This is the Nature Pastcast, each month raiding Nature's archive and looking at key moments in science.

In this show, nature is concerned with finding extraterrestrial life.

Nature, volume 365, 21 October, 1993.

So in October of 1993, so in October of 1993, Carl Sagan and several colleagues published a research article in nature with really a remarkable discovery.

They found highly suggestive, perhaps conclusive evidence for having found life on a planet in the universe.

The Galileo spacecraft found evidence of abundant gaseous oxygen, a widely distributed surface pigment, and atmospheric methane, an extreme thermodynamic disequilibrium.

Moreover, the presence of narrow band, pulsed, amplitude-modulated radio

transmission seems uniquely attributable to intelligence. My name is David Kaiser. I teach physics and

the history science at the Massachusetts Institute of Technology. Remarkably enough, in the 1990s, you could publish an article claiming you'd found life on Earth.

A search for life on Earth from the Galileo spacecraft.

Which was en route to Jupiter, but sort of fortuitously because of the particular orbital path that it would have to take to get there,

it had to sort of linger relatively close to the Earth.

So it was in space and looking at its near neighbor, the planet Earth.

So this is a quite classic paper that uses the Earth as a test case.

Can you detect life on the Earth?

So they used a Galileo spacecraft spun it round, pointed it back to the Earth,

and asked a very simple question, can we see any signatures of life?

And of course, they did.

I'm Charles Kakel and I'm an astrobiologist at the University of Edinburgh.

Now we're entering into a period of finding Earth-like planets around other stars.

It's obviously an area that should be of interest to biologists about the implications of finding Earth-like planets and looking for life on them.

Carl Sagan, W. Reed Thompson, Robert Carlson, Donald Gernett, and Charles Horde.

...