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Gone with the wind

posted Jul 25, 2017, 5:21 AM by Jeremy Drake   [ updated Nov 6, 2017, 8:16 AM ]
The discovery of a planet orbiting our nearest star, Proxima Centuri, is a jewel in the crown of exoplanetary science. Further gilded by its location in the "habitable" zone, where an Earth-like planet would not freeze or boil its oceans, Proxima b represents a chance that the nearest exoplanet to us could actually harbour life. 

Surface liquid water requires the presence of a substantial atmosphere - without atmospheric pressure, water simply boils to the gaseous state. Close proximity to Proxima poses problems in this regard: as noted previously, the intense stellar wind where Proxima b resides could strip away a planetary atmosphere.  A strong planetary magnetic field might provide some protection, but atmospheric escape can still occur through the magnetic poles.  This "polar wind" occurs on Earth, but is not strong enough to endanger our atmosphere.  It occurs because air molecules at high altitude get ionized by solar extreme ultraviolet and X-ray radiation, and the light electrons liberated in the process can rise to sufficient altitudes to begin to escape Earth's gravity.  The rise of electrons generates an electric field that pulls the positively charged heavier ions upward, essentially resulting in a flow into space.  

Proxima b experiences ionizing radiation levels hundreds of times higher than the Earth, and we expected its polar wind to be commensurately stronger.  In a study lead by Katie Garcia-Sage of the NASA Goddard Space Flight Center and the Catholic University of America, featured in a NASA release, a Forbes article, and published in the July 20th edition of the Astrophysical Journal Letters, we used supercomputer models to compute the ionospheric outflow that the Earth would experience were it in the location of Proxima b. A best guess for the thermospheric temperature suggests a polar wind escape rate sufficient to remove an Earth atmosphere in a few hundred million years - a short timescale compared with the planet's likely age of nearly 5 billion years.  While the atmosphere could plausibly be replenished by geological processes, such as volcanic activity, the loss rates we estimate are conservative and do not include other erosion effects from the stellar wind and coronal mass ejections.  In all likelihood, Proxima b is bare, devoid of an atmosphere that has gone with the wind.