The term "habitable zone" used to be an esoteric concept on the fringes of astrophysics, describing the region around a star in which a planet could support liquid water. The discovery over the last decade or so that planets are actually very common in the Universe has thrust habitable zones into the common lexicons of both astrophysics and popular science. While the classical definition of a habitable zone remains useful, it is not obvious that all planets in habitable zones will actually be habitable.
In several past works, we have argued that the space weather environment of a planet - the plasma and magnetic field conditions resulting from the host stellar wind - is likely to be crucial to habitability. The link to habitability is through the atmosphere that is continually eroded by the action of the host stellar wind. The key question is whether a planet can hold on to its atmosphere over billion year timescales. The Earth managed it, but evidence suggests that Mars, probably because of its lack of a strong protective magnetic field, has long since lost most of its surface water due to solar wind scouring.
The recently discovered planet in the habitable zone of our closest neighboring star, Proxima Centauri, has been one of the most exciting findings of the last decade. The planet is close enough that next generation telescopes will be able to observe it directly: Proxima b could become the first habitable planet studied in detail. But Proxima is a host star quite different to the Sun - cool and faint, shining with only 1/600th of the Sun's power. The habitable zone of Proxima is then very close to the star, and Proxima b orbits twenty times closer to Proxima than the Earth to the Sun. SAO scientist Cecilia Garraffo has lead a new study of the impact of this proximity to Proxima. Supercomputer model simulations of the wind from Proxima indicate it blows with similar strength to that of the Sun. Being so close though, Proxima b gets blasted by this wind, experiencing a wind pressure several hundred times that experienced by Earth. Twice each 11 day orbit it passes through more dense wind streams that raise the pressure to 2000 times that at Earth. If Proxima b has a magnetic field it might serve as some protection for its atmosphere, but it will be a highly dynamic magnetospheric environment, expanding and compressing like a bellows on timescales of less than a day. The much less severe dynamics of the Earth's magnetosphere are thought to be a factor in the Earth's atmospheric loss, as plasma is released by the forced opening and closing of the magnetic field. It seems doubtful under such conditions that Proxima b will have retained any sort of atmosphere capable of sustaining life.
Detailed atmosphere calculations are needed to assess the true habitability of Proxima b, but the property does not appear quite as appealing as it did in the first glossy brochures. This work was published in the 2016 December 10 edition of the Astrophysical Journal Letters.
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