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“IF YOU can imagine a world that is consistent with the laws of physics,” write planetary scientists James Trefil and Michael Summers, “then there’s a good chance it exists somewhere in our galaxy.”

The universe is mostly dark and empty, but the few pockets that are populated by matter are full of planets. Interstellar space is littered with hard-to-spot rogue worlds, ejected early in their solar system’s history, and they may outnumber orbiting planets two to one. Some experts put this figure at 1000 to one, which may explain why little green men have yet to land on the White House lawn.

So is our planet-cluttered galaxy full of life? Trefil and Summers are obviously primed to receive with open arms any visitors who happen by. In Imagined Life, their second book, they do a splendid job of explaining how tentative our thoughts on exobiology are. Their first book, Exoplanets (2013), is already rather dated, such is the pace of the field.

In just 14 pages of Imagined Life, the authors outline the physical laws constraining the universe. They rattle through how to define life, and why spotting it is so difficult. Most of the molecules identified as a potential biomarker of life have a “nonbiological production mechanism”, they write.

They list environments in which life may have evolved, from water worlds to mega-Earths (expect “normal fish… and stubby dinosaurs”). All this before the meat course: a tour, planet by imaginary planet, of otherworldly life and civilisation.

The authors want to believe in life that is “really not like us”, but have a hard time making it stick. Carbon-based life itself may be pressing against unexpected limits. Of the 140 amino acids, only 22 are central to Earth’s biome; it may be that the mechanisms of inheritance must converge on a narrow set of possibilities, which may also set limits on alien biology.

The trick to finding life in odd places is to dig. Scientists are beginning to abandon the idea life must evolve and persist on the surface, the authors say, as they imagine an aquatic alien civilisation for whom a mission to the surface would be akin to a Mars mission for us.

I’m not sure I buy their assumption that life most likely breeds the kind of intelligence that manufactures technology. Nothing in biology, or human history, suggests that. We may be a colossal oddity. Still, Imagined Life reminds me of my childhood books, full of artists’ impressions of oceans on Venus, only much, much better.