In August 2016 hints were confirmed that an Earth-like planet orbits the Sun’s nearest stellar neighbor, the red dwarf Proxima Centauri. The discovery excited astronomers. Proxima Centauri is close enough that the exoplanet, Proxima Centauri b, will be directly visible to the next generation of space telescopes and to the giant ground-based telescopes currently under construction. (The accompanying artist’s impression depicts Proxima Centauri as seen from the exoplanet surface.) Furthermore, the exoplanet’s orbit lies within the star’s habitable zone—that is, the volume of space within which a planetary surface can sustain liquid water given enough atmospheric pressure. Does the exoplanet have a thick atmosphere? To begin to address that question, Cecilia Garraffo of the Harvard–Smithsonian Center for Astrophysics and her colleagues modeled the hot wind of charged particles that emanates from Proxima Centauri. Because the precise values of the star’s magnetic field and the exoplanet’s orbital inclination and eccentricity are unknown, the researchers ran their magnetohydrodynamic model eight times with different sets of parameters. According to the model, Proxima Centauri’s wind is roughly as strong as the Sun’s, even though the red dwarf is smaller and dimmer. But because of Proxima Centauri’s low luminosity, its habitable zone lies much closer to the star than the Sun’s does. For all eight scenarios, the wind that buffets Proxima Centauri b is approximately 1000 times more powerful than solar wind is at Earth’s orbit. Even though the exoplanet likely has a magnetic field, whatever atmosphere the exoplanet was endowed with was likely blown away long ago. (C. Garraffo, J. J. Drake, O. Cohen, Astrophys. J. Lett.833, L4, 2016, doi:10.3847/2041-8205/833/1/L4.)
