Huge coronal mass ejections (CMEs) on the Sun produce extreme space weather effects at Earth. Models of solar CMEs have now been applied to M-dwarf and Sun-like stars a popular target in the search for Earth-like exoplanets, in research published in the Astrophysical Journal (arXiv.org preprint).

Artists conception of an exoplanets atmosphere being stripped by the radiation from its parent star. Image credit: Mark A. Garlick / University of Warwick.

CMEs are gigantic clouds of solar plasma drenched with magnetic field lines that are blown away from the Sun during solar flares and filament eruptions.

They are a fundamental factor in so-called space weather, and are known to disrupt the flow of the solar wind and produce disturbances that strike the Earth with sometimes catastrophic results.

However, astronomers have shown that the effects of space weather may also have a significant impact on the potential habitability of planets around cool, low mass stars.

Traditionally an exoplanet is considered habitable if its orbit corresponds to a temperature where liquid water can exist.

Low mass stars are cooler, and therefore should have habitable zones much closer in to the star than in our Solar System, but their CMEs should be much stronger due to their enhanced magnetic fields.

When a CME impacts a planet, it compresses the planets magnetosphere, a protective magnetic bubble shielding the planet.

Extreme CMEs can exert enough pressure to shrink a magnetosphere so much that it exposes a planets atmosphere, which can then be swept away from the planet.

This could in turn leave the planetary surface and any potential developing lifeforms exposed to harmful X-rays from the nearby host star.

We figured that the CMEs would be more powerful and more frequent than solar CMEs, but what was unexpected was where the CMEs ended up, said lead author Dr. Christina Kay, from NASAs Goddard Flight Center and Boston University.

Dr. Kay and co-authors modeled the trajectory of theoretical CMEs from V374 Pegasi, an M-class dwarf star located 19.6 light-years from Earth.

V374 Pegasi has a surface temperature of only 2,900 degrees Celsius, in contrast to the Suns 5,500 degrees Celsius. Its mass and radius are less than one-third the mass and radius of the Sun.

The researchers found that the strong magnetic fields of V374 Pegasi push most CMEs down to the Astrophysical Current Sheet (ACS), the surface corresponding to the minimum magnetic field strength at each distance, where they remain trapped.

While these cool stars may be the most abundant, and seem to offer the best prospects for finding life elsewhere, we find that they can be a lot more dangerous to live around due to their CMEs, said co-author Marc Kornbleuth, a graduate student at Boston University.

The results suggest that an exoplanet would need a magnetic field ten to several thousand times that of Earths to shield their atmosphere from the cool stars CMEs.

As many as five impacts a day could occur for planets near the ACS, but the rate decreases to one every other day for planets with an inclined orbit.

This work is pioneering in the sense that we are just now starting to explore space weather effects on exoplanets, which will have to be taken into account when discussing the habitability of planets near very active stars, said co-author Dr. Merav Opher, an associate professor at Boston University.

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C. Kay et al. 2016. Probability of CME Impact on Exoplanets Orbiting M Dwarfs and Solar-Like Stars. ApJ 826, 195; doi: 10.3847/0004-637X/826/2/195

This article is based on text provided by the Royal Astronomical Society.

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