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Check the space weather before packing

posted Jul 17, 2017, 7:36 AM by Jeremy Drake
The remarkable TRAPPIST-1 system is now know to host at least seven planets, three of which reside in the so-called temperate or "habitable zone", where surface temperatures should in principle be able to sustain water in liquid form.  But, as we have argued in earlier postings, there is a big difference between a planet being in the habitable zone and actually being habitable. Liquid water, in particular, can only exist within a relatively dense atmosphere. 

TRAPPIST-1 is an "utracool" red dwarf - a tiny star one tenth the size of the Sun with a surface temperature of about 2600 degrees.  It is so faint that only planets that orbit really close in can have temperatures warm enough for liquid water. The problem for the TRAPPIST-1 planets is that their star is not so weak at ultraviolet to X-ray wavelengths and also in regard to its magnetic field and wind. TRAPPIST-1 rotates once every 3.3 days, and this rapid rotation has built up its surface magnetic field to an immense strength of 100 times our Sun's.  Despite the star being so small, this magnetic field drives a stellar wind that ends up being of a similar strength to the solar wind.  Because the wind strength experienced by a planet decreases with the square of its distance from its parent star, the TRAPPIST-1 planets in their close-in habitable zone get blasted by a wind up to ten thousand times stronger than the Earth experiences.

On Earth, we are protected from most of the ravages of the solar wind by the Earth's magnetic field. However, any magnetic fields the TRAPPIST-1 planets have will be largely overwhelmed by the magnetism of their star.  The star-planet magnetic fields can directly connect over much of each planet, allowing the full force of the stellar wind to stream straight onto their surfaces.  The concept of atmospheric protection by a planetary magnetic field does not hold here. These conditions will likely strip off any atmospheres - and liquid water - the TRAPPIST-1 planets might have had on timescales of a hundred million years or so. Most habitable zone planets around similar low mass M dwarfs are likely to end up dry and barren - uninhabitable in the habitable zone.