The recent announcement that scientists have found possible signs of life in the clouds of Venus has resurrected the age-old question: Are we alone? It is a question that has intrigued scientists, philosophers and science fiction writers for centuries. If confirmed, any life on Venus would be a major discovery, akin to finding Earth-like planets around other stars.

It would also be another triumph for modern astronomers, who believe that the processes which gave rise to life on Earth are not unique, and should have happened elsewhere in the cosmos. Even if it were primitive life, as is probably the case with any life on Venus, a confirmation of life beyond Earth would shatter centuries of religious dogma, and be a major milestone in the revolutionary journey that started with Nicholas Copernicus and Galileo Galilei, in which the Earth lost its special place as the centre of the Universe.

Science fiction writers have been talking about life on Venus for centuries. In the second century AD, Lucian of Samosata depicted a war between the armies of the King of the Moon and the King of the Sun over the colonisation of Venus in his novel A True Story (probably the first known example of the genre of science fiction). The German Jesuit scholar Athanasius Kircher imagined Venus was populated with beautiful youth, who had golden hair and dressed in garments made of crystals in his 1656 work, Itinerarium Exstaticum.

In 1787, Swedish scientist-theologian Emmanuel Swedenborg published Earths in our Solar System and talked of two groups of men on Venus – one group being mild and humane, and the other being savage and brutal. And in the last century, multiple science fiction authors have imagined life on Venus.

The recent announcement doesn’t say astronomers have seen actual life on Venus. Instead what they have observed is evidence of a bio-marker called phosphine in the planet’s clouds.

On our planet, phosphine is very hard to produce by non-biological processes. The same would likely be the case for Venus. (Giant planets such as Jupiter have a lot of hydrogen, and are known to produce phosphine; however, on Venus and Earth, there is very little hydrogen and any mechanism for producing phosphine is likely associated with microbial life.) Plus any phosphine that is being produced on Venus is likely being continually replenished, because Venus’s atmosphere is mostly carbon dioxide and phosphine would chemically combine with it.

Venus is an obvious place to look for life beyond Earth. As a planet, Venus is Earth’s twin, with a similar size and structure. It is closer to us than Mars, one of the current favourites of astronomers looking for life elsewhere in the Solar System. (The icy moons of Jupiter and Saturn are also possible places.) Venus is also closer to the Sun, which provides the necessary warmth for life as we know it.

Over the years, some scientists have maintained that Venus was a source of primordial life. This theory, called lithopanspermia, says that life originated on Venus and was later seeded on Earth. It never gained popularity because Venus is presently very inhospitable for life.

Venus has a runaway greenhouse effect because of the high concentration of carbon dioxide in its atmosphere. Its surface is incredibly hot, hotter than ovens, and would kill life as we know it. And the clouds in its atmosphere are acidic. So scientists turned their attention elsewhere, but no definitive signs of life have been found anywhere else.

Now Sara Seager of MIT and her colleagues are saying that there could be some resilient microbes in the clouds of Venus that are producing the tell-tale phosphine. These microbes would live high above the scorching temperatures of Venus’s surface, but would have the ability to live in a very acidic environment. Hardy microbes are seen living in extreme conditions near geothermal vents on Earth, so this is not implausible. In fact, the astronomer Carl Sagan and biologist Harold Morowitz painted a similar scenario in the late 1960s.

Of course, it is also possible that there is some unknown chemical reaction that is creating the tantalising phosphine on Venus. Planetary scientists and chemists agree that more follow-up observations are needed of Venus.

A few decades ago, there was a lot of interest in extraterrestrial life, and there were listening programmes for alien broadcasts. The Search for Extraterrestrial Intelligence (SETI) was federally funded until 1993, and has continued with private support. In the heyday of SETI, its proponents argued that intelligent civilisations at least as intelligent as the human race must exist. A further premise was that those civilisations would be interested in communicating with us and we would have the technological ability to recognise the broadcasts.

Enrico Fermi, a Nobel prize winner in physics, had argued that if these premises were indeed true, we should have heard the first extraterrestrial broadcasts soon after the surveys were carried out, and the absence of any signal implies that the assumptions might be false.

One could argue that it’s easier to get to less advanced lifeforms like microbes, but so far we don’t have any proof. While the Venus observations have renewed interest in the question once again, for the moment, the issue of extraterrestrial life still remains what it has always been: pure speculation.