A previously overlooked heating factor may shrink in half current estimates of the habitable zone, or Goldilocks zone, of our own Milky Way, at least when it comes to red dwarfs or "M" stars. Gravitational heating by way of tides could be a major source of internal heat In our planetary system, the zone at which liquid water can exist goes from the inner edge of Mars's orbit to the outer edge Venus's orbit. However, for smaller, cooler stars such as red dwarfs, the liquid-water zone could be closer, the equivalent of Mercury for our solar system, or closer. But because tidal forces change drastically with the distance between a planet and its star, closer orbits effect massive tidal forces. Tidal forces cause the planet to stretch and contract, and those forces produce a massive amount of heating. Slight deviations from a perfectly circular orbit produce substantial tidal heating. According to calculations made by Rory Barnes, an astrobiologist at the University of Washington in Seattle, many of these planets could easily have already undergone sufficient heat to lose all of its water through evaporation. These planets would be, according to Barnes, "permanently sterilized." The findings are theoretical for now, but it may be an important consideration when trying to decide whether an Earth-like planet circling a red dwarf is our win or more like tidal Venus.
Overlooked factor suggests fewer habitable planets than thought.
A previously little-considered heating effect could shrink estimates of the habitable zone of the Milky Way’s most numerous class of stars — ‘M’ or red dwarfs — by up to one half, says Rory Barnes, an astrobiologist at the University of Washington in Seattle. That factor — gravitational heating via tides — suggests a menagerie of previously undreamt-of planets, on which tidal heating is a major source of internal heat. Barnes presented the work yesterday at a meeting of the American Astronomical Society’s Division on Dynamical Astronomy in Timberline Lodge, 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.
There's just one problem with finding habitable planets around such stars, says Barnes. Because tidal forces vary dramatically with the distance between a planet and its star, closer orbits also result in massively larger tidal forces.