In between are more squishy details like how many habitable planets there are per planetary system (one to five they said), and what fraction of those habitable planets develop life, intelligence and the technology to communicate with other worlds. The Dolphins pegged those last three probabilities optimistically as, respectively, 100 percent, 100 percent, and 10 percent to 100 percent (dolphins, for example, don’t build radio telescopes). Multiply all the factors together and you get the putative galactic census.

In the realms in which astronomers have actually gotten new data, the old guesses of the Dolphins have held up very well, Seth Shostak, an astronomer and spokesman at the institute, explained. In the more sociological and biological realms, where the data are ambiguous or nonexistent, you can’t prove they were wrong.

“These guys were either enormously lucky or amazingly prescient,” he said. One change, he said, was in the notion of a habitable world, that is to say, one with liquid water. In the old days “habitability” meant a planet had to be small and rocky and in a narrow Goldilocks zone around its star where the temperature would be just right.

Image Credit... Tom Gauld

Astronomers say space missions like NASA’s Kepler, scheduled to be launched next year, will determine the frequency with which these Goldilocks planets occur in our neck of the galaxy. But the possibilities have expanded since spacecraft discovered evidence of water on or in some of the moons of Jupiter.

At the same time, scientists discovered that life on Earth was tougher and more versatile than scientists had thought, thriving in weird places like boiling undersea vents. “There is so much evidence for lots of pathways to the origin of life,” Dr. Drake said.

But how often does intelligent and technological life actually emerge from such environments? Some evolutionists, like Stephen Jay Gould, who died in 2002, have argued that intelligence is not inevitable. The dinosaurs did just fine for 150 million years without getting appreciably brainier.