The habitable zone is the region surrounding a star in which surface temperatures are such that liquid water can exist on the surface of planets. Scientists expect that any extraterrestrial life that might exist most likely inhabits planets orbiting in this habitable zone. A new study by Rory Barnes of the University of Seattle suggests, however, that the habitable zone might be smaller than previously thought, and that previous estimates of the population of habitable zone planets might need to be halved.
For larger stars, the habitable zone exists so far from the star that gravitational effects are negligible. For smaller, cooler stars like red dwarfs (the most common kind of star) the habitable zone is much closer, in an area where the stars gravity has greater effects on the planet. Many planets orbiting red dwarfs have significantly elliptical orbits, cruising closer to their parent stars at some times, and farther at others. As the planet follows this elliptical path, the force of the star’s gravity on the planet changes enough to cause tides–not tides on the surface, but tides within the entire planet itself. The whole interior of the planet is pulled back and forth, producing friction. This planet-scale friction produces heat, and it is the effect of this heat that might reduce the habitability of such close-in planets. The surfaces of such planets would not be the moderately heated solid rock-and-water surfaces suitable for life, but rather hellish wastelands of intense geological activity, far too hot and tumultuous for life to survive.
In our own solar system, this tidal heating is seen not on any planets, but on the moon’s of Jupiter. Jupiter’s moon Io has a highly active surface, with numerous volcanoes and lava plains marking its surface. The moon itself is too small to hold a heated, geologically active interior like the Earth’s. Instead, this heat is caused by the tides pulling on Io.
The Jovian moon Europa has a somewhat different story. Unlike Io, Europa is covered with a thick, frozen crust of ice. Underneath this ice, however, is a mysterious ocean. Planetary scientists believe that this ocean–which ought to be frozen–is kept warm and liquid by tidal heating as Europa follows its elliptical orbit around Jupiter. Interestingly, while tidal heating is expected to be so intense as to sterilize the surface of many otherwise habitable planets around red dwarfs, it is precisely the liquid ocean warmed by tidal heating that makes Europa one of the likeliest locations of extraterrestrial life in the solar system. It’s also possible that, rather than limiting habitable surfaces on planets around red dwarfs, tidal heating might produce a Europa-like effect and render habitable or liquid otherwise frozen surfaces.
The takeaway, I think, is that all the headline-gathering announcements about estimates of habitable planets need to be taken for what they are: educated but highly speculative guesses. There’s always something we didn’t think of yet which will dramatically effect the actual conditions on the surfaces of these planets. For the same reason I’ve never been impressed by the Drake Equation — we just don’t know anywhere near enough to make with certainty the kinds of estimates these analyses are trying to make. The only way to really know about truly habitable, or truly inhabited, planets outside the solar system is to actually find them.