Carl Sagan’s 1993 Nature paper has, rather appropriately, a hint of science fiction about it. Twenty years ago this week, Sagan and a team of other astronomers announced that they had found life on a planet in the Galaxy. They used data from the Galileo space telescope to catch clear signatures of methane and carbon dioxide in the planet’s atmosphere and abundant water in frozen and liquid states on its surface. They even confirmed the presence of radio emissions emanating from it — the canonical autograph of intelligence.

This month’s Nature PastCast — one of a series of special audio treats available for free on the Nature website — recounts the tale. The twist, of course, was that this planet was Earth. Sagan and his team were trying out a method for finding life on other planets, using Earth as a calibration for future missions that might explore the depths of the Galaxy for signs of life.

Those were not friendly times for thinking about life elsewhere. At the time, the US Congress was debating whether to cut federal funding for NASA’s SETI programme, the search for extraterrestrial intelligence. So Sagan and his team set about their task in as objective a way as they could, notwithstanding their foregone conclusion. They were careful to declare that “life is the hypothesis of last resort”, and to show that this was a scientific question that needed an answer.

The bigger question, of course, goes unanswered, although not for want of trying. SETI was launched in the late 1950s, propelled by the optimism of the space age. In 1959, a paper in Nature by Giuseppe Cocconi and Philip Morrison suggested that if civilizations elsewhere wanted to contact Earthlings, they would probably use electro­magnetic signals. “We shall assume that long ago they established a channel of communication that would one day become known to us, and that they look forward patiently to the answering signals from the Sun which would make known to them that a new society has entered the community of intelligence,” they wrote.

Soon after, astronomer Frank Drake was preparing for one of the first conferences to address the search for extraterrestrial life. As a loose agenda, he came up with a list of unknowns that would need to be resolved in order to predict whether intelligent life exists elsewhere in the Universe. For example, how many star systems exist that are suitable for the development of intelligent life? How many Earth-like planets are in orbit around them? What is the probability of life sparking into existence on any of them? Drake then formulated an equation that created a mathematical framework for such unknowns.

Research ongoing since Sagan’s paper is making Drake’s equation more solvable today than it has ever been. The control test was performed, so astronomers know that their tests for life would work. Meanwhile, the first exoplanet was found in 1992, and hundreds have been spotted since.

Scientists can use variants of Sagan’s prescient control test to characterize the atmospheres and locations of exoplanets whizzing around their stars. Are we now in an era not of space-age optimism, but of realism? Life is still the hypothesis of last resort for astro­biologists. But if they find none, they will not be disillusioned. It would be just as interesting, they say, to find that habitable-looking environments do not all sprout life, and that Earth is unique in being so full of it.