How to Find a Habitable Planet

Are there other planets like the Earth in our Galaxy, and if so, do any of them harbour life? These are fundamentally important questions, and as James Kasting describes in How to Find a Habitable Planet, scientists are on the verge of being able to answer them.

Kasting, a geoscientist, has spent his career studying how the environment of the Earth has changed over the planet's history, and he has been involved in the search for habitable worlds beyond our solar system. In particular, he performed some of the seminal work in calculating the extent of the life-friendly region around a star, where an orbiting planet receives neither too much nor too little heat, but just the right amount of warmth for liquid water and life on its surface. This so-called "habitable zone" of a star is one of many crucial aspects that determine whether a planet could support life, and Kasting builds on his expertise in this area to lead the reader on a tour of exactly what makes a world habitable.

The right kind of star is important, as is a planet large enough to hold on to an atmosphere and regulate its climate over billions of years. But Kasting makes a stand against arguments such as those put forward in Peter Ward and Donald Brownlee's Rare Earth: Why Complex Life is Uncommon in the Universe, that many of the chance circumstances of the Earth render it special in the cosmos, perhaps even uniquely so. He counters the assertions that a habitable world must have a protective magnetic field, or possess a large moon to help keep its spin stable, or share its solar system with a large gas giant planet like Jupiter to spare it the pounding of too many comet impacts. With these arguments in mind, the probability that life is smeared across plentiful wet rocks throughout the Galaxy seems greatly improved.

Kasting explains how life itself has played a key role in maintaining the habitability of the Earth by regulating our planet's climate, even as the Sun has brightened by about 30 per cent over the history of the solar system. Paradoxically, life may hasten its own extinction long before the Sun exhausts its hydrogen fuel. Photosynthetic life, such as cyanobacteria and plants, are sucking more and more carbon dioxide out of the atmosphere until eventually it will become a limiting resource, causing the collapse of much plant life and their ecosystems.

More importantly for observational science, Kasting discusses how astronomers could actually detect other habitable worlds in our Galaxy. Several different planet-spotting techniques have already discovered more than 400 new worlds, and the discovery of a truly Earth-like twin is already within our grasp. The next step, he explains, is to look for telltale signatures of life in the planet's atmosphere: a mixture of oxygen and methane in the air. In this way, telescopes could discover a living world in the next two or three decades.

This is a very well-written book that serves perfectly as an introduction for the lay reader to astrobiology and the search for life beyond Earth, and it also presents some important and up-to-date material for the specialist researcher. Of particular note for me is Kasting's rebuttal of several aspects of the Rare Earth hypothesis and his discussion of the features that truly are crucial for planetary habitability.

This debate will surely only intensify as our telescopes discover more and more potentially habitable worlds. Could alien life ever emerge beneath a dim red sun or moonless skies?