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Supersonic bangs and atmospheric loss on exoplanets

posted Jul 8, 2015, 8:34 AM by Jeremy Drake
We are all familiar with the geological finding that the patterns of land and water on the surface of the Earth - the lithosphere - change over geological time as a result of plate tectonics.  But the lithosphere is not the only aspect to undergo substantial change.  Planetary atmospheres are also subject to evolution and change.  Change can be driven by the planet itself, through processes such as outgassing and dissolution, or biological action such as the "great oxygenation event' 2.3 billion years ago on Earth when the rise of cyanobacteria produced copious oxygen by photosynthesis.  Change can also be driven by the external planetary environment and "space weather" - the solar or stellar wind and coronal mass ejections.

The solar wind constantly scours and erodes the atmospheres of the planets in the solar system. Erosion is often not a direct process but can involve induced electric and magnetic fields, acceleration of atmospheric ions and the opening of magnetic field lines to release rarified, trapped plasma. Here, we use a sophisticated magnetohydrodynamic model to study the atmospheres of Venus-like (no strong magnetic field) exoplanets orbiting an M-dwarf star. We focus on the effect of the stellar wind and ionizing radiation on planets at different distances from the star.  We find the wind around close-in planets can be either supersonic or subsonic, and the transition between these conditions can deposit significant energy into the upper atmosphere - sort of like a colossal exoplanetary supersonic bang.  Without a planetary magnetic field, the stellar wind also penetrates much closer to the surface than the solar wind does on Earth.  Initial estimates suggest the atmospheric loss is not catastrophic, but there are several effects we still need to build into the model that could change this outcome, and perhaps the potential for such planets to sustain life.  This work was lead by SAO astrophysicist Ofer Cohen, and was published in the 2015 June 10 edition of the Astrophysical Journal.