Stanford University geophysicist Norman Sleep has outlined some new constraints on habitable planets.1 He explains how the possible existence of advanced life crucially depends upon a planet maintaining efficient plate tectonics for billions of years. Without such plate tectonics several nutrient-recycling processes, critical for advanced life, cannot be sustained. Efficient plate tectonics are also essential for transforming a planet’s surface into a mix of oceans and continents.
Both the nutrient recycling and the development of continental landmasses require a high rate of subduction. Subduction is the sliding of one tectonic plate under another. For subduction to take place, the tectonic plates need to slip in friction at the fault zones. Also, the lithosphere within the crustal slab that is slipping under another crustal slab needs to bend with a specified strain.
An overarching design requirement for advanced life, then, is that the rate of subduction must be fine-tuned. Too low of a subduction rate would lead to inadequate nutrient recycling and inadequate buildup of continents. (If the buildup rate is much less than the erosion rate, the continents will disappear.) Too high of a subduction rate would disturb the ecosystems of advanced life and challenge the development of global high-technology civilization.
To sustain the subduction rate at the just-right level that advanced life needs means that the sliding friction between crustal plates at the subduction zones must be maintained at just-right levels. Also, the crustal slabs undergoing subduction need to bend at the just-right levels and rates. All this fine-tuning adds to the growing weight of evidence that a supernatural, super-intelligent Creator is necessary to explain all the characteristics of Earth that must be present in order for the planet to be habitable by advanced life. It also implies that, unless the Creator has intervened in other places in the cosmos, astronomers will not find advanced-life habitable planets elsewhere in the Milky Way Galaxy or in any other galaxy.
- Norman Sleep, “Tectonics and Habitability of Super-Earths,” Astrobiology 8 (April 2008): 395.