increase over the past 100 years, with most improvements occurring in the past few decades. This is the subject of this book.
The field of exoplanets has developed into one of the most exciting and vibrant fields in astronomy, and that is all owed to the Doppler method. By discovering the first exoplanets, it essentially created the field. Although the detection efficiency of exoplanets using the Doppler method has been surpassed by the photometric transit method, the Doppler method still plays a vital role in confirming transit discoveries and giving a mass, one of the most fundamental parameters of a planet. It is one of the few methods (along with astrometry) that gives you a “dynamical” mass—dynamical in the sense that one derives the mass using the laws of Kepler and Newton, rather than statistics and theoretical models. The Doppler method still ranks as one of the most important exoplanet detection methods in use today.
The pioneering transit-search space missions COnvection ROtation and planetary Transits (CoRoT) and Kepler have produced a treasure chest of transiting planets. As of this writing, NASAʼs Transiting Exoplanet Survey Satellite (TESS) is performing a transit survey among the brightest stars in the sky, and the RV community has its hands full determining the mass for candidate transiting planets. For all of these missions, ground-based spectroscopic measurements, in particular RV measurements, have played a vital role in characterizing the planet discoveries. Within a decade, the PLAnetary Transits and Oscillations of stars (PLATO) mission of the European Space Agency will also search for transiting planets around bright stars, but with the goal of finding Earth-like planets in the habitable zone of stars. So, the Doppler method is poised to continue its important role in exoplanet studies well into the 2030s.
Although almost 1000 exoplanets have been discovered with the Doppler method, this book will not focus on the results from the various RV planet search programs. This can be gleaned from the literature or from Perrymanʼs The Exoplanet Handbook. Rather, this work will focus purely on the method. This includes how one can achieve a high RV measurement precision as well as the challenges, limitations, and potentials of this technique. It will include other aspects of the method, such as instrumentation, wavelength calibration, finding periodic signals in RV time series, interpreting the signals that you find, and Keplerian orbits. An important aspect is stellar variability, which has been known to trick more than a few astronomers (this author included) into thinking that they have discovered an exoplanet. In short, it will cover every aspect needed for one to detect exoplanets with the RV method, a sort of “handbook” for the Doppler method.
If the reader wants to purse RV follow-up of transiting planets from space missions or wants to perform exoplanet RV surveys, this book should be useful. When it comes to exoplanet discoveries, it is easy to fall into traps, to be misled, or to arrive at erroneous conclusions. As the physicist Richard Feynman once famously said, “Science is a way of trying not to fool yourself. The principle is that you must not fool yourself, and you are the easiest person to fool.” This is especially true in the field of exoplanets. I hope that this book will ease the path of those embarking on the use of the Doppler method for the detection and characterization of exoplanets and hopefully, to avoid pitfalls.
Acknowledgments
It is a pleasure to thank all of the scientists and students who helped in the preparation of this book. It would not have been possible without them.
I thank the Tautenburg Observing School: Jaime Avalos, Clark Baker, Dugasa Belay Zeleke, Richard Bischoff, Martin Blazek, Sireesha Chamarthi, Michael Debus, Jana Dvorakova, Vanessa Fahrenschon, Andreea Gornea, Sascha Grziwa, Engin Keles, Hannah Kellermann, Sarah-Jane Köntges, Oliver Lux, Priscilla Muheki, Eva Plávalová Jan Subjak, Jerusalem Tamirat, Fabian Wunderlich, and Jiri Zak. They were kind enough to give up observing time for some crucial tests that are presented in this book.
Silvia Sabotta provided me with an RV time series made with the iodine cell. Priyanka Chaturvedi produced the synthetic stellar spectra used in this work. Figures and data highlighting results from CARMENES were provided by Ansgar Reiners, Ignas Ribas, Guillem Anglada-Escudé, Mathias Zechmeister, and of course, the entire CARMENES consortium. Ulf Seeman provided valuable figures and input for the CRIRES+ gas absorption cell.
A special thanks goes to Michael Hartmann who provided me with results from his PhD. His analysis of two roAp stars nicely demonstrated how the use of different templates for calculating the RV can produce conflicting results. He also provided me with the typesetting of the GLS equations and a short description of the GLS periodograms. This saved me some work.
I thank Patrick Lenz who developed Period04 which was an indispensible tool for the preparation of this book.
I also thank my wife, Ingrid Schutzmann, who tolerated more than a few “book writing” weekends and helped clear time for me.
Finally, a sincere and heartfelt thank you to two very important people: Gordon Walker, who showed us all how to perform precise stellar RVs and whose work was an inspiration to me, and finally, William Cochran, who got me into the business of detecting exoplanets with the RV method. This book is dedicated to them.
Author biography
A. P. Hatzes
Artie Hatzes is one of the pioneers in searching for extrasolar planets and he brings over 30 years experience in the use of precise stellar radial velocity measurements. Besides searching for extrasolar planets, he has also extended the use of these types of measurements to the study of stellar oscillations in magnetic A-type and K giant stars. Hatzes received his Bachelor of Science with Honor from the California Institute of Technology and his Master of Science and PhD from the University of California in Santa Cruz. In 1988 he joined Bill Cochran at the University of Texas at Austin for the start of the McDonald Observatory Planet Search Program. He has been working exoplanets ever since. Hatzes is currently director of the Thuringian State Observatory and Professor of Physics and Astronomy at the Friedrich-Schiller-University in Jena, Germany.
Photo credit: Christian Högner
The Doppler Method for the Detection of Exoplanets
A P Hatzes
Chapter 1
Introduction
1.1 The Dawn of Doppler Measurements
In 1842, the Austrian physicist, Christian Doppler, published his treatise Über das farbige Licht der Doppelsterne und einiger andere Gesterne des Himmels (On the Colored Light of Binary stars and Some Other Stars in the Heavens). Doppler postulated that because the pitch of a sound wave depended on the relative speed between the source and the observer that the color of light of a moving star should also change. Doppler thought that this phenomenon could explain the colors of binary stars. Although wrong about the colors of stars, his hypothesis about the change in the frequency of waves relative to a moving source—and that the effect can be used to measure the velocity of stars—proved true. It was shown to be experimentally correct for sound waves and had an easy theoretical explanation, but not so for electromagnetic waves.
At the same time, Armand Hippolyte Louis Fizeau also became involved with aspects of the discovery of the Doppler effect (known as the Doppler–Fizeau effect in France). He focused his work on understanding the effect as applied to light rather than sound and developed the mathematical formalism underlying the principle. He
