12 April 2016


John Asher Johnson. How Do You Find an Exoplanet? Princeton University Press, 2016.

The author was asked to write this book because of the large number of students who want to study exoplanetary science for the same reasons that exoplanets are so popular among the general public, because their interest has been aroused by science fiction and by the possibility of "answering some of humankind's most ancient questions about our place in the universe".
The treatment of this subject is aimed at students who have a good knowledge of physics, which of course includes the possession of the appropriate mathematical skills. Johnson's teaching method is based on the idea of describing the orbits of planets as circular rather than elliptical so that " . . . at the outset the focus should be on the basic physics rather than the detailed mathematics".

To illustrate the kind of work he does, Johnson gives us a description of a night spent observing a star catalogued as HD94834, in the constellation Leo. Modern observational astronomers spend far less time at the telescope than earlier ones, as the guiding of the telescopes, and recording images of stars and other data, are now done by computers. Just as much time, though, is spent planning the observing schedule, analysing the data obtained, and publishing the results.

Johnson's interest in HD94834 was because of the possibility of detecting planets orbiting it. He first started observing it in 2007, when its velocity appeared to be constant. In 2008 a decreasing velocity was detected, which indicated the presence of an orbiting planet. He goes on to show the reader (at least, the mathematically competent reader) how it is possible to work out not only the mass of the planet, but its distance from the star and the eccentricity of its orbit.

He then goes on to try studying the planets that transit their stars as viewed from Earth. This might seem to be just the detection of a slight drop in brightness as the planets transit them, but we are told that by taking repeated accurate measurements of transits it is possible to discover information about the physical characteristics of the planet, as well as information about its orbit. Johnson's work led to improvements in the accuracy of these measurements and there were further improvements resulting from the use of the NASA Kepler space telescope, which removed the distortions caused by viewing through the Earth's atmosphere.

A particularly interesting finding from the studies described in this book is the data obtained about the distribution of planet sizes. This shows that small planets of 1 to 2 Earth radii are an order of magnitude more abundant than giant planets, those bigger than 4 Earth radii. The finding that Earth-sized planets are common throughout the galaxy was confirmed by different kinds of surveys, and indicates that planets on which life might be possible are quite common.

This book will probably appeal most strongly to science students with an interest in recent developments in astronomy. -- John Harney.

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