Alien Earths - Lisa Kaltenegger ****

After an introduction to exoplanets - planets orbiting stars other than the Sun - Alien Earths concentrates on the theory of and the search for life on these planets. Written by astronomy professor Lisa Kaltenegger, who runs the Carl Sagan Institute for the Search for Life in the Cosmos at Cornell University, it's a gentle guide to one of the most imaginative aspects of astronomy.

A fair amount of the content looks at what makes a habitable planet (which is not always an Earth-like situation), what life is and how we may be able to detect it a great distance. There are some good details here, though I would have appreciated more depth. Given this is such a speculative subject, there is also relatively little questioning of assumptions. For example, there's a description of the gold records sent out on Voyager 1 and 2, with detail of how to put a time interval across to aliens. 'The team [behind the records] solved this problem by using a time constant that any spacefaring civilisation should understand.' This involves the hyperfine transition 'spin flip' of hydrogen - but I've never met a human who could interpret the vague visual supposed to illustrate this, so I'm not sure how any alien could 

Kaltenegger's style is chatty - if anything, for me, it tries too hard in this respect. Each chapter starts with a fairly hefty chunk of supposedly engaging fluff. For example, the first proper chapter begins 'The foam on my Portuguese espresso tastes a little bitter, but I hardly notice. For the past hour, I've been staring at images on my computer screen, a live feed from NASA's recently launched James Webb Space Telescope (JWST).' Rather than make me feel warm and fuzzy, this just puts my back up a bit - I really don't care about someone else's coffee. A couple of paragraphs later, when she is watching the launch, we are told 'I kept reminding myself to breathe - in and out, in and out.' I think a little study of biology would suggest that no reminding is necessary with autonomous biological systems. It's just a bit overblown.

The other issue I have is about how speculative much of the theorising about alien life inevitably has to be. I appreciate we can't wait for flying saucers over Washington before we give any serious thought to what's out there, but I prefer my science to be more grounded in evidence. Nonetheless, lots of people do like a good dose of speculation (many science magazines thrive on it), so I appreciate that for others this is great fun.

All in all, Kaltenegger does a good job of making the topic approachable and interesting. It's not really for me (and if I were thinking only for myself I'd give it three stars), but for the right audience it does an excellent job.

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Review by Brian Clegg - See all Brian's online articles or subscribe to a weekly email free here