Since the first extrasolar planet was discovered in 1995, astronomers have found more than five thousand others in our cosmic neighbourhood. Planets are so common that they circle most stars, and the Milky Way hosts about two hundred billion stars. This has generated plenty of interest and excitement about alien worlds and the possibility of finding Earth-like planets with extraterrestrial life.

“Alien Earths: Planet Hunting in the Cosmos” is a book about the search for exoplanets written by Lisa Kaltenegger, the Founding Director of the Carl Sagan Institute at Cornell University. She has dedicated her life to this pursuit and exudes great enthusiasm and expertise about the subject.

Lisa begins by discussing the search for alien artificial radio signals and describes the Drake equation which tries to quantify the probability of finding extraterrestrial intelligence. With such a huge number of exoplanets, there is a high probability that extraterrestrial life exists, but the “Fermi Paradox” reflects the discrepancy between this high probability and the current absence of any evidence.

A more fruitful approach to searching for alien life is to carefully study the characteristics of exoplanets as they are discovered. This is more than just a consideration of their parent star, size, mass & orbit. Lisa notes that any widespread biosphere on a planet will change that planet as it did on Earth. Seen from space, our planet presents a combination of gases that scientists can only explain by the existence of life. This has led to the concept of a planet’s “light fingerprint”, i.e. the spectra and albedo of light reflected by its surface.

Lisa realised that astronomers needed what she calls a “colour catalogue of life”. This is a database of diverse Earth biota and information on how they reflect incoming starlight. This can be compared to observations of light reflected by exoplanets, and finding a planet with a very similar light fingerprint to Earth should indicate the presence of life. However, Lisa makes the important point that Earth and the chemical makeup of its atmosphere have changed significantly throughout its existence. For half of Earth’s history, its atmosphere will have shown no telltale signs of life at all. With input from scientists from different fields and with different training and skills, Lisa developed a model that describes how Earth’s light fingerprint has evolved through time. Armed with this and a reference catalogue of 19 diverse bodies in our solar system (all eight planets, nine moons and two dwarf planets), Cornell researchers are now better able to characterise and understand the exoplanets discovered.

In searching for an Alien Earth, it is important to determine exactly what you are looking for, not only having a light fingerprint but beginning with a proper understanding of our own planet, how it formed, how it has evolved and how life has developed on it. Lisa provides all the necessary background information on these matters in the chapters “How to Build a Habitable World” and “What is Life?”.

This book describes the detection methods from the radial velocity method, which led to the discovery of the first exoplanet (51 Pegasi b) by Mayor and Queloz in 1995, to the transit method and direct imaging, which are useful for examining light fingerprints. From the start, there have been surprising discoveries showing planetary systems significantly different to our own. Described as a “Hot Jupiter”, the characteristics of 51 Pegasi b were unlike any planet in the Solar System. The weird and wonderful amongst the growing menagerie of exoplanets are discussed in the chapter “Worlds That Shook Science”, whereas planets most similar to Earth are covered in “No Place Like Home”.

The scientific information is accompanied by personal anecdotes and recollections from relevant events in Lisa’s life, which make the text accessible and give it a conversational tone. Although the information can sometimes appear to be presented in a strange sequential order, Lisa’s enthusiasm for planet hunting shines through, and that makes for a pleasant and informative read. There is no end to this story, of course, as we have not yet found an Earth analogue or another planet with clear evidence of life. Lisa’s epilogue imagines a journey to a hypothetical first alien Earth, although she emphasises that no other planet will ever be as perfectly suited to us as Earth is. A timely reminder to look after the precious planet we inhabit.