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The planets with an electric heater turned to high

posted Nov 4, 2018, 9:19 AM by Jeremy Drake   [ updated Nov 4, 2018, 9:22 AM ]
We now know that planets are common around M dwarfs, and the 100 billion or so M dwarfs in our own Galaxy mean the chance of there being "habitable" worlds with liquid water on their surface should be extremely high.  But liquid water is not the only consideration for habitability. The magnetic fields of M dwarfs and their associated energetic radiation - both in the form of a plasma "wind" and UV to X-ray emission - are generally much stronger in comparison to their visible light than for solar-like stars. As a consequence, close-in planets around M dwarfs endure a distinctly more hostile space environment than the Earth does ,and an environment that might strip planet's atmosphere and surface water. 

The wind from a star like the Sun - including M dwarfs - is not just plasma; it also carries with it magnetic field that originates within the star in a magnetic dynamo driven by the star's rotation. When encountering a planet, the moving magnetic field and stellar wind plasma generate electric currents within the planetary ionosphere called "Birkeland currents", after Norwegian explorer and physicist Kristian Birkeland who first proposed them.  The ionosphere of a planet is the very outer atmosphere where ionization by the Sun's UV, EUV and X-ray light creates a significant number of ions and free electrons. In the Earth's case, the ionosphere is the region between 60 and 1000 km or so in altitude.  The currents that flow heat up the ionosphere in a process called "Joule heating", just like in an electric heater.  Enough energy is injected by Joule heating to expand and make a significant difference to the extent of the Earth's very outer atmosphere.

University of Massachusetts researcher, Prof. Ofer Cohen, lead a study published in the Astrophysical Journal Letters to estimate the heat injected into the the ionospheres of the planets around the M dwarf TRAPPIST-1.  The TRAPPIST-1 planets experience stellar wind conditions up to 100,000 times stronger than Earth, and Cohen's team found that, if the TRAPPIST-1 planets have managed to retain an atmosphere, the Joule heating rate is commensurately elevated by enormous amounts.  The net effect will be to increase the planet's atmospheric loss rate, likely further jeopardising their capability to harbour and sustain life over billion year timescales.