SEARCH

SEARCH BY CITATION

inline image

The exoplanet handbook , edited by M. Perryman . Cambridge, UK : Cambridge University Press , 2011 , 410 p . , hardcover (ISBN: 978-0-521-76559-6 )

The modern exoplanet research is a relatively young and quickly evolving discipline that is rarely discussed in a global context. This is probably because, in less than two decades, this field has produced more than 6000 papers on very different issues, but all related to exoplanet findings. The well-known exoplanet researcher Michael Perryman, visiting professor at the University of Bristol (UK), has recently completed an updated compilation of what we know about the existence of planets beyond our solar system. The story about the discovery of first planetary mass objects started about twenty years ago when first real detections were reported. Surprisingly, the first confirmed planetary system was discovered around a pulsar, just at the place where astrophysicists never had thought to discover planets due to the catastrophic origin of such a star (due to the collapse and later explosion of a giant star). The official start of this promising discipline was also marked in 1995 with the discovery of 51 Pegasi b around the star of the same name using the radial velocity technique. Two additional planets were found that same year and thirty-four at the end of past century. A new field of research was born in which about 500 additional planets were discovered during the first decade of the twenty-first century. Such a success, despite the current bias and intrinsic limitations of most of the techniques, is clear evidence that planets are as common as stars. It is a fascinating achievement that is explained in this book.

As Prof. Perryman explains well in the preface, his book tries to tie the different fields of exoplanet research together with a descriptive compilation of the key aspects of observation, technology, and theory. In my opinion, he achieved his goal and, despite having in our hands a handbook, he wrote a coherent and methodic compendium of the current state of exoplanet research.

The book is divided into eleven chapters, including a synthetic introduction; the first chapter nicely compiles the most relevant techniques and aspects concerning this field.

The second chapter deals with the radial velocity method. A very well-conceived theoretical description is usually given in each chapter, also explaining the pros and cons of each observational technique. When describing the radial velocity method, an overview of early, ongoing, and future search programs is made, and also points out the current limitations of the technique and the future goals. It ends with a description of the capabilities of radial velocity to detect multiple planets in other stars as a function of component masses, the mass and spectral type of the star, etc.

The third chapter describes another successful technique to detect planets by measuring the transverse component in the displacement of the host star due to gravitational perturbations of orbiting planets. Accurate astrometry of stars is able to measure subtle positional variations, also providing key information about the planetary mass and orbital parameters. This chapter ends describing the current achieved accuracy of the most powerful interferometers on the ground together with the astonishing results obtained from space by the Hipparcos satellite (that operated between 1989 and 1993) or by using the fine guidance sensors on the Hubble Space Telescope.

Chapter 4 deals with the timing technique that is applicable to sources that also exhibit periodic signatures and that consequently experience a change in the measured period as a consequence of the movement around the system’s barycenter. In this category, radio pulsars, pulsating stars, and eclipsing binaries can be sampled for exoplanets in their neighborhoods. In fact, as stated before, the first unequivocally identified planetary system (reported in 1992) was orbiting the millisecond pulsar PSR B1257+12. It revealed that the massive emission of stellar material produced during that violent supernova explosion made the accretion of planetary bodies possible around the remaining pulsar. Such a scenario is not unique, and so far about ten planets around six stars have been found using this technique.

Microlensing is the focus of the fifth chapter. This technique is based on the distortion of space-time by discrete sources located between the observer (ourselves) and the targeted star. It is a one-chance event: when we observe a distant stellar field there is a possibility that a nearby star crosses that monitored field. The gravitational field of this casually aligned star participates in magnifying the signal received from the background source. Depending of the geometry of this alignment, we can recognize different regimes. Strong lensing allows us to discern an individual object and constrain its properties. In the most favorable cases, it is possible to define with accuracy the mass of such an object. So far, eleven planets ranging from three Earth masses to about four Jupiter masses have been detected. Despite the complexity of this technique and the geometrically limited achievements, the very wide planetary mass range discovered with this technique makes it encouraging to the community.

The continuous monitoring of star luminosity, also called photometry, allows detecting exoplanet transits in those systems where the planet’s orbital plane is aligned with the observer. This casual configuration geometrically happens with a chance of about 1‰. During a transit, the light from the host star is attenuated by the planet crossing in front of it. Usually it produces a rather weak drop of the star flux that for a solar-mass star and a planet like Jupiter is about 0.01 magnitude. Obviously, such a minimal attenuation could have been only achieved in the past by precise photometers, but modern CCD cameras with accurate photometric procedures are able to detect variations of about one thousandth of magnitude, even for medium-sized telescopes. Increasing technological capabilities have made such systems affordable to observatories all around the world, and also to amateurs who are cooperating successfully in professional tasks. The first planet found with this technique, HD 209458, occurred in 1999 and was a Jupiter-mass planet orbiting very close to its star. Chapter 6 describes in great detail the different photometric searches performed so far, the ongoing programs on the ground and in space, the theory associated with the variable geometric configurations that produce the transit light curves, and the transmission and emission spectroscopy that allows us to learn about the atmospheres of transiting planets.

The seventh chapter deals with the direct imaging of exoplanets. This technique is really challenging because of two crucial factors. First, we are observing other planetary systems in our own galaxy from typical distances of hundreds of thousands of parsecs. At these distances, the exoplanets typically lie very close in angular terms to their host stars being consequently swamped by the bright stellar glare. The second one is the extremely low ratio of the planet to stellar brightness as the planets are only reflecting a minimum part of the light received from their host stars. Typically, this mentioned ratio ranges from 10−5 in the infrared to 10−10 in the optical. To avoid this second problem, different imaging techniques have been developed, e.g., coronographic masks, and the author gives abundant information about all of them. Finally, the chapter provides a compilation of different results obtained in these different environments.

Chapter 8 summarizes our knowledge about the host stars of exoplanets. Currently, we know that the planets form around a wide variety of stars, not only around main sequence stars like the Sun. A key related issue is associated with the element abundances of these host stars. Astrophysicists are looking for common features that make some stars especially favorable to produce planets, e.g., their metal abundance.

Chapter 9 discusses substellar objects that are too low in mass to develop a stable hydrogen fusion, but in which lower threshold nuclear reaction can occur due to the presence of lithium and deuterium. This category includes brown dwarfs whose masses range between 13 and 80 Jupiter masses, and these objects occupy the transitional dominion between planets and stars. The free-floating objects with planetary masses that are found in star forming regions or young stellar clusters are also discussed.

Chapter 10 deals with the formation and evolution of stars. It begins with the description of disk formation processes and the current classification of Young Stellar Objects (YSOs). Later on, the chapter discusses protoplanetary and debris disks, and the most important concepts that must be known about them, e.g., the minimum mass solar nebula approach. After that, the chapter deals with the different stages in terrestrial and giant planets formation. Finally, it also discusses orbital migration, tidal effects, and their respective consequences to the dynamic evolution of planets.

Chapter 11 focuses on planetary constituents. It is a nice compilation of what can be inferred about the interiors and the atmospheres of exoplanets from remote studies. It starts with the description of the compositions of planets in our solar system, but quickly goes into the increasing complexity of the giant worlds discovered so far. This compilation of the models of giant planet interiors, and how the composition and equations of state allow us to make important predictions about their structure is very interesting. It also nicely describes several new concepts such as the super-Earths, or the ocean planets. The author’s discussion and inferences on the expected atmospheric chemistry for these worlds are fascinating. Finally, the chapter defines many other key concepts, e.g., the habitability, habitable zone, life signatures, etc.

Chapter 12 is a brief compilation of what we know about our own solar system and the implications for other planetary systems. It includes discussions about the composition and structure of solar system planets, the importance of minor bodies, the origin of water, and issues on planetary habitability. The chapter ends with considerations about the remote search of spectral signatures of life in exoplanets. The book ends with an appendix listing the known exoplanets as of November 1, 2010, and a meticulous compilation of references in this field of research.

To summarize, The exoplanet handbook is, in my opinion, a very useful book for researchers interested in the study of planet and star formation. The text nicely outlines our knowledge about extrasolar planets and the many pathways to their formation in the cosmos. Consequently, I also recommend it to graduate students looking for a general description of the importance of dynamics in the formation of stars and planets, and to those interested in an update on the latest exoplanet discoveries.