Probability of Cme Impact on Exoplanets Orbiting M Dwarfs and Solar-Like Stars

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.Subject headings: Sun: coronal mass ejections (CMEs) M DwarfsSmall, low mass stars, such as M dwarfs, vastly outnumber their more massive counterparts within the Galaxy. M dwarfs' great number, combined with their long main sequence lifetime, and small mass and radius, which results in a low luminosity, have made them popular targets in the search for habitable exoplanets.However, many of the aspects that make M dwarf exoplanets easier to find may also be a detriment to their habitability. The cool, low luminosity nature of M dwarfs leads to close habitable zones, as defined using the traditional requirement of planetary temperatures conducive to the existence of liquid water. M dwarf habitable zones range between 0.03 Astronomical Units (AU) and 0.4 AU, with the distance being the smallest for late-type M dwarfs (Khodachenko et al. 2007a). The largest of these orbital distances is roughly the same as Mercury's orbit. For the early-and mid-type habitable zone exoplanets, we have no analogue orbit at such small distances within our own solar system.

The MUSCLES Treasury Survey. IV. Scaling Relations for Ultraviolet, Ca ii K, and Energetic Particle Fluxes from M Dwarfs

“…There are many unknowns here, including the interplanetary magnetic field topology that charged particle trajectories will follow (Parker 1958); the opening angle of the accelerated particles; the planet's cross section, which may be larger than R p 2 p owing to the presence of a magnetosphere; and the direction of the particle ejection with respect to the planet's orbital plane (see Kay et al 2016). A thorough treatment of this issue is beyond the scope of this work.…”

Frequent Flaring in the TRAPPIST-1 System—Unsuited for Life?

“…Kay, Opher, & Kornbleuth (2016) found that for typical coronal mass ejection (CME) masses and speeds measured on M dwarfs, the orbiting rocky planets would need magnetic fields between tens to hundreds of Gauss, while hot Jupiters would only require magnitudes between a few and 30 G. The authors concluded that rocky exoplanets possibly could not generate sufficient magnetic field to shield their atmosphere from these eruptions (e.g., Earth exhibits a magnetic field of ≈ 0.5 G). According to Vidotto et al (2013) planetary magnetic fields should be stronger.…”

The habitability of planets orbiting M-dwarf stars

“…Planetary magnetic moments may be weakened on tidally-locked M-dwarf planets (Lammer, 2007;Khodachenko et al, 2007). Recent work found that magnetic field strengths of tens to hundreds of Gauss may be required to protect the atmospheres of habitablezone planets orbiting mid M-dwarf stars from coronal mass ejections, or CMEs (Kay et al, 2016). While it could be challenging for rocky exoplanets to attain that strong of a magnetic field, it would likely be easier for similar planets orbiting early M dwarfs to retain their atmospheres, given the lower expected CME impact rates (Kay et al, 2016).…”

“…Recent work found that magnetic field strengths of tens to hundreds of Gauss may be required to protect the atmospheres of habitablezone planets orbiting mid M-dwarf stars from coronal mass ejections, or CMEs (Kay et al, 2016). While it could be challenging for rocky exoplanets to attain that strong of a magnetic field, it would likely be easier for similar planets orbiting early M dwarfs to retain their atmospheres, given the lower expected CME impact rates (Kay et al, 2016). Studies have found that tidally-induced heating of an Earth-mass planet with a fixed, eccentric orbit in the habitable zone of a low-mass star can cause a weakened magnetic field, among other effects, if the tidal heating is strong enough (Driscoll & Barnes, 2015).…”