How Do You Find an Exoplanet?,
In How Do You Find an Exoplanet? John Asher Johnson explains the four key methods that have been used to discover exoplanets—planets that orbit around stars other than the Sun. Those are the radial velocity method, observations of transits, analysis of gravitational microlensing, and direct imaging. As Johnson states in his preface, many books explore the subject matter in much greater depth and complexity. His aim, however, is to make it accessible to anyone with an understanding of freshman physics. In that aim he succeeds. Readers with an understanding of Kepler’s laws or even just Newtonian gravity will find the topics covered to be comprehensible and the progression of topics easy to follow.
The book starts with a short description of how Johnson first became interested in astronomy and, in particular, how he “discovered” his first planet—Jupiter—shining like a bright star in the night sky. The story helps readers identify with the author and points out that even those who grew up in a city with hardly any awareness of the nighttime stars above could be bitten by the astronomy bug. The introduction then gives a brief history of astronomy that includes a concise overview of the Copernican revolution. Even if the historical material is familiar to many readers, Johnson’s perspective serves to illustrate how scientific ideas and discoveries progress and, in particular, how they can challenge our perceptions of our place in the universe. The introduction is also a good review for readers who need a brief refresher on the basics of Kepler’s laws.
Johnson writes in a familiar tone and includes anecdotes from his own career as a professional astronomer on the hunt for exoplanets. At times, I could imagine myself in one of Johnson’s classes, listening as he explains in his conversational style the theory behind the exoplanet discovery techniques and points to his whiteboard sketches to illustrate the basic concepts. That said, I should clarify that the diagrams in the book are by no means straight off a whiteboard. Rather, they are mostly clear, simple, well-labelled line diagrams.
I particularly like how Johnson starts from basic principles and explains jargon terms; he even includes a glossary at the back of the book in case readers forget. Reading a book that actually starts with simple approximations is a refreshing change. Once the general concept is clear and examples are given to provide approximate numerical values of observables, Johnson adds to the foundation by introducing terms such as eccentricity and angles of inclination to better describe real-life systems.
The ordering of the chapters and the number of pages dedicated to each discovery method approximate the order of how successful the technique has been in finding exoplanets. Currently, microlensing and direct imaging are competing for third place, although I expect direct imaging to become the dominant discovery method as scientists overcome the technological hurdles. And that brings me to my only quibble with the book: I would have liked a few more pages spent on direct imaging. That method had the fewest pages dedicated to it and, as a consequence, the discussion of it felt overly concise. For example, Johnson does not mention the giant telescopes expected to come on line in the next 10 years. A discussion of those telescopes, whose primary mirrors have diameters of 20–40 m, would not only be an inspiring glimpse into future endeavors, it would also be a relevant addition to the description of the resolution and sensitivity required to directly image exoplanets.
From its cover, you may not immediately realize what How Do You Find an Exoplanet? is about. You’ll find no depictions of imagined far-off star systems and alien-looking worlds, as is common for books on exoplanets. The paper is not glossy and the text and diagrams are black and white. The minimalist styling works in the book’s favor; no distractions or superfluous images divert readers’ attention from the important key concepts. I will certainly recommend the book to students interested in understanding exoplanet discovery.
Samantha J. Thompson is a research associate in the exoplanets research group at the University of Cambridge, UK. She is currently overseeing the building of HARPS3, a high-resolution spectrograph that will be used in the 10-year Terra Hunting Experiment designed to find Earth-mass planets orbiting nearby dwarf stars.