An extrasolar planet or exoplanet is a planet orbiting a star (or remnant of a star) beyond our Solar System. As of autumn 2007, about 250 exoplanets had been discovered around 220 different stars, including nearly two dozen multiple planet systems. No less than five exoplanets have been discovered orbiting the star 55 Cancri; one of the planets has nearly four times the mass of Jupiter, another is comparable with Jupiter in mass, two are slightly less massive than Saturn, while the innermost planet has a mass similar to that of Uranus.
Around 2300 years ago, the Greek philosopher Epicurius reflected on the existence of planets around other stars, and of life on those planets:
"There are infinite worlds both like and unlike this world of ours... We must believe that in all worlds there are living creatures and plants and other things we see in this world."
And in the 16th Century, the medieval scholar Giordano Bruno, in his work De l'infinito, universo e mondi, speculated:
"There are countless suns and countless Earths all rotating around their suns in exactly the same way as the seven planets of our system. We see only the suns because they are the largest bodies and are luminous, but their planets remain invisible to us because they are smaller and non-luminous. The countless worlds in the universe are no worse and no less inhabited than our Earth."
Extrasolar planets became a subject of scientific investigation in the mid-19th Century, and although there were some unsubstantiated claims as to their discovery, it was not known how common they were, how similar they were to the planets of the Solar System, or indeed how typical the make up of our Solar System was in comparison with planetary systems around other stars. There was also the question of the habitability of such planets. Were there Earth-like planets orbiting other stars and, if so, could they have the necessary surface conditions to support some form of life?
What actually constitutes a planet? In February 2003, the Working Group on Extrasolar Planets (WGESP) of the International Astronomical Union produced a reasonable working definition of a "planet", agreeing to revise the definition as and when necessary, and as our knowledge improves. The WGESP considered that objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be ~13 Jupiter masses (~13 Mj) for objects of solar metallicity) that orbit stars or stellar remnants are "planets", no matter how they formed. As it happens, this deuterium-burning limit at ~13 Mj resides near the upper-end of the observed exoplanet mass distribution.
The WGESP also decided that the minimum mass/size required for an extrasolar object to be considered a "planet" should be the same as that used in our Solar System. Here, of course, there has been very considerable deliberation and debate arising out of the resolutions passed at the IAU General Assembly in Prague in August 2006, mainly in relation to the status of "dwarf" bodies such as Pluto, Eris and Ceres within our own Solar System. As far as detected exoplanets are concerned, the minimum mass object detected to date is the 0.00007 Mj object (40 per cent the mass of Mercury) orbiting the pulsar PSR 1257+12, but the lowest mass companions to ordinary stars which have been discovered to date are Gl 876 d, which has a minimum mass of 0.0185 Mj (about 5.9 Earth masses), 0GLE-05-390L
b, which has an estimated mass of 0.017 Mj (about 5.4 Earth masses) and Gl 581
c, which has a minimum mass of 0.0158 Mj (about 5 Earth masses).
The WGESP also decided that substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed nor where they are located. Furthermore, free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).
The first confirmed detections of exoplanets were made in early 1992, by the radio astronomers Aleksander Wolszczan and Dale Frail, but rather surprisingly these were not found around an ordinary star, but a pulsar - the superdense remnant of a massive star that has exploded as a supernova. The first definitive detection of an exoplanet orbiting an ordinary main-sequence star came in October 1995 with the announcement, by Michel Mayor and Didier Queloz of the University of Geneva, of an exoplanet orbiting the star 51 Pegasi. This discovery ushered in the modern era of exoplanet discovery, and since 2000 about 20-30 exoplanets have been discovered every year, with the most detections, by far, during 2007.
New discoveries and significant developments in exoplanet research continue at a frenetic pace, and it is difficult to keep up with progress in this exciting field. This multi-author volume comprises a collection of eleven topical reviews, each presented as a separate chapter, and covering an important aspect of exoplanet studies. The contributions have been written by scientists at the forefront of research in the selected areas, in a style which, we hope, will be accessible not only to advanced undergraduate students and beginning graduate students, but also to professional astronomers working in the field.
Although the direct imaging of exoplanets is extremely difficult at the present time, a variety of indirect detection methods are available. In Chapter 1, Patrick Irwin provides an overview of exoplanet detection techniques. The most successful take advantage of the fact that a planet orbiting a distant star can make its presence known through small, regular variations in the radial velocity or position of its parent star. However, exoplanets are increasingly being detected by observing the minute decrease in the light of the host star if an exoplanet happens to pass in front of it (in transit), or through techniques such as gravitational microlensing. So many exoplanets have now been found that it is possible to consider the statistics of the mass and orbital parameter distributions, and Chapter 1 includes a collection of plots showing the exoplanet mass distribution, their orbital period and orbital radius distribution, distributions of mass and radius and of eccentricity and radius for known exoplanets, and the distribution of host star metallicity. Chapter 1 concludes with a discussion of selection effects for different exoplanet detection programmes, and a look ahead to planned transit surveys and the techniques being developed for direct optical detection.
In Chapter 2, Jian Ge takes a detailed look at the most successful method employed to date for exoplanet detection, that of Doppler planet surveys. Of the roughly 250 exoplanets discovered to date, over 90 per cent have been detected by single object Doppler techniques. This chapter outlines the theory of the two principal Doppler methods: one using high resolution cross dispersed echelle spectrographs (the echelle method) and the other using dispersed fixed-delay interferometers (the DFDI method). Both methods have been successfully used for detecting new exoplanets. The main results of Doppler planet surveys over the past decade are then summarised, together with early results in the development of new Doppler techniques, especially multiple object techniques. Chapter 2 presents the scientific motivation for the next generation large-scale multi-object Doppler planet surveys and possible new science which will be addressed. Past experience has shown that the ability to move from single-object to multi-object observations has facilitated large-scale astronomical surveys (e.g. the Sloan Digital Sky Survey), and has consistently led to dramatic new discoveries. It is anticipated that similar advances will result from multi-object Doppler planet surveys in the next decade.
Another important exoplanet detection technique, that of gravitational microlensing, is reviewed by David Bennett in Chapter 3. This method relies upon chance alignments between background source stars and foreground stars which may host planetary systems. The background source stars serve as light sources that are used to probe the gravitational field of the foreground stars and any planets that they might host. The author explains how the microlensing method is unique among exoplanet detection methods in a number of respects, particularly in its ability to find low-mass planets at separations of a few AU. The basic physics of the microlensing method is reviewed together with typical planetary microlensing events. The author shows how such microlensing events may be used to enable the measurement of planetary orbital parameters, and he reviews early observational results highlighting the exoplanets discovered by microlensing to date. Finally, the author demonstrates that a low-cost, space-based microlensing survey can provide a comprehensive statistical census of extrasolar planetary systems with sensitivity down to 0.1 Earth-masses at separations ranging from 0.5 AU to infinity.
As George Rieke explains in Chapter 4, exoplanets move within tenuous disks of dust (and early-on, gas) that are relatively easy to detect. The dust intercepts energy from the parent star more efficiently than a planet can, and thus scatters and reradiates energy in far larger amounts than a planet could. In the process, it imposes its own signatures on this output. We know of hundreds of planetary systems through observation of circumstellar disks of dust, and we can learn indirectly about them if we can read these signatures. The author discusses the formation and evolution of protoplanetary disks in the context of terrestrial planet formation. He shows that although there is a well-defined overall pattern of protoplanetary disk characteristics, there is a wide range of starting conditions, e.g. disk masses, along with some variation in evolutionary timescales. Such differences presumably translate into a wide range of properties for the planetary systems that develop within these disks. The process of terrestrial planet formation continues well beyond the protoplanetary stage, and produces disks of debris from the planetesimal collisions. The observed behaviour of these debris disks can test many hypotheses regarding the evolution of the Solar System. Debris disks also enable astronomers to probe many different examples of how planetary systems evolve, since there are ^150 known examples within 50pc.
The interesting connection between brown dwarfs and exoplanets is explored by I. Neill Reid and Stanimir Metchev in Chapter 5. Brown dwarfs form like ordinary stars but, with masses below 0.075 solar masses, or 1.5 x 1029 kg, they fail to ignite core hydrogen fusion. Lacking a central energy source, they cool and fade on timescales that are rapid by astronomical standards. Consequently, the observed characteristics of old, cold brown dwarfs provide insight into the expected properties of gas-giant exoplanets. The chapter focusses on brown dwarfs as companions to main-sequence and evolved stars. Following a brief introduction to the intrinsic properties of brown dwarfs, including their observed characteristics and classification, the authors examine the different observational techniques used to identify very low mass companions of stars and review the advantages and challenges associated with each method. The authors summarise the results of various observational programs, particularly those regarding companion frequency as a function of mass and separation, and discuss the so-called 'brown dwarf desert'. The implications of these results for brown dwarf and planetary formation mechanisms are considered. The chapter concludes with a discussion of future surveys for low mass companions, particularly direct imaging programs that will have sufficient sensitivity to detect objects of planetary mass.
The detection of the first exoplanet around the G2V star 51 Pegasi in 1995 was a landmark discovery. The presence of this Jupiter mass planet in a very close 4.2-day orbit around the host star was quickly confirmed, and corroborated by Doppler evidence for more of these close-orbiting Jupiter mass planets (dubbed 'hot Jupiters') around a number of other nearby stars. Developments in experimental capabilities have meant that so called 'hot Saturns' and 'hot Neptunes' have also been discovered, and these close-orbiting planetary systems are discussed in detail by Hugh Jones, James Jenkins and John Barnes in Chapter 6. As the authors explain, although 51 Pegasi-like objects dominated early discoveries, other types of planets are considerably more common. The 51 Pegasi class were found first because they were the easiest to detect by the radial velocity method. In addition to being favoured by radial velocity surveys, the bias is even stronger in transit surveys. All known transiting exoplanets have periods less than a week. Although our overall knowledge of exoplanets has been fuelled by the growth in the sheer number and also by the broad range of parameter space now populated, close-orbiting planets characterised with a combination of precise radial velocity measurements and transit photometry have played a key role. In these close-orbiting systems it is possible to determine the mass and radius of the planet, which in turn yields constraints on its physical structure and bulk composition. The transiting geometry also permits the study of the planetary atmosphere without the need to spatially isolate the light from the planet from that of the star. This technique (known as transit spectroscopy or occultation spectroscopy) has enabled photometric and spectroscopic measurements of exoplanets to be made. As the authors of Chapter 6 make clear, the wide range of properties of close-orbiting planets has stimulated a plethora of physical models to explain their properties. They provide the sharpest test for theories of formation, e.g., gravitational instability versus core-accretion, the role of stellar metallicity in determining planetary core mass and how an irradiating star influences planetary contraction and migration, e.g., type I, type II and delayed migration. With the continuous development of experimental techniques, close-orbiting terrestrial-mass exoplanets are the exciting new frontier in astrophysics and will test a wide range of theoretical predictions.
The dynamical properties of multiple planet systems are reviewed by Rory Barnes in Chapter 7. As the author explains, the study of exoplanet dynamics is severely hampered by observational uncertainties. Although the detections themselves are robust, the orbital elements have significant uncertainties. The most problematic aspect of the Doppler technique is the mass-inclination degeneracy. If the inclination, the angle between the plane of the orbit and a reference plane, can be determined by a complementary method, such as astrometry or transits, this degeneracy may be broken, and the planetary masses and full three dimensional orbits identified. The mass-inclination degeneracy therefore makes many simulations, analyses, and hypotheses unreliable. Generally, in the dynamical studies discussed in Chapter 6, the masses are assumed to be the "minimum mass" - the mass if the orbit was exactly edge-on. Statistically, this choice is expected to be reasonably accurate. The Doppler technique also limits the ranges of planetary masses and orbital radii that may be observed, and so the observed planets may not be all the planets in a system. Consequently, the conclusions presented in Chapter 6 are subject to revision as additional planets may exist in each system that are either low-mass or orbit at large distances, and these unseen companions may significantly alter the best-fit orbits of the known planets. The author describes how the orbits of planets evolve due to tidal, resonant, and/or secular (long-term) effects. Basic analytical and numerical techniques can describe these interactions, and the author reviews orbital theory and analytical methods (secular theory and resonant interactions), and shows how N-body integrations are used to determine the evolution of a system. Multiple planet systems may also evolve chaotically, and some principles of chaos theory are described. Finally, the author discusses the current distributions of dynamical properties of known multiple exoplanetary systems, possible origins of these distributions, and compares exoplanetary systems with the Solar System.
There is increasing evidence that planets are ubiquitous, and may form around stars over a wide range in stellar masses. After a star dies, the planets may remain, and in some circumstances there may be a new epoch of planet formation after the main sequence. In Chapter 8, Steinn Sigurdsson discusses scenarios for the retention and formation of planets after the death of the parent star, and the prospects for detection, including current known post-main sequence systems. Planets in the so-called 'stellar graveyard' are, in many cases, easier observational targets than planets around main sequence stars, and different detection techniques may also be brought to bear, in some cases with much higher sensitivity, allowing the detection of low mass planets. This is particularly true in the case of the three exoplanets detected around the millisecond pulsar PSR 1257+12, which at 0.00007, 0.13 and 0.12 Jupiter masses are the lowest mass exoplanets discovered to date. The author discusses theories as to the origin of planets around pulsars, including the pulsar planet in the globular cluster Messier 4, before turning his attention to the detection of planets around white dwarfs. He also describes the recent exciting discovery of a giant planet around the extreme horizontal branch star V391 Pegasi. This is a well known pulsating subdwarf, a star that has terminated core hydrogen fusion on the stellar main sequence and evolved through a red giant branch phase. The planet must originally have been closer to the star, but moved outwards as the star lost mass, avoiding being swallowed by the red giant envelope as the star expanded. As the author explains, planets detected in the stellar graveyard reflect the 'live' population of planets, and in some cases provide potentially strong constraints on planet formation processes, and the general planet population.
A survey of currently known planet-hosting stars indicates that approximately 25 per cent of extrasolar planetary systems are within dual-star environments. Several of these systems contain stellar companions on moderately close orbits, and the existence of exoplanets in such binary systems has confronted dynamicists with many new challenges, as Nader Haghighipour explains in Chapter 9. Questions such as how are these planets formed, whether binary-planetary systems host terrestrial and/or habitable planets, how habitable planets form in such dynamically complex environments, and how such planets acquire the ingredients necessary for life, are among major topics of research in this area. Chapter 9 begins with a review of the dynamics of a planet in a binary star system, and in particular whether the orbit of a planet around its host star would be stable. The author then examines the formation of planets in binary star systems. In spite of the observational evidence that indicates the majority of main and pre-main sequence stars are formed in binaries or clusters, and in spite of the detection of potentially planet-forming environments in and around binary stars, planet formation theories are still unclear in explaining how planets may form in multi-star environments. The author then discusses the formation of giant and terrestrial planets in moderately close binary-planetary systems, and reviews the current status of planet formation theories in this area. The habitability of a binary system is then examined. Models of habitable planet formation in and around binary systems are presented, and their connections to models of terrestrial planet formation and water-delivery around single stars are discussed. Chapter 9 ends with a discussion of the future prospects for research in the field of planets in binary star systems.
The theme of the habitability of planets and the search for life beyond the Solar System is explored in detail by Victoria Meadows in Chapter 10. In its most conservative definition, a 'habitable world' is a solid-surfaced world, either a planet or moon, which can maintain liquid water on its surface. This definition is based on the fact that water is the one common constituent used by an enormous array of life forms on the Earth. Life may also be present in the atmospheres of planets, or in subsurface water tables or oceans, even in our own Solar System. However, as the author explains, when searching for life beyond our Solar System, we adopt the more conservative definition of the presence of surface water, because this definition also has the advantage of describing worlds that would be more detectable as habitable, even over enormous distances. After introducing the concept of habitable zones around stars which may harbour planets, the author explains how even a conservative definition of habitability still encompasses a vast array of potential worlds that could be considered habitable, without being similar to the present-day Earth. The techniques and space missions which will enable the direct detection of Earth-sized planets are then described, and aspects of the remote detection of planetary characteristics are outlined. Although characterising a planet for the ability to support life is an exciting first step, it is a precursor to the search for any indications that the planet already harbours life. Such signs of life, either past or present, when inferred from very distant measurements are called 'remote-sensing biosignatures'. As the author carefully explains, the search for these is based on the premise that widespread life will modify the atmosphere and surface of its planet, and that such modifications will be detectable on a global scale. The chapter concludes with a look at how such biosignatures might be detected.
There is good reason to hypothesise that giant exoplanets will be attended by significant moon systems. Moon systems exhibit diverse characteristics, and present unique environments - possibly even suitable habitats for life. As Caleb Scharf outlines in the final chapter, Chapter 11, such exomoons may share many characteristics with those in our own Solar System, as well as represent alternatives - possibly including temperate Mars- or Earth-sized bodies. In our own Solar System the majority of giant planet moons harbour substantial water ice mantles. The inferred internal structure and observed activity of many suggests the potential for extensive subsurface liquid water, both currently and in the past. A well known example of this is Jupiter's icy moon, Europa. Liquid water is vital for all forms of terrestrial life, through its integrated roles in biochemistry and geophysics. By contrast, the thick atmosphere and rich, low-temperature, hydrocarbon chemistry of Saturn's largest moon, Titan, points towards a highly complex surface environment paralleling some of the conditions on the early Earth, and conceivably offering alternative pathways for complex phenomena such as life. As the author concludes, detecting the presence of moons in exoplanetary systems is rapidly approaching feasibility, and will open a new window on such objects and their potential habitability.
This book has benefited from the support and assistance of a large number of people. I would like to offer my sincere thanks to all of the contributing authors for their considerable efforts, perseverance and enthusiasm for this project. I am indebted to Frank Herweg of Springer, Heidelberg for his invaluable support and advice in the preparation of the LaTeX files for this book, including his work on a number of the illustrations prior to publication. I am also most grateful to my wife Jane Mason for her assistance in the preparation of the Index, and to John and Margaret Dowling for help with proof reading. Finally, I am indebted to Imogen Millard, Sue Peterkin and Romy Blott of Praxis Publishing for their very considerable assistance at all stages in the organization and coordination of this project, and to Clive Horwood, Publisher, for his encouragement, advice and patience throughout.
Barnham, November 2007
John W. Mason
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