Author(s): S. Alan Stern1
Institution(s): 1. SwRI
The Fermi Paradox (e.g., ) asks the question about extraterrestrial civilizations, “Where are they?” Given speculations based on numerical evaluations of the Drake Equation that would seem to indicate that the likelihood of precisely N=1 communicating extraterrestrial civilizations in the Universe is small, i.e., that we are unique, the Fermi Paradox remains a puzzle. Many possible explanations have been proffered. We suggest another—namely that the great majority of worlds with biology and civilizations are interior water ocean worlds (WOWs). Interior WOWs appear to be particularly conducive to the development of life owing to several key advantages, including these two: (1) Environmental Independence to Stellar Type, Multiplicity, and Distance. Owing to the several to hundreds of kilometers depth of typical Type II liquid water oceans, and the overlying thermal insulation provided by the planetary lid atop these oceans, the energy balance, temperature, pressure, and toxicity in Type II ocean worlds is only weakly coupled to their host star’s stellar type, stellar multiplicity, stellar distance, and stellar evolutionary stage (i.e., from protostars with winds and high activity through the main sequence to stellar remnants). (2) Environmental Stability. Again owing to the depth of typical Type II oceans and the overlaying thermal insulation provided by the planetary lid atop these oceans, these environments are protected from numerous kinds of external risks to life, such as impacts, radiation, surface climate and obliquity cycles, poisonous atmospheres, and nearby deleterious astrophysical events such as novae and supernovae, hazards stellar flares, and even phenomena like the Faint Early Sun. Interior WOWs are naturally cut off from communication by their interior nature below a thick roof of ice or rock and ice, therefore do not easily reveal themselves. In this talk I will examine this new idea in more detail.  Hart, M.H., 1975. Explanation for the Absence of Extraterrestrials on Earth. Quarterly Journal of the Royal Astronomical Society, Vol. 16, p.128-135.