The concept, widely used as a rule of thumb by astronomers, was codified last year by Dr. Mendez and his colleagues into a more formal Habitable Zone Distance scale.
It also said the units would create businesses which were "of sufficient scale to be efficient, but which are easier to manage as they will be operating in the same time zone".
Researchers at the U.K.'s University of East Anglia now are working on a scale they call the "dwell time index" to calculate how much time a planet has orbited in a habitable zone, compared with the time it took life to evolve on Earth.