The Drake equation estimates the number of active, extra-terrestrial civilizations that might be contactable. It was proposed by astronomer Frank Drake in 1961, as a way to stimulate scientific dialogue at a meeting on the search for extra-terrestrial intelligence (SETI). At that time, only a few of its parameters were known with any real accuracy. Recent advances in exoplanet studies provide new constraints on all astrophysical terms in the equation.
Adam Frank and Woody Sullivan use these new constrains to set a lower bound on the probability that more than one (us) technological species have evolved anywhere and at any time in the history of the observable Universe(1). Instead of focusing on how many technological species currently exist, they ask: How often in the history of the Universe has evolution ever led to a technological species, whether short or long-lived?
They estimate that, as long as the probability that a habitable zone planet develops a technological species is larger than ~10-24, then humanity is not the only time technological intelligence has evolved in the observable universe.
(1) Frank, Adam, and W. T. Sullivan III. “A New Empirical Constraint on the Prevalence of Technological Species in the Universe.” arXiv:1510.08837 [astro-Ph, Physics:physics], October 19, 2015. http://arxiv.org/abs/1510.08837.