Tidal Heating Shrinks the Goldilocks Zone
A previously little-considered heating effect could shrink estimates of the habitable zone of the Milky Wayâ€™s most numerous class of stars â€” â€˜Mâ€™ or red dwarfs â€” by up to one half, says Rory Barnes, an astrobiologist at the University of Washington in Seattle. That factor â€” gravitational heating via tides â€” suggests a menagerie of previously undreamt-of planets, on which tidal heating is a major source of internal heat. Barnes presented the work at a meeting of the American Astronomical Societyâ€™s Division on Dynamical Astronomy in Oregon.
The habitable zone is the orbital region close enough to a star for a planet to have liquid water, but not so close that all of the water evaporates. For our Sun, the zone extends roughly from the inner edge of the orbit of Mars to the outer edge of that of Venus. For smaller, cooler stars, such as M-class dwarfs, the zone can be considerably closer to the star than Mercury is to the Sun. And because close-in planets are easier to spot than more distant ones, such stars have been a major target for planet hunters seeking Earth-like worlds.