Are there inhabitable worlds elsewhere in the Universe? Or better, inhabited worlds? Or is the third planet of the solar system really special? More than two thousand years ago, some Greek philosophers were already speculating about the existence of Earth-like planets. For the atomists like Epicure, it was a matter of principle: there should be planets around every star. But despite these high expectations and several research programs, only nine planets orbiting a main sequence star were known at the beginning of the nineties: the nine planets of the solar system.

Since then, the situation has changed drastically. While the nine planets of the solar system are now only eight (Pluto having lost this appellation), more and more extrasolar planets are now being detected, observed and catalogued by the astronomers.

Indeed, extrasolar planet, or exoplanet, is the name given by astrophysicists to these new worlds—a name not yet in all dictionaries—as well as exoEarth, exo-Jupiter, exoUranus, depending on the mass of the exoplanet. In mid-2008, there were more than three hundreds of these extrasolar planets, many of them belonging to planetary systems with at least two planets orbiting the same star. The solar system is thus far from being the only one of its kind: a revolution that both astronomy and planetary science have awaited for a long time.

And, indeed, it is a revolution. Exoplanets are not at all what astronomers expected. Actually, they do not look like if they were twins, or even cousins of the Earth, Jupiter and solar system planets. Most of them are giant, likely gaseous planets. This is not completely surprising since our detection methods, at least in the first years of this story, were not sensitive enough to detect less massive planets. But what was a real shock was the discovery of dozens of such exoJupiters 50 or 100 times closer to their star than Jupiter is from the Sun—a really hot place to live. How were these hot Jupiters formed? Another mystery is the rather elliptical orbits of many extrasolar planets, while planets in our solar system are on rather circular orbits. In the end, could it be that our solar system has exceptional features? The discovery of extrasolar planets has then led researchers to reconsider all theories of planetary formation and evolution. For example, the role of a phenomenon called migration is now widely recognized: most planets do not stay in the region of the planetary system where they were formed. With the discovery of more and more "superEarths," with masses between several Earth masses and the Uranus mass, it also seems that the distinction between telluric and giant planets is fading away.

Exoplanets were a challenge not only for the theoreticians, but also for observers. These tiny dots are not yet directly observable except in very exceptional cases. It is an indirect method (velocimetry) that has yielded the majority of the discoveries, but astrometry, microlensing, transits, and pulsar timing are among the methods that observers have invented to circumvent this problem. The ultimate goal for the next 20 years, however, is to detect exoEarths—planets with a mass close to that of Earth, and located at the right distance from their star for life to be able to develop. Thanks to innovative methods, detecting exoEarths should be feasible in the next 10 years, while detecting signs of life (biosignatures) in their atmospheres is still an immense challenge, and the goal of extremely ambitious space projects.

This book attempts another challenge, which is to draw a picture as complete as possible of this field while it is still quickly evolving. The first chapters describe what is currently known of exoplanets, from a description of the detection methods and of the observed properties of the known objects to the dynamics of planetary systems and the structure and evolution of planets in general. It appears that the solar system planets are still the reference for all models. The last two chapters deal with current and future detection projects, and the final goal—the search for life on exoplanets.

One could hope that the field of exoplanet research has reached a mature state and the major results that one can get with present-day techniques are known. However, the "other worlds" are still capable of amazing us. In such a dynamic field, the foreword is the best place for the latest news. Indeed, three months after this book was completed, the space mission COROT discovered an enigmatic object between a star and a planet with a density twice that of platinum. The team using the HARPS spectrograph on the 3.6 m telescope at the European Southern Observatory announced that a system of three superEarths orbits the star HD40307. When will be the first announcement of the discovery of a true Earth twin? Two years, five years, ten years from now? Let us guess that exoplanets will surprise us again.


We are grateful to J. Lequeux for his very helpful comments.

We also wish to thank Mr. Storm Dunlop for his collaboration and his excellent translation of the manuscript.


1 Planetary Systems 1

1.1 Introduction 1

1.2 The Plurality of Worlds: A Question as Old as the Hills 1

1.2.1 From Antiquity to the Copernican Revolution 1

1.2.2 The First Theories on the Formation of the World 3

1.3 First Searches for Other Worlds 5

1.3.1 The First Astrometric Searches 6

1.3.2 The Velocimetry Method 6

1.3.3 The First Results and the Problems Raised 9

1.3.4 Planets Around Pulsars 10

1.3.5 The Search for Protoplanetary Disks 12

1.4 The Solar System: A Typical Planetary System? 13

1.4.1 The Sun as an Average Star 14

1.4.2 Brown Dwarfs: Between Stars and Planets 15

1.4.3 A Specific Planetary System: The Solar System 15

1.4.4 The Formation of the Planets by Nucleation 15

1.4.5 Terrestrial and Giant Planets 18

Bibliography 20

2 Detection Methods 21

2.1 The Extent of the Problem 21

2.1.1 Contrast Between Star and Planet 21

2.1.2 Angular Separation Between the Objects 22

2.1.3 Environment of the Earth and Exoplanets 23

2.2 The Indirect Detection of Exoplanets 24

2.2.1 The Effect of a Planet on the Motion of Its Star 24

2.2.2 The Effect a Planet has on Photometry of Its Star 36

2.2.3 Comparison of the Different Indirect Methods 46

2.3 Direct Detection of Exoplanets 46

2.3.1 Choice of Spectral Region 47

2.3.2 Coronagraphic Methods and Adaptive Optics 48

2.3.3 Interferometry 55

2.3.4 Interferometry and Imagery: Hypertelescopes 62

2.3.5 Detection by Radio 65

Bibliography 67

3 Extrasolar Planets, 12 Years After the First Discovery 69

3.1 Exoplanets and Exoplanetary Systems 70

3.2 The Mass-Distribution of Exoplanets 70

3.3 The Distance-Distribution of Exoplanets 74

3.4 The Relationship Between the Mass of Exoplanets and Their Distance from Their Star 76

3.5 Orbital Eccentricity Among Exoplanets 78

3.6 Exoplanets and Their Parent Stars 80

3.7 Mass/Diameter Ratio 82

3.8 Characteristics of Extrasolar Planetary Atmospheres 83

Bibliography 84

4 What we Learn from the Solar System 85

4.1 Observational Methods 85

4.2 The Observational Data 87

4.2.1 Orbits that are Essentially Co-Planar and Concentric 87

4.2.2 Terrestrial Planets and Giant Planets 87

4.2.3 The Small Bodies 88

4.2.4 Dating the Solar System Through Radioactive Decay 90

4.3 The Emergence of a'Standard Model' 91

4.3.1 The Nebular Theory 91

4.3.2 The Standard Model: The Chronology of Events 92

4.4 The Physical and Chemical Properties of Solar-System Objects 100

4.4.1 The Electromagnetic Spectrum of the Objects in the Solar System 100

4.4.2 Planetary Atmospheres 101

4.4.3 The Terrestrial Planets 105

4.4.4 The Giant Planets 110

4.4.5 Rings and Satellites in the Outer Solar System 120

4.4.6 Small Bodies in the Solar System 124

4.5 Conclusions: The Solar System Compared with Other Planetary Systems 129

4.5.1 The Scenario for the Formation of the Solar System 129

4.5.2 Objects in the Planetary Systems Observable from Earth ... 130 Bibliography 131

5 Star Formation and Protoplanetary Disks 133

5.1 The First Stages in Star Formation 133

5.1.1 Properties of the Interstellar Medium 133

5.1.2 The Formation of Molecular Clouds 135

5.1.3 Collapse of a Molecular Cloud 136

5.1.4 Observation of Young Stars 136

5.2 Structure and Evolution of Protoplanetary Disks 139

5.2.1 Observation of Protoplanetary Disks 139

5.2.2 Stellar Accretion Flux 142

5.2.3 The Rotation of T-Tauri Stars 143

5.2.4 The Formation of Binary Systems 144

5.2.5 The Principal Stages of Star Formation 145

5.2.6 Later Stages of Stellar Evolution: Evolution

Towards the Main Sequence 149

5.2.7 The Structure of Protoplanetary Disks 151

5.2.8 Composition of the Gas and Dust 155

5.3 Planetary Disks and Debris Disks 156

5.3.1 Observation of the Disk of HR 4796A 157

5.3.2 Observation of the Disk of 3 Pic 158

5.4 The Formation of Planetesimals and Planetary Embryos 161

5.4.1 From Microscopic Particles to Centimetre-Sized Grains 161

5.4.2 From Centimetre-Sized Grains to Kilometre-Sized Bodies.. 162

5.4.3 From Protoplanets to Planets 163

Bibliography 165

6 The Dynamics of Planetary Systems 169

6.1 Characteristics of the Orbits 169

6.1.1 Calculation of Radial Velocities 169

6.1.2 Orbital Characteristics from Radial-Velocity Curves 170

6.1.3 Multiple Systems Case 172

6.1.4 Exoplanets and Known Multiple Systems 173

6.1.5 Rotation of the Planets 176

6.2 Migration 177

6.2.1 Migration in the Solar System 177

6.2.2 Migration in Exosystems 179

6.2.3 The Different Migration Mechanisms 180

6.2.4 Observational Indications 182

6.2.5 The End of the Migration and Tidal Effects 184

6.3 Stability of Planetary Systems 185

6.3.1 Dynamical Categories 185

6.3.2 The GJ 876 System 187

6.3.3 The HD 82943 System 188

6.3.4 The v Andromedae System 188

6.3.5 The HD 202206 System: A Circumbinary Planet? 189

6.3.6 The HD 69830 System: Three Neptunes and a Ring of Dust 191

6.4 Planetary Systems Around Pulsars 191

6.5 The Dynamics of Debris Disks 193

Bibliography 196

7 Structure and Evolution of an Exoplanet 197

7.1 The Internal Structure of Giant Exoplanets 198

7.1.1 The Observable Features 198

7.1.2 The Equations of Internal Structure 199

7.1.3 Rotation Effects 201

7.1.4 Equations of State 201

7.1.5 Construction of Models of Internal Structure 203

7.1.6 Evolutionary Models 206

7.2 The Internal Structure of Terrestrial-Type Exoplanets and Ocean Planets 208

7.2.1 Terrestrial-Type Exoplanets 209

7.2.2 Ocean Planets 211

7.3 The Atmospheres of Exoplanets: Their Structure,

Evolution and Spectral Characteristics 214

7.3.1 Giant Exoplanets 214

7.3.2 Terrestrial Planets and Habitable Planets 225

7.3.3 Hot Neptunes, Super-Earths, and Ocean Planets 239

Bibliography 242

8 Present and Future Instrumental Projects 245

8.1 Indirect Methods of Detection 246

8.1.1 Velocimetry 246

8.1.2 Astrometry 250

8.1.3 The Study of Planetary Transits 255

8.1.4 Searching for Microlensing Events 266

8.2 Direct Methods of Detection 270

8.2.1 Imaging 270

8.2.2 Interferometry 279

8.2.3 Direct Detection of Radio Waves 288

Bibliography 291

9 The Search for Life in Planetary Systems 293

9.1 What is Life? 293

9.1.1 How Should Life be Defined? 293

9.1.2 The Role of Carbon and of Liquid Water 294

9.1.3 The Building-Block of Life: Macromolecules 296

9.1.4 Nucleic Acids 297

9.1.5 The Role of the Cell 298

9.2 Prebiotic Material in the Universe 299

9.2.1 Organic Material in the Universe 299

9.2.2 The Synthesis of Organic Molecules: Miller and Urey's Experiment 301

9.2.3 Transport of Complex Organic Molecules to the Primordial Earth 303

9.3 Stages on the Road to Complexity 306

9.3.1 Polymers and Macromolecules 306

9.3.2 The Formation of Membranes 307

9.3.3 RNA and DNA 307

9.4 The Appearance of Life on the Primitive Earth 308

9.4.1 Favourable Conditions 308

9.4.2 The Environment of the Primitive Earth: The Hydrosphere and Atmosphere 309

9.5 The Search for Habitable Locations in the Solar System 311

9.5.1 The Planet Mars 311

9.5.2 The Satellites of the Outer Planets 315

9.6 The Search for Life on Exoplanets 319

9.6.1 Exoplanets'Habitable Zones 319

9.6.2 How May Life on an Exoplanet be Detected? 321

9.7 The Search for Extraterrestrial Civilisations 324

9.7.1 The Drake and Sagan Equation 324

9.7.2 Communication by Radio Waves 325

9.7.3 The State of SETI and CETI Searches 325

Bibliography 327

Appendix A 329

A.1 Star or Planet? 329

A.2 Gravitation and Kepler's Laws 330

A.3 Black-Body Emission - Planck's Radiation Law - Stefan's Law ... 330 A.4 The Hertzsprung-Russell Diagram and the Spectral Classification of Stars 332

A.5 Resonances 334

Index 337

Chapter 1

Planetary Systems

1.1 Introduction

The question of the existence of inhabited worlds outside the Solar System goes back to antiquity. Even at the time of the Greeks, who, following the tradition of Aristotle, placed the Earth at the centre of the Universe, voices were raised, suggesting a heliocentric system. History has preserved the name of one celebrated pioneer, Aristarchos of Samos. However, his works remained forgotten for more than a millennium, until the Copernican revolution at the end of the 15th century. 'An infinite number of suns exist, an infinite number of earths turn around those suns just as the seven planets turn around our sun': These visionary views were held, four centuries ago, by Giordano Bruno. This intuitive view continued to grow among scientists, along with the astronomical discoveries of recent centuries: with the discovery of the nature of stars; of the place of the Sun in the Galaxy; and of the extragalactic Universe. Because the Sun is just one unremarkable star among the 100,000 million that populate our galaxy, why should it be the only star to have a train of planets? And why should life be confined to our own Earth?

1.2 The Plurality of Worlds: A Question as Old as the Hills 1.2.1 From Antiquity to the Copernican Revolution

We can trace the concept of the existence of other worlds as far back as ancient Greece. It is described by the philosopher Epicurus (341-270 BC), in particular, in the following words, written to Herodotus: 'There are infinite worlds similar to and different from our own.' In his work De Caelo, the philosopher Aristotle (384-322 BC) also wondered about the existence of other worlds. Two centuries later, in his work De natura rerum, Lucretius, the Roman philosopher, also subscribed to the concept of the plurality of worlds.

In the Middle Ages, the question of the plurality of worlds was the subject of debate, with a succession of confrontations between theologians in the 13th and 14th

M. Ollivier et al., Planetary Systems. Astronomy and Astrophysics Library, DOI 978-3-540-75748-1.1, © Springer-Verlag Berlin Heidelberg 2009

centuries. According to Albert Magnus (1193-1280), 'the concept of the plurality of worlds represents "one of the most marvellous and noble questions in Nature"'. His disciple Thomas Aquinas (1224-1274) also favoured the concept of the existence of other worlds. Jean Buridan (c. 1295-1358), Rector of Paris University, and William of Okham (c. 1280-1347) also both maintained that other worlds could exist.

In 1543, the year of the death of its author, the publication of De revolutionibus, by Nicholas Copernicus, signalled, with the advent of the heliocentric system, the end of the Aristotelian system which located the Earth at the centre of the Universe. A few decades later, Giordano Bruno (1548-1600) became its ardent champion, as well as the passionate advocate for an infinite number of possible worlds, which he described in certain of his works: De l'infinito universo et mondi, which appeared in 1584, and then De immenso e innumerabilibus, published in 1591. In 1600, he paid for his writings with his life, condemned as a heretic by the Inquisition. The heliocentric view did, however, come to convince the world of scholars and philosophers (Fig. 1.1), thanks to the work of observers such as Tycho Brahe (1541-1601), Johannes Kepler (1571-1630) and Galileo Galilei (1564-1642). With the discoveries of Galileo, published in 1610 in his famous work Sidereus Nuncius, on the presence of mountains on the surface of the Moon and the existence of satellites

Fig. 1.1 The Perfit description of the caelestiall orbes by Thomas Digges (1576). A supporter of the Copernican system, the author described, beyond the orbit of Saturn 'an orb of the fixed stars whose dimensions are infinite in altitude' (After Verdet, 2002)

of Jupiter, the question of the existence of other inhabited worlds arose yet again. Kepler, in the Dissertatio, wondered about the habitability of the Moon and Jupiter's satellites; but there were still many opponents, notably among the Church.

1.2.2 The First Theories on the Formation of the World

The first attempt to account for the formation of the Earth and the Universe came with Rene Descartes (1596-1650). In Principia philosophiae, which appeared in 1644, Descartes described the latter as a set of whirls, or vortices, the centres of which were occupied by the Sun and other stars. To Descartes, the stars played exactly the same role as the Sun. Descartes' concept therefore opened the door to the possibility of an infinite number of possible worlds.

In 1686, Bovier de Fontenelle (1657-1757), published his Entretiens sur la pluralite des mondes (Fig. 1.2). In this work, Fontenelle supports the idea of the habit-ability of the planets and the satellites in our Solar System, but he also puts forward the idea of an infinite number of planets. 'Our Sun has planets that he illuminates, f't'i'r.iM','. ri*./

Fig. 1.2 An illustration from the first edition of Entretiens sur la pluralité des mondes, by Fontenelle (1686), showing planets outside the Solar System, orbiting other fixed stars (After Dick, 1982)

why should each fixed Star not also have some that it illuminates?' A few years later, Christiaan Huygens (1629-1695), in his work Cosmotheoros published in 1698, took up Fontenelle's suggestions, but this time based them, not on the theory of vortices, but on the physical analogy between the Earth and the other planets in the Solar System.

The year 1687 saw the start of a new era. The publication by Isaac Newton (1642-1727) of the Philosophiae naturalis principia mathematica laid down the fundamental basis of the laws of universal gravitation, and provided an irrefutable demonstration of the Copernican theory. It opened the way for the nebular theory, which was proposed by Immanuel Kant (1724-1804, Fig. 1.3) in his Allgemeine Naturgeschichte und Theorie des Himmels [Universal Natural History and Theories of the Heavens], which appeared in 1755. In this work, Kant explains the origin of the Solar System by the collapse of a rotating cloud, which flattens into a disk, within which planets subsequently form. Subsequently finalized by the work of Pierre-Simon de Laplace (1749-1804, Fig. 1.4), which was published in 1796 in his Exposition du systeme du monde, this model is the basis for the view of planetary formation that is generally accepted today. Like Huygens, Kant saw the stars of the Milky Way as objects comparable with the Sun. He put forward the idea that these

Fig. 1.3 Immanuel Kant (1724-1804)

Fig. 1.4 Pierre-Simon de Laplace (1749-1827). Together with Immanuel Kant, he proposed the nebular theory, explaining the origin of the Solar System, which is generally accepted today

Fig. 1.4 Pierre-Simon de Laplace (1749-1827). Together with Immanuel Kant, he proposed the nebular theory, explaining the origin of the Solar System, which is generally accepted today

stars might be endowed with planets, and that these could be habitable. The idea was taken up by 19th-century astronomers, in particular by Camille Flammarion (1842-1925) in his Astronomie Populaire.

1.3 First Searches for Other Worlds

The idea of the probable existence of extrasolar planets (or 'exoplanets') was therefore definitely in the minds of astronomers, but until the end of the 20th century, they did not have the requisite observational methods to detect them. The sought-after exoplanets do, after all, give off very weak radiation, but this is drowned by that from their parent stars. Before the advent of large telescopes in the 8-10-m class, exoplanets could not be detected by direct imaging because of their intrinsically weak flux, and their angular proximity to their parent stars. For an observer located outside the Solar System, the ratio of the flux emitted by Jupiter and the Sun, in visible light, is of the order of 10~9. Note that in the mid-infrared (10 |m) it is of the order of 10~6, which is more favourable, but still beyond the range of the instruments available at the end of the 20th century. As for the angular distance of Jupiter from the Sun, it is 0.5 as for an observer at 10 pc1 from the Sun. The light from Jupiter is thus utterly negligible compared with the light emitted by the Sun.

1.3.1 The First Astrometric Searches

At the beginning of the 20th century, astronomers wondered about the presence of possible companions around nearby stars. Because it is accepted that the Sun is a perfectly ordinary star in the Galaxy, nothing ruled out the existence of numerous stellar systems, similar to our own. The whole problem, however, lay in detecting these systems. We have seen that direct detection was impossible. Another method had to be used, namely measurement of the periodic motion of the central star caused by the companion, relative to the centre of mass of the overall system.

In the middle of the 19th century, the German astronomer Friedrich Bessel (1784-1846) had become famous by detecting, using this indirect method, the presence of a low-mass companion orbiting Sirius. It was still not a planet; in 1930 Chandrasekhar would show that the mysterious object is a white dwarf, the final stage in the evolution of low-mass stars.

Bessel's discovery opened the way to the search for low-mass stellar companions by using the astrometric technique. During the course of the 20th century, with more powerful telescopes, and more sensitive detectors, several astronomers embarked on a search for exoplanets. Piet Van de Kamp (1901-1995, Fig. 1.5), from the astrometric curve of Barnard's Star, announced the discovery of one or more companions. Announced in 1944, this result was finally disproved by other investigations which showed the presence of systematic errors that were linked to the instrumentation used for Van de Kamp's measurements. He announced another discovery in 1974, this time orbiting the star Epsilon Eridani but, once again, this result was challenged by other measurements. At the beginning of the 1980s, the prevailing view within the astronomical community was that astrometric techniques were not sufficiently accurate to allow the detection of exoplanets around nearby stars, bearing in mind the performance of the instruments that were then available.

1.3.2 The Velocimetry Method

The second indirect method to detect exoplanets, known as velocimetry, consists of measuring, by means of the Doppler Effect, the periodic fluctuations in the velocity of a star that has a companion, relative to the centre of gravity of the system

1 1 parsec (pc) is the distance at which one AU (the average distance between the Sun and Earth,

149.6 million kilometres) subtends an angle of one second of arc. Because of the Earth's revolution around the Sun, a star that lies at a distance of 1 pc describes a small ellipse, whose semi-major axis is one second of arc (1 pc = 2.05 x 105 AU = 3.26 light-years).

Fig. 1.5 The astronomer Piet Van de Kamp (1901-1995) believed that he had discovered the first extrasolar planet orbiting Barnard's Star. The observations were later explained by a systematic error linked to the telescope that was used

(Fig. 1.6). To detect the equivalent of a Jupiter-sized planet orbiting the Sun, it is necessary to be able to measure velocity differences of around 10m/s. To obtain such precision, astronomers construct spectrographs, operating in the visible spectral range with very high spectral resolution (>105 in resolving power), ca-

Fig. 1.6 The principles on which the velocimetric method is based. (a) The star and its companion orbit their common centre of gravity of the system. (b) The Doppler Effect allows the observer to detect the motion of the star (provided that the orbit of the planet is not in a plane perpendicular to the line of sight) (After Casoli & Encrenaz, 2005)

G = centre of gravity spectrum of the star zero point zero point velocity (m/s)

zero point zero point blue

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