Hart published in 1975 a detailed examination of the Fermi paradox:[1] the contrast between the extreme likelihood of extraterrestrial life somewhere in the universe and the total absence of any evidence for this.
[5] Hart is an advocate of the Rare Earth hypothesis; he proposed a very narrow habitable zone based on climate studies.
He advocated for this hypothesis in the influential book which he co-edited, "Extraterrestrials: Where are They",[6] in particular in the chapter he contributed to it "Atmospheric Evolution, the Drake Equation and DNA: Sparse Life in an Infinite Universe".
[6]: 215–225 Hart's first book was The 100: A Ranking of the Most Influential Persons in History (1978), which has sold more than thousand copies and been translated into many languages.
"[12] Invited speakers included: Lawrence Auster, Peter Brimelow, Steven Farron, Julia Gorin, Lino A. Graglia, Henry C. Harpending, Roger D. McGrath, Pat Richardson, J. Philippe Rushton, Srdja Trifković, and Brenda Walker.