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. 

Tidal heating shrinks the 'goldilocks zone'

• Richard A. Lovett