Probability of CME Impact on Exoplanets Orbiting M Dwarfs and Solar-Like Stars (1605.02683v1)

Abstract: Solar coronal mass ejections (CMEs) produce adverse space weather effects at Earth. Planets in the close habitable zone of magnetically active M dwarfs may experience more extreme space weather than at Earth, including frequent CME impacts leading to atmospheric erosion and leaving the surface exposed to extreme flare activity. Similar erosion may occur for hot Jupiters with close orbits around solar-like stars. We have developed a model, Forecasting a CME's Altered Trajectory (ForeCAT), which predicts a CME's deflection. We adapt ForeCAT to simulate CME deflections for the mid-type M dwarf V374 Peg and hot Jupiters with solar-type hosts. V374 Peg's strong magnetic fields can trap CMEs at the M dwarfs's Astrospheric Current Sheet, the location of the minimum in the background magnetic field. Solar-type CMEs behave similarly, but have much smaller deflections and do not get trapped at the Astrospheric Current Sheet. The probability of planetary impact decreases with increasing inclination of the planetary orbit with respect to the Astrospheric Current Sheet - 0.5 to 5 CME impacts per day for M dwarf exoplanets, 0.05 to 0.5 CME impacts per day for solar-type hot Jupiters. We determine the minimum planetary magnetic field necessary to shield a planet's atmosphere from the CME impacts. M dwarf exoplanets require values between tens and hundreds of Gauss. Hot Jupiters around a solar-type star, however, require a more reasonable <30 G. These values exceed the magnitude required to shield a planet from the stellar wind, suggesting CMEs may be the key driver of atmospheric losses.

• The paper models Coronal Mass Ejection (CME) trajectories and impact probabilities on exoplanets around M dwarfs and solar-like stars, emphasizing CME deflection.
• For M dwarfs, strong magnetic fields cause significant CME deflection, increasing impact frequency on planets up to 5 per day, far exceeding impacts in solar systems.
• To prevent atmospheric erosion from frequent CME impacts, planets orbiting M dwarfs require magnetic fields significantly stronger than Earth's, posing challenges for habitability in such systems.

Analyzing CME Impact Probabilities on Exoplanets around M Dwarfs and Solar-Like Stars

The paper by Kay et al. presents an analysis of the probability and effects of coronal mass ejection (CME) impacts on exoplanets, specifically those orbiting M dwarfs and solar-like stars. The paper employs a model named "Forecasting a CME's Altered Trajectory" (ForeCAT) to account for CME deflections and assess their impact potential on planetary atmospheres.

Key Findings

1. CME Simulation and Deflection: ForeCAT is used to simulate the trajectory of CMEs originating from a mid-type M dwarf (V374 Peg) and solar-like stars, with an emphasis on planets situated within habitable zones or in close orbits, such as hot Jupiters. Notably, the paper identifies a strong dependence of CME deflection on mass, with lower mass CMEs being more susceptible to significant deflections toward the Astrospheric Current Sheet (an analog to the solar Heliospheric Current Sheet).
2. Mid-Type M Dwarfs and CME Deflection: The simulations reveal that for V374 Peg, the strong magnetic field can highly influence CME paths by trapping lighter CMEs at the Astrospheric Current Sheet, significantly increasing the likelihood of CME impacts on planets with orbits aligned close to the stellar equator. These findings suggest that CMEs on M dwarfs exhibit deflections that are much more pronounced than those observed with solar CMEs.
3. CME Impact Frequency on Exoplanets: For mid-type M dwarf exoplanets, the impact likelihood due to frequent CME deflection is shown to be as high as 5 impacts per day, which is considerably higher compared to Earth during solar maximum conditions. For solar-type stars with hot Jupiters, the expected CME impacts vary from 0.05 to 0.5 per day, indicating a less frequent interaction compared to M dwarf systems.
4. Minimum Magnetic Fields for Atmospheric Protection: The paper quantifies the stellar and CME-induced pressures and establishes the minimum planetary magnetic field necessary to maintain a stable magnetosphere and shield from frequent CME impacts. Exoplanets orbiting M dwarfs may require magnetic field strengths on the order of tens to hundreds of Gauss, far exceeding Earth's magnetic field, whereas hot Jupiters need less than 30 Gauss.
5. CME Impact on Atmospheric Erosion: The prolonged exposure to frequent CME impacts, particularly for M dwarf orbiting planets, highlights concerns about atmospheric retention and habitability. High CME frequencies could lead to significant atmospheric erosion unless exoplanets possess extraordinarily strong magnetic fields, which are typically uncharacteristic of terrestrial-like bodies.

Implications and Future Research Directions

The implications of this research are substantial, particularly for the assessment of exoplanet habitability in systems with active stellar environments. The necessity of strong magnetic fields for atmospheric protection against frequent CME impacts poses challenges for rocky exoplanets within the habitable zones of M dwarfs.

Future research could focus on verifying the model predictions with observational data, if obtainable, and exploring the impacts of CME-induced space weather on planetary atmospheres using detailed magnetohydrodynamic (MHD) simulations. Further exploration of planetary magnetic field generation and its interaction with stellar and interplanetary magnetic environments would also provide valuable insights into the long-term retention of atmospheres in these extreme space weather conditions.

Overall, this paper lays the groundwork for understanding the dynamics of CMEs in extrasolar contexts and offers critical insights into the habitability criteria for exoplanets in magnetically active stellar systems.