THE SPACE WEATHER OF PROXIMA CENTAURI b

Abstract

A planet orbiting in the "habitable zone" of our closest neighboring star, Proxima Centauri, has recently been discovered, and the next natural question is whether or not Proxima b is"habitable". Stellar winds are likely a source of atmospheric erosion that could be particularly severe in the case of M dwarf habitable zone planets that reside close to their parent star. Here we study the stellar wind conditions that Proxima b experiences over its orbit. We construct 3-D MHD models of the wind and magnetic field around Proxima Centauri using a surface magnetic field map for a star of the same spectral type and scaled to match the observed ∼ 600 G surface magnetic field strength of Proxima. We examine the wind conditions and dynamic pressure over different plausible orbits that sample the constrained parameters of the orbit of Proxima b. For all the parameter space explored, the planet is subject to stellar wind pressures of more than 2000 times those experienced by Earth from the solar wind. During an orbit, Proxima b is also subject to pressure changes of 1 to 3 orders of magnitude on timescales of a day. Its magnetopause standoff distance consequently undergoes sudden and periodic changes by a factor of 2 to 5. Proxima b will traverse the interplanetary current sheet twice each orbit, and likely crosses into regions of subsonic wind quite frequently. These effects should be taken into account in any physically realistic assessment or prediction of its atmospheric reservoir, characteristics and loss.

Is Proxima Centauri b Habitable? A Study of Atmospheric Loss

“…This is important because the stellar wind pressure is highly variable, as noted in Garraffo et al (2016). Hence, the fact that the stellar wind variability does not greatly influence the lower regions lends some credence to the hypothesis that the surface biology may not be significantly affected (Grießmeier et al 2016).…”

“…Table 2. The stellar wind input parameters at PCb (Garraffo et al 2016) for two case studies in the PSO coordinate system. Case 1 (C1) corresponds to the maximum P dyn and Ptot over one orbital period of PCb.…”

“…The dynamic pressure P dyn = m p n sw v 2 sw (m p is proton mass), magnetic pressure P mag = B 2 IMF /(2µ 0 ) and total pressure P tot = P dyn + P mag are also introduced. The relevant values have been listed in Table 2, and have been chosen from Garraffo et al (2016) with either maximum or minimum total pressure (and dynamic pressure), over one orbit of PCb. The stellar wind MHD model is driven at its inner boundary by the observable stellar surface magnetic field (i.e., magnetogram).…”

On the Age of the TRAPPIST-1 System

“…Based on current estimates of XUV-driven mass loss, the high energy emission of TRAPPIST-1 is likely sufficient to have evaporated an Earth's ocean of water mass from each of the TRAPPIST-1 planets except g and h over the system's lifetime (Bolmont et al 2017;Bourrier et al 2017). Moreover, the stripping of atmospheres and oceans may be enhanced by direct interaction between stellar and planetary magnetic field lines, which could funnel stellar wind particles directly to the planets' surfaces (Garraffo et al 2016(Garraffo et al , 2017. On the other hand, current estimates of the planets' densities are generally below Earth's average density Wang et al 2017), suggesting volatile-rich worlds that may have ample reservoirs; while ocean evaporation and hydrogen loss could result in an oxygen-and ozone-rich atmosphere that could shield the surface from high UV fluxes (Luger & Barnes 2015;O'MalleyJames & Kaltenegger 2017).…”

Electromagnetic Forces on a Relativistic Spacecraft in the Interstellar Medium

“…The magnetic field may be locally amplified due to a bowshock around the stellar wind. The magnetic field and stellar wind around Proxima Centauri were studied in Garraffo et al (2016). However, the path within the stellar neighborhood is rather short.…”