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Fig. 4.23 Measurement of D:H in Jupiter and Saturn from ISO observations (After Lellouch etal.,2001)

The predictions were confirmed by the D:H measurements carried out by the ISO satellite, notably from the HD:H2 ratio (Fig. 4.23). The D:H enrichment in Uranus and Neptune is about 3, relative to the protosolar value (Fig. 4.24), which constrains the value in the proto-Neptunian ices which served as building blocks for the initial cores. A remarkable fact is that this value is less, by a factor of 2, than the D:H ratio measured in comets, and this sets constraints on the conditions under which the different types of objects formed. In fact, the D:H ratio is an indication of the temperature at which the body being observed formed (the former is lower, the higher the ratio), and thus of the heliocentric distance at the time of its formation.

Fig. 4.24 The D:H ratio in the outer Solar System (Encrenaz et al., 2004)

4.4.5 Rings and Satellites in the Outer Solar System

The formation of the giant planets, following the collapse of the protoplanetary disks around their initial cores, possibly suggests an analogy with the Solar System itself, the result of the collapse of the presolar nebula. Numerous satellites of the giant planets were thus formed in their equatorial planes (known as regular satellites), just as the planets formed in the plane of the ecliptic. Such a comparison soon reveals its limits, however: whereas Mercury, the closest planet to the Sun, lies at about 80 solar radii, Io, the closest Jovian satellite, is at only about 6 planetary radii from the planet. The existence of rings in the equatorial planes of the four giant planets, in their immediate proximity, bears witness to the presence of material that is even closer. Below a limit at about 2.5 planetary radii (the Roche limit), the tidal forces to which the regular satellites are subject, created by the planet's gravitational field, lead to the destruction of the former. This fragmented material is the reason for the existence of planetary rings.

Another family of satellites exists around the giant planets, alongside the regular satellites. These are satellites that have been captured by the planetary gravitational field. These characteristically have high orbital inclinations relative to the planet's equatorial plane, and high eccentricities.

4.4.5.1 The Rings of the Giant Planets

The rings consist of a myriad grains and rocky blocks, whose dimensions may vary from a millimetre to several tens of metres. They occupy quasi-circular orbits, in planes that are very close to the equatorial plane of the parent planet. Each particle follows its own individual path, and permanently interacts with the particles in neighbouring orbits.

The rings have been studied by analyzing sunlight that they scatter. The observations made by the Voyager and later Cassini missions (Fig. 4.25) have shown the great complexity of the rings' dynamics. Their behaviour does not just depend on the planet's gravitational field, but also on that of the small satellites that are located within the outer rings.

The particles in planetary rings are subject to two effects:

• solar-radiation pressure which acts to push them away from the Sun, and which is proportional to the surface area of the grain; because gravitational attraction is proportional to volume, radiation pressure is particularly effective when acting on small-sized grains, about a micrometre across;

• the Poynting-Robertson effect, which arises when a particle in orbit around the Sun receives the solar radiation along the radius vector, but radiates it away isotropically. The result is a braking force which constantly decreases the eccentricity of the grain's orbit, and leads to a spiral path heading towards the Sun. This effect is particularly important for grains that are a few centimetres in size.

Fig. 4.25 Saturn's rings as observed by the Cassini probe (© NASA)

Fig. 4.25 Saturn's rings as observed by the Cassini probe (© NASA)

Particles in rings are thus subject to two types of forces that tend, either to brake them, or to eject them from the system. Calculations show that their lifetimes are very short, around a few million years. Rings are therefore systems that are permanent over time, but which consist of transient bodies. They are permanently fed by the small, inner satellites, while part of their material escapes from the system.

One cannot fail to be surprised by the extraordinary diversity of the ring systems surrounding the four giant planets. While Saturn's has been known since Galileo's time, the other three ring systems, far more tenuous, were discovered only in the last few decades. Jupiter's rings, discovered by the Voyager 1 probe in 1979, are extremely tenuous and contain heavy elements (Si, S, etc.) derived from Jupiter's innermost small satellites Saturn's rings primarily consist of water ice, as was shown by spectroscopic observations carried out in the infrared from Earth some twenty years ago. Observations made by the Cassini probe, which has been orbiting Saturn since 2004, have enabled the rings to be studied in immense detail and for their chemical composition to be analyzed as a function of their distance from Saturn. Finally, the rings around Uranus and Neptune, initially detected from observations of stellar occultations from Earth in 1977 and 1984, respectively, and subsequently analyzed by the Voyager 2 probe during its fly-bys of 1986 and 1989, have an extremely low albedo. This suggests the presence of organic material, which might result from the irradiation of ices containing hydrocarbons. The presence of ring systems around the four giant planets in the Solar System seems to suggest that giant exoplanets might also possess such ring systems. The diversity in the rings observed in the Solar System suggests an equal diversity of rings around exoplanets.

4.4.5.2 The Satellites of the Giant Planets

Even within the regular satellites of an individual planet, there is an extremely wide variety of objects. Most of the satellites are without a permanent atmosphere. The most notable exception is Titan, the largest satellite of Saturn, which has a nitrogen atmosphere whose surface pressure (1.5 bar) is close to that of the Earth. The number of satellites known continues to increase, as a result of advances in instrumentation on ground-based telescopes. Currently, more than one hundred are known. The diameters of the smallest are no more than about ten kilometres.

The four largest satellites of Jupiter (known as the Galileans after their discovery by Galileo in 1610) exhibit major differences in composition and morphology, which may be explained primarily through their different distances from Jupiter. Io (Fig. 4.26), the satellite closest to Jupiter, is subject to intense tidal forces, which, in association with Io's elliptical orbit that is created by the interaction with the other Galilean satellites, leads to continuous remodelling of the surface and active volcan-ism. This volcanism feeds a permanent, but extremely tenuous, atmosphere, mainly consisting of SO2. The surface pressure is around ten nanobars at the equator, and it decreases significantly towards the poles as a result of the condensation of sulphur dioxide. The other three Galilean satellites (Europa, Ganymede, and Callisto) have surfaces that primarily consist of water ice. In the case of Europa, which is also subject to Jupiter's tidal effects, the satellite's internal energy might be sufficient to maintain a liquid ocean beneath the surface. The depth of the surface layer of ice

Fig. 4.26 The satellite Io (image credit: courtesy NASA, JPL Caltech)

may reach several tens of kilometres. The surfaces of the Galilean satellites are less evolved, the farther they are away from Jupiter.

Most of the regular satellites of Saturn are also covered with water ice, and without atmospheres. Lying at a distance of four Saturn radii, Enceladus is exceptional since the discovery by the Cassini probe of active cryovolcanism at the south pole. The other exception, already mentioned, is Titan (Fig. 4.27). Orbiting at a distance of 25 Saturn radii, Titan has a dense atmosphere of molecular nitrogen, with a

Fig. 4.27 The surface of Titan as seen by the Huygens probe (image credit: courtesy ESA; NASA, JPL University of Arizona)

few per cent of methane. The methane is dissociated in the upper atmosphere and forms complex hydrocarbons that fall back onto the surface. The nitrogen is also dissociated by the energetic particles in the planet's magnetosphere, and becomes incorporated into nitriles. The methane, continuously photodissociated, is constantly regenerated in the atmosphere by an internal source. It is probably the same for NH3, degassed from the surface by cryovolcanism (that is, by being ejected as liquid or gas which immediately solidifies on the surface), and transformed into N2 by photodissociation. Unlike those of the giant planets, Titan's atmosphere was not produced from the surrounding nebula, but has been degassed from the body itself.

At a greater heliocentric distance from the Sun, the satellites of Uranus are darker than those of Jupiter and Saturn. As with the rings, this dark colour is undoubtedly the result of irradiation of ices containing hydrocarbons by energetic particles. Among Neptune's satellites, the most remarkable is Triton, an irregular satellite, whose features closely resemble those of Pluto when the latter, which has a very eccentric orbit, is at a comparable heliocentric distance. It has a tenuous atmosphere of molecular nitrogen, with a surface pressure of a few microbars, and methane is present at rate of about one per cent. In the light of the discoveries made in the last ten years about the trans-Neptunian (TNO) family, of which Pluto is one of the largest representatives, it seems that Triton itself is a TNO that has been captured by Neptune.

4.4.6 Small Bodies in the Solar System

Several classes of objects orbit the Sun in interplanetary space: the asteroids, comets, and the trans-Neptunian objects. Investigation of them provides information about the formation process and dynamical evolution of the outer Solar System. Comets have been known since antiquity, because their highly eccentric orbits may bring them close to the Sun and the Earth, when they become observable. The asteroids were first discovered at the beginning of the 19th century, and the trans-Neptunian objects at the end of the 20th century. As with the outer satellites, the number of small bodies discovered continuously increases, thanks to the commissioning of deep probes using increasingly powerful cameras in conjunction with large telescopes on the ground.

4.4.6.1 The Asteroids

The asteroids are objects with no atmospheres, with diameters less than 1000 km, most of which have quasi-circular orbits at heliocentric distances between 2 and 3.5 AU; this is the Main Belt (Fig. 4.28). Their accumulation within this region suggests that they are the remnants of a swarm of planetesimals, which were not able to accrete into a single planet, because of Jupiter's immense gravitational field. Other families of asteroids lie inside and outside the main asteroid belt: The NEA

Fig. 4.28 The orbits of several asteroids

objects (Near-Earth Asteroids) have one specific feature in that they come close to the Earth's orbit; the Trojans lie on Jupiter's orbit at the L4 and L5 Lagrangian points, 60 degrees ahead of, and behind, Jupiter. Farther from the Sun, the Centaurs are on more eccentric orbits, between the orbits of Jupiter and Neptune. Some of these objects are surrounded by a transient gaseous envelope, resembling those found around comets.

We know the chemical and mineralogical composition of the asteroids through spectroscopic observations in the near infrared, made from the ground. Several spectral types are recognized: the principal ones are the silicaceous asteroids (S), the metallic (M), and the carbonaceous (C). Their distribution as a function of heliocentric distance reveals that the densest asteroids (S and M) are, on average, closer to the Sun that the carbonaceous ones, the latter being more primitive. We find stratification resulting from the condensation sequence, in agreement with the model of planetary formation. A few asteroids have been the targets of fly-bys by space probes, and it has therefore been possible to study their surface properties in detail. Gaspra and Ida were examined by the Galileo probe, Mathilde and Eros by NEAR, Braille by Deep Space 1, Hayabusa by MUSES-C, etc.

Another source of information about asteroids comes from laboratory studies of meteorites, whose parent bodies are the asteroids. Such measurements have enabled the D:H ratio in particular to be measured for asteroids. We have seen that this ratio is greater, the lower the temperature, because low temperatures favour the formation of deuterated molecules. It has thus been possible to show that the D:H ratio in the terrestrial oceans (SMOW, Standard Mean Ocean Water) is close to that in asteroids of type D, primitive carbonaceous objects populating the outer region of the Main

Asteroidal Belt. The water in terrestrial oceans thus primarily stems from that population, with a minor component (with a lower D:H ratio than SMOW), originating from degassing by the globe, and another tiny component (with a higher D:H ratio than SMOW) originating in comets (see below).

4.4.6.2 Comets

Comets are objects with diameters generally less than some ten kilometres, orbiting the Sun on very eccentric orbits. Far from the Sun, they consist of a nucleus of ice and dust. Their periods range from a few years for the closest comets (such as Encke) to several thousand years (e.g., Hale-Bopp). When a comet approaches the Sun, its surface sublimes and we observe the ejection of gas and dust: this is the coma, which reflects and scatters sunlight, rendering the comet visible from Earth, sometimes even with the naked eye. The parent molecules that are ejected from the nucleus, observable from the ground in the infrared and millimetric regions, are, in turn, themselves dissociated into daughter molecules, radicals and ions. These species are detectable in the visible and ultraviolet regions through their fluorescent emissions.

Cometary physics has benefited greatly from the exploration of Comet Halley, with a period of 76 years, known since antiquity, and which is particularly bright. When it returned in 1986, several space probes were directed to encounter it, allowing the first in-situ analysis of a cometary nucleus. Since Halley's return, other very bright new comets (Hyakutake in 1996 and Hale-Bopp in 1997) (Fig. 4.29) have been the subject of profound study, in particular thanks to infrared and millimetric spectroscopy.

Fig. 4.29 Comet Hale-Bopp

Comets consist of 80 per cent water, but numerous other molecules have also been detected within them, generally at abundances that are less than a few per cent: CO, CO2, HCN, CH3OH, HCO, etc. The degassing rate for these molecules depends on the heliocentric distance. Degassing of CO, at several AU (11 AU in the case of Hale-Bopp) is probably the first sign of a comet's activity. Degassing of water occurs at around 2 AU from the Sun. The list of parent molecules (Table 4.3) contains more than twenty. All have similarly been observed in the interstellar medium. The nucleus of a comet has a very low albedo (less than 0.10). It appears to be covered in carbonaceous material, probably the result of irradiation of hydrocarbon-rich ices by cosmic rays. This carbonaceous material has been observed to be abundant in cometary dust released from the nucleus. Finally, isotopic ratios measured in comets, highly enriched in deuterium, bear witness to their formation at extremely low temperatures. The D:H ratio measured for water in particular, is twice as high as the terrestrial ratio.

All these signs bear witness to a common parentage between cometary material and interstellar material. Comets are some of the most primitive objects in the Solar System. Their small size has protected them from any effects of differentiation. Because they evolved for most of the time far from the Sun, in a cold and extremely tenuous environment, they also avoided thermal effects and collisions. Comets are therefore precious witnesses to the conditions surrounding the formation of the outer Solar System.

We have seen (Sect. 4.2.3) that the study of the orbits of comets has enable us to determine that high-inclination comets originate in the Oort Cloud, a vast shell lying some 40 000 AU from the Sun. Injected into this reservoir by perturbations by Jupiter, at the very beginning of their history, on rare occasions they return as the result of other local perturbations and may, in certain cases, stabilize on orbits within the Solar System following other planetary perturbations. Certain comets approach sufficiently close to the Sun to disappear into it. The SOHO satellite has shown that more than one hundred comets are swallowed up by the Sun every year. We may note the similarity here with that of the regular fall onto the star P Pictoris of comets from its disk of debris (see Chap. 6.) Finally, alongside comets from the Oort Cloud, other comets, with low inclinations and short periods, originate in another reservoir, the Kuiper Belt, the location of the trans-Neptunian objects.

4.4.6.3 The Trans-Neptunian Objects

Around the middle of the 20th century, the astronomers K. Edgeworth and G. Kuiper suggested, on the basis of dynamical studies, the existence of a toroidal belt of small bodies lying beyond Neptune - known as the trans-Neptunian objects (TNOs) -where low-inclination comets originated. In 1992, D. Jewitt and J. Luu detected the first member of this new family of objects. About 15 years later, more than a thousand are known, and the number continues to increase, with objects that are even farther away becoming detectable with the constant improvements in observational techniques (Fig. 4.30).

Fig. 4.30 The paths of a number of TNOs (After Encrenaz et al., 2004)

Fig. 4.30 The paths of a number of TNOs (After Encrenaz et al., 2004)

The TNOs are large icy bodies, with an average diameter of a few hundred kilometres, and whose albedo is higher than that of comets. At present we know little about their physical and chemical properties, because spectroscopy of these faint objects is at the limits of current techniques. The first spectra obtained seem to indicate a resemblance to the spectra of Triton, Pluto, or with the Centaur, Pholus, with the presence of water ice in some cases, and of hydrocarbon (methanol) ices. The difference in composition may reflect different stages in the alteration of the surface, caused by different rates of bombardment by cosmic rays.

In contrast, we know more about the dynamical characteristics of the TNOs. The latter may be divided into three major classes:

• 'classic' objects, which includes more than half of these bodies, have quasi-circular orbits with low inclinations. Their average heliocentric distance is 42 to 47 AU, and they are not associated with any resonances;

• resonant objects (between 10 and 15 per cent of the total population) are, like Pluto, in the 3:2 resonance with Neptune, at a distance of 39.2 AU; these are known as Plutinos. This property confirms that Pluto, once classed as the ninth planet in the Solar System, is in fact one of the largest representatives of the TNO family.

• scattered objects have a perihelion outside the orbit of Neptune and a high eccentricity. It is in this category that astronomers may well expect to make new discoveries of objects that are more and more distant and larger in size, such as the object 2003 UB313, discovered by M. Brown, which has a diameter larger than that of Pluto.

The Kuiper Belt therefore has a well-defined inner boundary that is determined by the 3:2 resonance with Neptune, but its outer boundary is far less well-defined.

To conclude, the Kuiper Belt may be considered, together with the comets in the Oort Cloud, as remnants of the initial protoplanetary disk. Studies of its dynamical families appears to show that the giant planets have migrated to a certain extent (see Sect. 4.3.2.6 and Chap. 6). Continuing exploration of the TNOs over the next few years should enable us to gain a better understanding of the overall dynamical history of the outer Solar System.

4.5 Conclusions: The Solar System Compared with Other Planetary Systems

What lessons may we draw from our study of the Solar System that are relevant to our exploration of planetary systems? We may obtain two forms of information from this comparison. The first concerns the scenario covering the formation of the Solar System: it is the exception or the rule? At present, do we have sufficient information on planetary systems to answer that question? The second lesson to be extracted from study of the Solar System concerns the study of the different classes of objects, with all their diversity. Can we gain a better idea of the atmospheres of exoplanets by studying planetary atmospheres? Finally, apart from exoplanets, what other bodies might be detectable in other planetary systems, and to what extent can we carry out remote sensing to investigate them?

4.5.1 The Scenario for the Formation of the Solar System

We have seen that the Solar System formed through the collapse into a disk of a protoplanetary nebula, and that the planets formed by nucleation within this disk, starting with solid material. The observational facts on which this model is based are primarily:

• the quasi-coplanar and quasi-concentric orbits of the planets

• the enrichment on the giant planets in heavy elements, which is a factor that is strongly in favour of their formation through nucleation

• the existence of systems of rings and numerous regular satellites in the equatorial planes of the giant planets, and close to the latter, which bear witness to the collapse of surrounding subsidiary nebulae onto an initial solid core

• the existence of the main asteroid belt, of the Kuiper Belt, and the Oort Cloud, all vestiges of the accretion process. They represent the overall population of planetesimals that were not incorporated into more massive objects.

How do these pieces of data compare with the data that we currently have regarding exoplanetary systems? Their properties are, on the face of it, very different:

• Many giant exoplanets occur in close proximity to their star. The model of the formation of the Solar System does not explain their presence at such a location, unless they formed farther away and subsequently migrated to the immediate neighbourhood of their parent star;

• Many giant exoplanets, farther away from their parent star, have very eccentric orbits, which does not agree well with their formation from a disk that was produced by the collapse of a protostellar nebula;

• The number of exoplanets detected around a star seems to increase with the latter's metallicity rate, which seems to favour the theory of planetary formation by nucleation; however, the metallicity of the Sun is not particularly high, and the Solar System does not conform to this property.

Must we therefore conclude that the Solar System is an exceptional case? It is probably too soon to maintain this view. There are giant exoplanets at considerable distances from their parent star, in orbits with very low eccentricities. Nothing precludes the existence of exo-Earths or small exoplanets in the inner regions of a planetary system. We do not currently have the means to detect them. From the ubiquity of disks around young stars, we can infer that the method by which the Solar System formed is probably not unique, but that it does not seem to be widespread in the solar neighbourhood. Another possibility is that the method by which the Solar System formed might, under limiting conditions, evolve into very different systems. An important step in understanding this problem will be the detection of exoplanets of the terrestrial type. We will have to await the results of the CoRoT and KEPLER space missions to know if small exoplanets exist, and where they are located.

4.5.2 Objects in the Planetary Systems Observable from Earth

Our knowledge of the physical and chemical properties of exoplanets is still very poor. A comparative study of the planets in the Solar System should enable us to understand them better. In addition, apart from exoplanets, other bodies belonging to planetary systems are observable, or could become so with improvements in methods of observation. These include comets (by their possible fall into their parent stars), the Kuiper Belt (through the large quantity of water that it contains), the rings and satellites of exoplanets (by observation of transits or gravitational microlensing).

4.5.2.1 The Atmospheres of Exoplanets

Study of the planets in the Solar System teaches us one lesson. This is the astonishing diversity of objects that may be found within a single family. The properties of each planet are the result of the way in which it formed, but also of the evolutionary processes it has undergone. The most striking example is the diversity of the surface conditions of the terrestrial planets. When we are able to detect exo-Earths, we can thus expect to find a similar diversity. The atmospheric composition of an exoplanet will be identifiable from its infrared spectrum. An exoplanet having an abnormally high temperature (like Venus) could be detected from its night side through the thermal radiation. A surface temperature less than that of the atmosphere would be detectable through a thermal emission spectrum. Finally, numerous studies, based on models of stellar and planetary atmospheres, have been carried out to model the composition and thermal structure of an exoplanet as a function of its distance from its parent star (see Chap. 7).

4.5.2.2 Comets and the Kuiper Belt in Planetary Systems

We have seen that numerous comets that approach the Sun end their lives by disintegrating and becoming swallowed up within it. This phenomenon would be observable from outside the Solar System, through the observation, in the visible and UV regions, of transient, fluorescent emission lines, created when these object are volatilized. Observations of P Pictoris have shown transient phenomena that suggest the fall of comets onto the central star (see Sects. 5.3.2 and 6.5). Beta Pictoris is surrounded by a disk of dust, and the presence of a gap in this disk suggests the existence of at least one exoplanet. Taking account of the relatively late age of the star, the disk may not correspond with the planetary formation stage, but may consist of a residue of planetesimals that did not lead to the formation of planets. It would thus be the equivalent - albeit with a much greater volume - of our Kuiper Belt. Other observations of disks of dust (such as HR 4796A and Epsilon Eridani) suggest the presence of a Kuiper Belt around other stars. Similarly, observation of an abnormally large quantity of water vapour around a late star might be explained by a massive vaporization of its Kuiper Belt, in which the star has swollen as it passes into the red-giant stage. We will then be witnessing the scenario that is scheduled to take place when the Sun finally dies, in a few thousand million years.

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