Editing Drake equation (section)

===Possible former technological civilizations===
In 2016, Adam Frank and Woodruff Sullivan modified the Drake equation to determine just how unlikely the event of a technological species arising on a given habitable planet must be, to give the result that Earth hosts the ''only'' technological species that has ''ever'' arisen, for two cases: (a) this Galaxy, and (b) the universe as a whole. By asking this different question, one removes the lifetime and simultaneous communication uncertainties. Since the numbers of habitable planets per star can today be reasonably estimated, the only remaining unknown in the Drake equation is the probability that a habitable planet ''ever'' develops a technological species over its lifetime. For Earth to have the only technological species that has ever occurred in the universe, they calculate the probability of any given habitable planet ever developing a technological species must be less than {{val|2.5|e=-24}}. Similarly, for Earth to have been the only case of hosting a technological species over the history of this Galaxy, the odds of a habitable zone planet ever hosting a technological species must be less than {{val|1.7|e=-11}} (about 1 in 60 billion). The figure for the universe implies that it is extremely unlikely that Earth hosts the only technological species that has ever occurred. On the other hand, for this Galaxy one must think that fewer than 1 in 60 billion habitable planets develop a technological species for there not to have been at least a second case of such a species over the past history of this Galaxy.<ref>{{cite news |url= http://phys.org/news/2016-04-limits-uniqueness.html |title= Are we alone? Setting some limits to our uniqueness |date= 28 April 2016 |publisher= phys.org }}</ref><ref>{{cite episode |title= Are We Alone? Galactic Civilization Challenge |series= PBS Space Time |date= 5 October 2016 |network= PBS Digital Studios }}</ref><ref>{{cite news |author=Frank |first=Adam |date=10 June 2016 |title=Yes, There Have Been Aliens |work=The New York Times |url=https://www.nytimes.com/2016/06/12/opinion/sunday/yes-there-have-been-aliens.html}}</ref><ref>{{cite journal |author=Frank |first1=Adam |last2=Sullivan III |first2=W. T. |date=22 April 2016 |title=A New Empirical Constraint on the Prevalence of Technological Species in the Universe |journal=Astrobiology |publication-date=13 May 2016 |volume=16 |issue=5 |pages=359–362 |arxiv=1510.08837 |bibcode=2016AsBio..16..359F |doi=10.1089/ast.2015.1418 |pmid=27105054}}</ref><ref>''Bioverse: How the Cellular World Contains the Secrets to Life's Biggest Questions'' William B Miller Jr. {{ISBN|9781633887992}} p50</ref>