Around half a century ago, when NASA's Apollo missions were taking brave men to play golf on the moon, the idea of extraterrestrials was the province of science-fiction authors and acid-tripping dreamers.

How times change. Today, the press is full of stories about planets and moons, near and far, pregnant with the possibilities of life.

We read about abundant hydrogen on Saturn's moon Enceladus, a sign, perhaps, of microbial activity. We note another of Saturn's moons, Titan, likely boasts a subterranean ocean, surely the harbour of something twitching.

Then we thrill to the suggestion that the planet Proxima b, four light years away, could support life if its climate is right. And we wonder, enthralled, about the Trappist-1 system, seven planets orbiting a star 39 light years distant, a couple of which, astronomers tell us, are in just the right orbits to be warm and wet and friendly.

The expectation now is palpable: surely, within our lifetimes, we will detect at least a bacterium, some weird arsenic-based extremophile by a thermal vent in an underground sea, or a smear of purple anaerobic slime beneath a rock on the edge of a liquid methane lake.

But is this expectation based on solid science, or romance, or a barely sublimated need to assert that we are not alone in a vast and godless universe?

"The change is for no reason that I can put my finger on," says Paul Davies. "I've lived through this pendulum swing. Now the whole scientific community is charging down the direction that holds the universe is teeming with life, and I'm now the voice of caution, saying: 'Hold on, hold on, you have no basis for that belief'."

Davies' objections are not to be lightly dismissed. He is, after all, perhaps the most distinguished astrobiologist alive, not to mention a quantum physicist of such import that two of the field's fundamental constructs bear his name (the Fulling-Davies-Unruh effect, and the Bunch-Davies vacuum state, for those taking notes.)

Based these days at the University of Arizona's Centre for Fundamental Concepts in Science, he is also the head of the Search for Extraterrestrial Intelligence (SETI) Post-Detection Science and Technology Task Group of the International Academy of Aeronautics.

And years ago he used to teach at Adelaide and Macquarie universities. So, even though he lives in the United States and was born in England, we can kind of, sort of, claim him as an Aussie. It's not a sustainable claim, of course, but, he would suggest, it has more evidential weight behind it than the notion that the cosmos is crawling with amoeba-things and all that's left to do is find them.

Back in the '60s, Davies says, most serious astronomers were convinced there were planets throughout the universe, even though technology at the time was insufficient to detect them.

Indeed, in 1961, an astronomer called Frank Drake came up with an equation to predict the probability of intelligent life emerging anywhere in the universe. In his 2010 book, The Eerie Silence, Davies calls it "not so much an equation in the conventional mathematical sense, more of a way to quantify our ignorance".

Seven years later, he maintains that we're still thick.

"Nothing has changed except our expectations," he says. "The big unknown is the fraction of planets on which life emerges, and we have no idea what that number is and no way to quantify it because we don't know what the process was. You can't estimate the odds of an unknown process. We know almost no more about it today than we did in the '60s."

The difficulty in all this – and perhaps the only point of consensus among astrobiologists – is that we don't know how groups of molecules, non-living matter, coalesce and combine to become living. What is the animating spark? Or, more prosaically, what is the minimum level of complexity for a living organism?

The main reason for this shortcoming is that the only life we have to study is the stuff found here on Earth. Given that everything, from cholera bacteria to Bactrian camels to Justin Bieber, descended from a single common ancestor, we have a sample size of just one – which is a hopeless number from which to draw generalised conclusions.

Without another evolutionary tree to compare, Davies points out, we have no idea what aspects of Earth life are specific just to our mob, and what aspects are "law-like" – common to all life right across the universe.

This is very much his field: he is tasked with examining the fundamentals of science (he also advises space agencies, and pursues cancer research, but let's stick to the big picture here).

Lately, this work has been taking him in a fascinating, if controversial, direction. In a recently published paper – a sketch, really, for a book he is working on – he and colleague Sara Imari Walker suggest that the real law-like essential for life to emerge anywhere in the universe is not biology or even chemistry, but information.

The processing of information, the pair wrote, might be the causal agent at the bottom of all life. Things such as water, carbon and DNA might be just local-level stuff, not required by extraterrestrial life forms.

Davies and Walker also make the point that if their assumption is correct then, even if we do encounter alien organisms, we quite possibly won't recognise them. The problem of life, they conclude, might "ultimately not be reducible to known physical principles".

"That's principles not yet known, rather than principles that can never be known," Davies is quick to point out. "We're open to the suggestion that there may be new physics that will be uncovered by studying biology. That's different to saying that all of biology can be explained by using physics already known."

It is entirely reasonable, he says, to define all living things as information-processing systems. It is also reasonable to define them, as physicists do, in terms of energy and matter and the forces that govern their interactions.

But suggesting the two explanations are simply different ways of expressing the same thing is wrong.

"Where you can't get away with that is with the origin of life," he says. "Because, somehow, out of blind and purposeless forces at the molecular level, somehow out of that sort of melee, an informational system had to emerge."

Probing that mystery, he suggests, is where new physics principles will be found.

Other things, too, may emerge, not the least of which could be a profoundly new discussion around the relationship between science and religion.

Most of Davies' colleagues are atheists, if for no other reason than because physics seems by definition to exclude the supernatural. He himself, however, has long had a subtler, more nuanced position. In his 1992 bestseller, The Mind of God, he suggested that conscious beings manifest self-awareness in the universe and that, thus, "we are truly meant to be here".

The use of the word "meant" occasioned much spirited debate, on the not unreasonable grounds that it implied a creative intention – and a creator, perhaps – buried deep in the workings of the cosmos.

For Davies, however, it is not he but those who work beside him who are falling prey to religious thought – precisely because they believe the universe is teeming with life. "Once we get rid of the idea of miracles, are we happier with the idea of life as a freak accident on one planet alone," he asks, "or are we more comfortable in a universe that is fundamentally bio-friendly?"

It's a critical question, he points out. The odds of inanimate molecules transforming into living matter are gazillions to one. The odds of it happening more than once are gazillions-squared to one, and the odds of it happening billions of times are infinitesimal.

For that to come to pass, there has to be something more, a life-friendly principle at work that rises above brute probability.

"That's a religious position," he says.

"Now, I would feel very comfortable living in such a universe, knowing that we're not the result of some freak chemical quirk. We would know that we're actually deeply embedded in the great cosmic scheme of things – part of the grand cosmic plan, if you like.

"So, for me, the notion that life is intrinsically built into the nature of the universe is deeply pleasing, and close to a religious belief. But that's also the reason that I'm so sceptical."

Paul Davies will be one of the speakers at an event titled Life on Mars: NASA's 2020 Rover Mission, on August 17 at the Sydney Opera House. Tickets on sale now.