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days pable of measuring several hundred spectral lines simultaneously. Such a method started to be employed at the beginning of the 1980s, with the principal targets being solar-type stars, and was eventually crowned with success. In 1977, the Swiss astronomer Michel Mayor started a systematic search for stellar companions with the 1.93-m telescope at the Observatoire de Haute Provence (Fig. 1.7).

At the same time, other teams were carrying out similar programmes, notable ones being those of G. Marcy and P. Butler in the United States, and of G. Walker and B. Campbell in Canada. The first few years of searching failed to produce any results. Finally, in 1995, Michel Mayor and Didier Queloz announced the discovery of the first exoplanet orbiting a solar-type star, 51 Peg. With a mass that is at least half that of Jupiter, it orbits its star in just 4 days! (Fig. 1.8). The reason why 51

Haute Provence Observatory
Fig. 1.7 The 193-cm telescope at the Observatoire de Haute Provence. Equipped with the highresolution Elodie spectrograph, it enabled the first exoplanet orbiting a solar-type star, 51 Peg, to be discovered in 1995
Haute Provence Observatory
Fig. 1.8 The velocity curve of the star 51 Pegasi, measured by the team led by M. Mayor at the Haute Provence Observatory (After Mayor & Queloz, 1995)

Peg b had not been discovered sooner it that the various teams carrying out these searches did not envisage the existence of giant exoplanets so close to their parent stars. This was the start of a long series of discoveries. In January 1996, G. Marcy announced the discovery of two new exoplanets, one orbiting 47 UMa, the other 70 Vir. Twelve years later, the number of exoplanets exceeds 250; most of which have been discovered by velocimetry.

1.3.3 The First Results and the Problems Raised

Given the instrumentation available at the beginning of the 1990s, the velocime-try method was only able to detect giant exoplanets, with masses comparable to that of Jupiter. Less massive exoplanets did not sufficiently perturb the motion of the star for that perturbation to be detectable. Astronomers therefore searched for 'exo-Jupiters' and, by analogy with the giant planets in the Solar System, for ob jects with orbital periods of a few years, or even more. This is why the discovery of 51 Peg b caused such surprise. This exoplanet, whose mass is about half that of Jupiter, lies at a distance of just 0.05 AU from its star and its orbital period is just 4 days! These surprising characteristics were to be found in many exoplanets, discovered in the months and years that followed (Fig. 1.9).

Astronomers thus found themselves faced with an unexpected situation. There were certainly exoplanets around solar-type stars, but for a considerable number of them, their properties did not resemble in any way those of the planets of the Solar System.

1.3.4 Planets Around Pulsars

51 Peg b was the first exoplanet to be discovered around a main-sequence star, i.e., a star in its hydrogen-burning phase (like the Sun). However, the first true exoplanet, which is orbiting a neutron star, was discovered in 1992 by a technique known as pulsar timing.

In 1967, systematic observation of radio sources led J. Bell and A. Hewish to discover a new form of stars at the end of their lifetimes: neutron stars. Endowed with an intense magnetic field and an extremely high rate of rotation, these objects periodically emit - every 1.3 s for the first to be discovered - the intense radio signals that led to their detection, whence the name pulsar (pulsating star, Fig. 1.10). In 1969, a similar object was discovered in the Crab Nebula. This established the link between pulsars and the explosions that mark the final stages of the evolution of the most massive stars.

The periodic signal emitted by pulsars has another advantage: if the pulsar is orbited by one or more companions, the periodic signal is perturbed by them, and its analysis enables us to work back to the characteristics of the one or more companions. Several tentative detections were announced in the 1970s, but were soon discounted: some perturbations in the emission curves of pulsars are caused by pulsations in the neutron stars. Ten years later, in 1985, the team led by A. Lyne announced the discovery of a planet orbiting the pulsar PSR 1829-10. But, once again, appearances were deceptive: the observed effect is an artefact related to the period of the Earth's revolution around the Sun.

Finally, in 1992, the first discovery of an exoplanet was announced. The Polish astronomer, Alexander Wolszczan discovered two planets orbiting the pulsar PSR 1257+12, which is notable for its extremely fast rotation (1.5 ms). A third planet, the size of the Moon, appears to complete the system (Fig. 1.11). After this first discovery, other millisecond pulsars with planets appear to have been detected. Although these pulsars do not have the slightest resemblance to the Sun, the exoplanets discovered around them are certainly the first planetary systems ever detected.

Fig. 1.9 Exoplanets that have been observed (up to 2005), shown as a function of their semi-major axes (After Casoli & Encrenaz, 2005)

Fig. 1.10 The radio emission from a pulsar is caused by the intense magnetic field that produces it, coupled with an extremely high rate of rotation. Observers periodically receive the radio emission radiated along the axis of the magnetic field if the latter, as it sweeps round, passes close to the direction of the Earth (After Mayor & Frei, 2001)

A pulsar Rotation axis

Magnetic field

Magnetic axis

Beam of radio waves

Magnetic field

Fig. 1.11 The planetary system around PSR 1257+32, compared with the inner Solar System. (The size of the bodies is not to scale.) (After M. Mayor & Frei, 2001)

The PSR 1257 + 12 planetary system

The PSR 1257 + 12 planetary system

Fig. 1.11 The planetary system around PSR 1257+32, compared with the inner Solar System. (The size of the bodies is not to scale.) (After M. Mayor & Frei, 2001)

1 astronomical unit (AU) = 150 million km Mt = Earth mass

1 astronomical unit (AU) = 150 million km Mt = Earth mass

(The size of the bodies is not to scale)

(The size of the bodies is not to scale)

1.3.5 The Search for Protoplanetary Disks

In parallel with the first discoveries of exoplanets, the study of circumstellar disks around young stars allowed us to gain a better understanding of the first stages in the life of a star, and the formation of the Solar System. According to the scenario generally accepted today, the Solar System formed from a disk, resulting from the collapse of a rapidly rotating nebula of dust and gas. Once the planets formed, the disk subsequently dispersed, probably during the phase of intense activity (known as the 'T-Tauri' phase) that the young Sun experienced at the very beginning of its lifetime. Such disks could exist around young stars neighbouring the Sun. Cooler than the star, they would emit radiation in the near infrared, distinct from that of the star, whose maximum is at shorter wavelengths. Their detection from Earth would therefore be far easier than that of possible exoplanets.

The discovery of protoplanetary disks has a rich history, which extends back to the 1970s, with, in particular, optical and infrared observations of T-Tauri stars (Herbig, 1977) and other young objects (Cohen and Kuhi, 1979), as well as theoretical work (Lynden-Bell and Pringle, 1974).

In 1983, the first disk surrounding a young star was actually discovered around Vega by the IRAS satellite. This Earth-orbiting satellite, designed to carry out a deep survey of the sky in the mid and far infrared, was able to measure the temperature and the dimensions of the disk. However, it was not, strictly speaking, a protoplane-tary disk of the sort that existed in the Solar System, given Vega's spectral type, and its age (some 100 million years). IRAS discovered that about 20 per cent of stars of type A are surrounded by a disk.

A year later, the first image of another disk was obtained by coronagraph observations from the ground. This disk lies around the star 6 Pictoris, a type-A star less than 100 million years old. These observations were subsequently repeated using the Hubble Space Telescope. The images showed a highly flattened disk, with a radius of several hundred Astronomical Units (AU). Spectroscopic observations in the UV and visible regions then revealed the presence of atomic absorption lines shifted by the Doppler effect towards the red, that were variable over time, which were attributed to episodes, in which comets, captured by the star, fell into it. 6 Pictoris was therefore the first system that revealed certain analogies with the early history of the Solar System.

Since the discovery of 6 Pictoris, numerous protoplanetary disks have been discovered, most notably by the Hubble Space Telescope. In many cases, observations undertaken in parallel from the ground, in the visible, infrared, and at millimetre wavelengths, have revealed bipolar flows, emitted symmetrically, perpendicular to the plane of the disk. This is the case with what are known as Herbig-Haro objects (observable in the visible) and YSO (young stellar objects), which are hidden within a cocoon of dust and are detectable only in the infrared. Such objects are found, in particular, in the Orion Nebula. The typical age of these objects, deduced from observations, is no more than a few million years, which corresponds to the phase of planetary formation in the Solar System. In the case of older stars (such as Vega and 6 Pictoris), the circumstellar disks are called debris disks, and may consist of the remnants following a phase of planetary formation. The discovery of a gap in the disk around 6 Pic may be the signature of a planet having accreted the material missing from the gap (see Chap. 6).

1.4 The Solar System: A Typical Planetary System?

Let us see why the discovery of planets as massive as Jupiter orbiting very close to their stars was such a surprise to astronomers, by studying the case of the Solar System.

1.4.1 The Sun as an Average Star

The Sun's characteristics make it an 'average' star. It was born 4.6 billion years ago from a cloud that was rich in gas (primarily hydrogen) and dust, which collapsed into a disk. The star is now in the main-sequence phase (several thousand million years) during which it transforms the hydrogen that it contains into helium (Fig. 1.12). In a second phase, the helium is itself transformed into carbon, nitrogen, and oxygen. A star with the mass of the Sun is destined to become, in about 5 billion years, a red giant, and its outer envelope will finally be ejected to form a planetary nebula, while the core of the star will collapse to form a white dwarf. Stars that are far more massive than the Sun undergo a different evolutionary path: they become red super-giants, and then explode as supernovae. During this final phase, the heavy elements (magnesium, silicon, metals, etc.), which exist in the Universe and particularly on Earth, are formed from C, N, and O.

Spectral type

Spectral type

20,000

10,000

5,000

2,500

Fig. 1.12 The Hertzsprung-Russell diagram shows the relation between the temperature and the luminosity of stars. Most of the stars are located along the Main Sequence, where they convert their hydrogen into helium. Later they evolve into giants and white dwarfs, or into supergiants and supernovae, depending upon their initial mass

20,000

10,000

5,000

2,500

Fig. 1.12 The Hertzsprung-Russell diagram shows the relation between the temperature and the luminosity of stars. Most of the stars are located along the Main Sequence, where they convert their hydrogen into helium. Later they evolve into giants and white dwarfs, or into supergiants and supernovae, depending upon their initial mass

1.4.2 Brown Dwarfs: Between Stars and Planets

What is the boundary between stars and planets? A star is a body in which thermonuclear reactions are taking place, the primary one being the conversion of hydrogen into helium. Calculations of stellar internal structure show that a mass of 74 Jupiter masses (74 MJ) is required to initiate the process.

A planet, in contrast, is an object orbiting the Sun (or a star), which does not emit any visible radiation in its own right; but only reflects the light from the star that it accompanies. Historically, the term 'planet' described the nine largest bodies in the Solar System, recognized in the 20th century as orbiting the Sun. This definition excluded the asteroids (also known as 'minor planets') because of their smaller size, as well as the trans-Neptunian bodies, other than Pluto, recently discovered in the Kuiper Belt. Since 2006, Pluto is officially no longer a planet; it is classed as a 'dwarf planet' and is a particularly massive trans-Neptunian object (TNO).

So a planet has insufficient mass to start thermonuclear reactions in its core. Nevertheless, there are two possible scenarios. If the mass of the object is less than 13 MJ, no thermonuclear reaction is possible. For a mass lying between 13 and 74 MJ, the internal temperature is sufficient to allow the onset of the first cycle of reactions, which involve deuterium only, and which last about 10 million years. The object is then known as a brown dwarf. During its short active phase, it appears like a cool star; subsequently, it becomes as difficult to detect as a planet. Like stars, brown dwarfs may, in theory, form as the result of gravitational collapse of a molecular cloud, either as an isolated object or as companion to a star.

1.4.3 A Specific Planetary System: The Solar System

The model for star formation beginning with the collapse of an interstellar cloud became established initially through our knowledge of the Solar System itself. This has one primary characteristic: all the planets orbit the Sun in the same direction (direct revolution, as seen from the Sun's north pole) on almost circular orbits, and all very close to the plane of the Earth's orbit (Table 1.1). The latter (the ecliptic) is chosen as the reference plane (Fig. 1.13). Most of the planets also rotate in the same direction, with the axis of rotation almost perpendicular to the ecliptic. This property strongly supports formation from a protoplanetary disk, as was suggested by Kant and Laplace in the 18th century.

1.4.4 The Formation of the Planets by Nucleation

According to the standard model of formation that is generally accepted nowadays, the Sun and the Solar System formed 4.55 ± 0.10 x 109 years ago (Fig. 1.14). We have information about the ages of the planetary material from measurements of

Table 1.1 Orbital properties of the planets in the solar system (After Encrenaz et al., 2004)

Name Semi-major axis (AU) Eccentricity Inclination/ ecliptic Sidereal period (years)

Mercury 0.38710 0.205631 7.0048 0.2408

Venus 0.72333 0.006773 3.39947 0.6152

Earth 1.00000 0.01671 0.0000 1.0000

Mars 1.52366 0.093412 1.8506 1.8807

Jupiter 5.20336 0.048393 1.3053 11.856

Saturn 9.53707 0.054151 2.4845 29.424

Uranus 19.1913 0.047168 0.7699 83.747

Neptune 30.0690 0.008586 1.7692 163.723

the abundance of radioactive atoms with very long half-lives, which have been applied to terrestrial rocks, lunar samples, and meteorites. A fragment of an interstellar cloud, which was turbulent and unstable, and in rapid rotation, collapsed into a disk. This disk consisted primarily of gas (most of which was hydrogen and helium), as well as dust particles consisting of heavier elements.

The material in the centre, under the influence of its own gravity, collected together to form a star, the future Sun. At greater distances from the centre, the solid particles clumped together, at first under the influence of local instabilities, and then as the result of multiple collisions. The phenomenon subsequently tended to accelerate. The planetoids that arose attracted the surrounding particles through the effects of their gravity. Computer simulations of the dynamical evolution of such a system of particles show that these multiple collisions act to cause the orbits of the fragments that are thus formed to become circular and co-planar.

Fig. 1.13 The orbits of the giant planets and Pluto. Like those of the terrestrial planets, they are very close to the plane of the ecliptic (defined by Earth's orbit), whereas Pluto's orbit lies away from it (After Encrenaz, 2003)

Fig. 1.14 A schematic representation of the formation of the Solar System. (a-b) Collapse into a disk; (c-d) formation of planetoids; (e-f) accretion by collisions; (g) growth by the gravitational attraction of the larger bodies; (h) because of the effects of multiple collisions, the orbits slowly become circular and co-planar (After Encrenaz, 2003)

1.4.5 Terrestrial and Giant Planets

From a physical point of view, the planets in the Solar System exhibit one particular characteristic. They may be classified into two distinct categories (see Table 1.2). At a heliocentric distance Rh less than 2 AU, the four terrestrial planets (Mercury, Venus, Earth, and Mars) are relatively small and dense; with the exception of Mercury, which lacks one, they have atmospheres whose masses are negligible relative to their overall mass; and the number of satellites varies between 0 and 2. Beyond 5 AU, the four giant planets (Jupiter, Saturn, Uranus, and Neptune) have large volumes and are low in density. Their atmospheres are dense, and they do not have a 'surface' as we understand it; they all have systems of rings and numerous satellites.

How may the classification of the planets into two such distinct classes be explained? According to the standard model described below, this is purely the natural consequences of the method by which the planets were formed.

Actually, the chemical composition of the protoplanetary disk should reflect (to a first approximation) the cosmic abundances. Hydrogen is the most abundant element, then helium, followed by oxygen and nitrogen, and then the heavier elements (Table 1.3). Close to the Sun (Rh < 2 AU), the temperature is sufficiently high for the elements C, N, and O, which are bound to hydrogen in the form of CH4, NH3, H2O, CO2, etc., to be in gaseous form. The only solid materials available to form planetary nuclei were the refractory materials, in particular silicates and metals. Because these elements are far less abundant than the lighter elements (see Table 1.3), the mass available to form planetary nuclei was limited. In contrast, beyond 4 AU, the temperature was sufficiently low for the elements C, N, and O to be in solid

Table 1.2 Physical characteristics of the planets in the solar system (after Encrenaz et al., 2004). The obliquity is the angle between the rotation axis of the planet and the perpendicular to the ecliptic (where the North Pole of the ecliptic is 0°)

Name

Diameter

Mass

Density

Surface

Sidereal

Obliquity

Atmosph.

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