The first evidence that water might have existed on Mars goes back to 1972 when Nasa's Mariner 9 orbiter took images of a series of canyons over about 4,020 kilometres. Later missions supported this theory, much to the delight of scientists.
Of course, no direct evidence of extraterrestrial life has been found, but scientists at Washington State University have now devised two rating systems of the moons and planets that have the highest probability of harbouring little green men.
Among them are Saturn's moon Titan and the exoplanet Gliese 581g, a mere 20.5 light-years away in the constellation Libra, the report in the journal Astrobiology proposed. Should any of these cases be confirmed, the existence of aliens is suddenly not such a ridiculous notion.
The author Arthur C Clarke often lamented fellow science fiction writers' lack of imagination when it came to imagining extraterrestrial beings. There was no reason, he argued, that they would be in humanoid form, irrespective of the size of their heads or the colour of their skin.
It would seem that scientists are similarly widening their horizons. It was always assumed all life was carbon-based. Now, however, the possibility that some might be based on other elements, such as silicon, or even on entities unknown to man is openly debated. Admittedly the new classifications are still light-years away from actually finding alien lifeforms. But until ET comes calling at our door, we will remain sceptical.