Planets in the CHZ remain of paramount interest to researchers looking for intelligent life elsewhere in the universe. The Drake equation, sometimes used to estimate the number of intelligent civilizations in our galaxy, contains the factor or parameter , which is the average number of planetary-mass objects orbiting within the CHZ of each star. A low value lends support to the Rare Earth hypothesis, which posits that intelligent life is a rarity in the Universe, whereas a high value provides evidence for the Copernican mediocrity principle, the view that habitability—and therefore life—is common throughout the Universe. A 1971 NASA report by Drake and Bernard Oliver proposed the "water hole", based on the spectral absorption lines of the hydrogen and hydroxyl components of water, as a good, obvious band for communication with extraterrestrial intelligence that has since been widely adopted by astronomers involved in the search for extraterrestrial intelligence. According to Jill Tarter, Margaret Turnbull and many others, CHZ candidates are the priority targets to narrow waterhole searches and the Allen Telescope Array now extends Project Phoenix to such candidates.