrogen or helium envelopes, planets in the habitable zone of this class of stars would be more likely, because photoevaporation would remove enough of these gases and will retain what is essential to the population in the end.
For example, the team used a file TRAPPIST-1 . system, consists of an M dwarf star with seven planets in its orbit, six of which are close enough to be in its habitable zone. The simulations showed that stellar winds can indeed generate seepage flows from the planet’s atmosphere, thanks to the magnetic field forces of these stars and the planet itself.
In the simulations, scientists found that the stellar winds of the M dwarf and the distribution of plasma emitted by it can damage the planet in its habitable zone. The leak results in a variety of planetary magnetosphere configurations and affects the distribution of stellar plasma depending on stellar wind conditions. These conditions, in turn, can change frequently along the planet’s orbit.
These mechanisms can be observed and studied using lines of atomic hydrogen in the ultraviolet, so astronomers can rely on telescopes equipped with observing filters at this wavelength to better understand these interactions and their consequences for candidate worlds for habitable exoplanets. But it is not easy to interpret the observations.
Systems such as TRAPPIST-1 undergo a display of “a variety of atmospheric properties and some physical conditions that can vary over short time scales,” within an hour, the authors explain in A study published in the Astrophysical Journal. This makes it difficult to understand what is observed in transits of exoplanets – the main way of discovering a world orbiting a star other than the Sun and observing its atmosphere.
To help solve the problem, the researchers point out that in observations of planetary transits in M dwarf stars, and more specifically in studies of the atmospheres of these worlds, it is necessary to use magnetic effects in 3D models of stellar systems.
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