Planetesimals and Satellitesimals: Formation of the Satellite Systems

The origin of the regular satellites ties directly to planetary formation in that the satellites form in gas and dust disks around the giant planets and may be viewed as mini-solar systems, involving a number of closely related underlying physical processes. The regular satellites of Jupiter and Saturn share a number of remarkable similarities that taken together make a compelling case for a deep-seated order and structure governing their origin. Furthermore, the similarities in the mass ratio of the largest satellites to their primaries, the specific angular momenta, and the bulk compositions of the two satellite systems are significant and in need of explanation. Yet, the differences are also striking. We advance a common framework for the origin of the regular satellites of Jupiter and Saturn and discuss the accretion of satellites in gaseous, circumplanetary disks. Following giant planet formation, planetesimals in the planet’s feeding zone undergo a brief period of intense collisional grinding. Mass delivery to the circumplanetary disk via ablation of planetesimal fragments has implications for a host of satellite observations, tying the history of planetesimals to that of satellitesimals and ultimately that of the satellites themselves. By contrast, irregular satellites are objects captured during the final stages of planetary formation or the early evolution of the Solar System; their distinct origin is reflected in their physical properties, which has implications for the subsequent evolution of the satellites systems.     

Water Loss from Young Planets

Good progress has been made in the past few years to better understand the XUV evolution trend of Sun-like stars, the capture and dissipation of hydrogen dominant envelopes of planetary embryos and protoplanets, and water loss from young planets around M dwarfs. This chapter reviews these recent developments. Observations of exoplanets and theoretical works in the near future will significantly advance our understanding of one of the fundamental physical processes shaping the evolution of solar system terrestrial planets.     

Water and the Interior Structure of Terrestrial Planets and Icy Bodies

Water content and the internal evolution of terrestrial planets and icy bodies are closely linked. The distribution of water in planetary systems is controlled by the temperature structure in the protoplanetary disk and dynamics and migration of planetesimals and planetary embryos. This results in the formation of planetesimals and planetary embryos with a great variety of compositions, water contents and degrees of oxidation. The internal evolution and especially the formation time of planetesimals relative to the timescale of radiogenic heating by short-lived ²⁶Al decay may govern the amount of hydrous silicates and leftover rock–ice mixtures available in the late stages of their evolution. In turn, water content may affect the early internal evolution of the planetesimals and in particular metal-silicate separation processes. Moreover, water content may contribute to an increase of oxygen fugacity and thus affect the concentrations of siderophile elements within the silicate reservoirs of Solar System objects. Finally, the water content strongly influences the differentiation rate of the icy moons, controls their internal evolution and governs the alteration processes occurring in their deep interiors.     

Evolution of the Protosolar Nebula and Formation of the Giant Planets

The formation of planetary systems is intimately tied to the question of the evolution of the gas and solid material in the early nebula. Current models of evolution of circumstellar disks are reviewed here with emphasis on the so-called “alpha models” in which angular momentum is transported outward by turbulent viscosity, parameterized by an dimensionless parameter α. A simple 1D model of protoplanetary disks that includes gas and embedded particles is used to introduce key questions on planetesimal formation. This model includes the aerodynamic properties of solid ice and rock grains to calculate their migration and growth. We show that the evolution of the nebula and migration and growth of its solids proceed on timescales that are generally not much longer than the timescale necessary to fully form the star-disk system from the molecular cloud. Contrary to a widely used approach, planet formation therefore can neither be studied in a static nebula nor in a nebula evolving from an arbitrary initial condition. We propose a simple approach to both account for sedimentation from the molecular cloud onto the disk, disk evolution and migration of solids. Giant planets have key roles in the history of the forming Solar System: they formed relatively early, when a significant amount of hydrogen and helium were still present in the nebula, and have a mass that is a sizable fraction of the disk mass at any given time. Their composition is also of interest because when compared to the solar composition, their enrichment in elements other than hydrogen and helium is a witness of sorting processes that occured in the protosolar nebula. We review likely scenarios capable of explaining both the presence of central dense cores in Jupiter, Saturn, Uranus and Neptune and their global composition. This revised version was published online in August 2006 with corrections to the Cover Date.     

Feedstocks of the Terrestrial Planets

The processes of planet formation in our Solar System resulted in a final product of a small number of discreet planets and planetesimals characterized by clear compositional distinctions. A key advance on this subject was provided when nucleosynthetic isotopic variability was discovered between different meteorite groups and the terrestrial planets. This information has now been coupled with theoretical models of planetesimal growth and giant planet migration to better understand the nature of the materials accumulated into the terrestrial planets. First order conclusions include that carbonaceous chondrites appear to contribute a much smaller mass fraction to the terrestrial planets than previously suspected, that gas-driven giant planet migration could have pushed volatile-rich material into the inner Solar System, and that planetesimal formation was occurring on a sufficiently rapid time scale that global melting of asteroid-sized objects was instigated by radioactive decay of ²⁶Al. The isotopic evidence highlights the important role of enstatite chondrites, or something with their mix of nucleosynthetic components, as feedstock for the terrestrial planets. A common degree of depletion of moderately volatile elements in the terrestrial planets points to a mechanism that can effectively separate volatile and refractory elements over a spatial scale the size of the whole inner Solar System. The large variability in iron to silicon ratios between both different meteorite groups and between the terrestrial planets suggests that mechanisms that can segregate iron metal from silicate should be given greater importance in future investigations. Such processes likely include both density separation of small grains in the nebula, but also preferential impact erosion of either the mantle or core from differentiated planets/planetesimals. The latter highlights the important role for giant impacts and collisional erosion during the late stages of planet formation.     

Beta Pictoris and Other Solar Systems

Planetary systems come in a bewildering variety of shapes and sizes. In addition to the exoplanetary systems with giant planets, found in surveys of stellar radial velocity variations, an overlapping class of dusty disk-containing solar systems exists. The disks include large quantities of meteoroids and dust, and a varying complement of gas. Their solid material represents `replenished' dust born in the collisions/sublimation of planetesimals perturbed by planets. We present several such systems, including HR 4796A, HD 141569, HD 100546, and the prototypical replenished disk of Beta Pictoris. We discuss the composition, physical processing, and migration of dust in the disks, their evolutionary status, and the evidence of embedded planets. This revised version was published online in June 2006 with corrections to the Cover Date.     

Below One Earth: The Detection, Formation, and Properties of Subterrestrial Worlds

The Solar System includes two planets—Mercury and Mars—significantly less massive than Earth, and all evidence indicates that planets of similar size orbit many stars. In fact, one of the first exoplanets to be discovered is a lunar-mass planet around a millisecond pulsar. Novel classes of exoplanets have inspired new ideas about planet formation and evolution, and these “sub-Earths” should be no exception: they include planets with masses between Mars and Venus for which there are no Solar System analogs. Advances in astronomical instrumentation and recent space missions have opened the sub-Earth frontier for exploration: the Kepler mission has discovered dozens of confirmed or candidate sub-Earths transiting their host stars. It can detect Mars-size planets around its smallest stellar targets, as well as exomoons of comparable size. Although the application of the Doppler method is currently limited by instrument stability, future spectrographs may detect equivalent planets orbiting close to nearby bright stars. Future space-based microlensing missions should be able to probe the sub-Earth population on much wider orbits. A census of sub-Earths will complete the reconnaissance of the exoplanet mass spectrum and test predictions of planet formation models, including whether low-mass M dwarf stars preferentially host the smallest planets. The properties of sub-Earths may reflect their low gravity, diverse origins, and environment, but they will be elusive: Observations of eclipsing systems by the James Webb Space Telescope may give us our first clues to the properties of these small worlds.     

Disks, Extrasolar Planets and Migration

We review results about protoplanetary disk models, protoplanet migration and formation of giant planets with migrating cores. We first model the protoplanetary nebula as an α–accretion disk and present steady state calculations for different values of α and gas accretion rate through the disk. We then review the current theories of protoplanet migration in the context of these models, focusing on the gaseous disk–protoplanet tidal interaction. According to these theories, the migration timescale may be shorter than the planetary formation timescale. Therefore we investigate planet formation in the context of a migrating core, considering both the growth of the core and the build–up of the envelope in the course of the migration. This revised version was published online in June 2006 with corrections to the Cover Date.     

Formation of the Outer Planets

Models of the origins of gas giant planets and ‘ice’ giant planets are discussed and related to formation theories of both smaller objects (terrestrial planets) and larger bodies (stars). The most detailed models of planetary formation are based upon observations of our own Solar System, of young stars and their environments, and of extrasolar planets. Stars form from the collapse, and sometimes fragmentation, of molecular cloud cores. Terrestrial planets are formed within disks around young stars via the accumulation of small dust grains into larger and larger bodies until the planetary orbits become well enough separated that the configuration is stable for the lifetime of the system. Uranus and Neptune almost certainly formed via a bottom-up (terrestrial planet-like) mechanism; such a mechanism is also the most likely origin scenario for Saturn and Jupiter.     

Formation of Giant Planets– An Attempt in Matching Observational Constraints

We present models of giant planet formation, taking into account migration and disk viscous evolution. We show that migration can significantly reduce the formation timescale bringing it in good agreement with typical observed disk lifetimes. We then present a model that produces a planet whose current location, core mass and total mass are comparable with the one of Jupiter. For this model, we calculate the enrichments in volatiles and compare them with the one measured by the Galileo probe. We show that our models can reproduce both the measured atmosphere enrichments and the constraints derived by Guillot et al. (2004), if we assume the accretion of planetesimals with ices/rocks ratio equal to 4, and that a substantial amount of CO₂ was present in vapor phase in the solar nebula, in agreement with ISM measurements.     

Formation of the Cores of the Outer Planets

The formation of the giant planets seems to be best explained by accretion of planetesimals to form massive cores, which in the case of Jupiter and Saturn were able to capture nebular gas. However, the timescale for accretion of such cores has been a problem. Accretion in the outer solar system differs qualitatively from planetary growth in the terrestrial region, as the larger embryo masses and lower orbital velocities make bodies more subject to gravitational scattering. The planetesimal swarm in the outer nebula may be seeded by earlier-formed large bodies scattered from the region near the nebular “snow line”. Such a seed body can experience rapid runaway growth undisturbed by competitors; the style of growth is not oligarchy, but monarchy.     

Formation of Planetesimals and Accretion of the Terrestrial Planets

Planetesimals formed in the solar nebula by collisional coagulation. Dust aggregates settled toward the central plane, the larger ones growing by sweeping up smaller ones. A thin, dense layer of particles formed; shear-generated turbulence and differential motions induced by gas drag inhibited gravitational instability. Growth proceeded by collisions, producing planetesimals on a timescale of a few thousand years in the terrestrial zone. For bodies smaller than about a kilometer, motions were dominated by gas drag, and impact velocities decreased with size. At larger sizes gravitational interactions became significant, and velocities increased due to mutual perturbations. Larger bodies then grew more rapidly, this ``runaway' led to formation of tens to hundreds of lunar- to Mars-sized planetary embryos in the zone of terrestrial planets. The final accretion of these bodies into a few planets involved large impacts, and occurred on a timescale of 10⁷ to 10⁸ years. This scenario gives a reasonably consistent picture of the origin of the terrestrial planets, but does not account for the anomalously low eccentricities of the Earth and Venus. This revised version was published online in June 2006 with corrections to the Cover Date.     

Isotopic Signatures of Volatiles in Terrestrial Planets - Working Group Report

Variations in the isotopic ratios of volatile elements in different reservoirs on the terrestrial planets carry information about processes that operated on the planets since their formation. Comparisons between primordial planetary compositions, to the extent they can be determined, may help us understand the planetary formation process. This working group report summarizes our knowledge of terrestrial planet volatile inventories. This revised version was published online in August 2006 with corrections to the Cover Date.     

Dynamo Models for Planets Other Than Earth

Observations from planetary spacecraft missions have demonstrated a spectrum of dynamo behaviour in planets. From currently active dynamos, to remanent crustal fields from past dynamo action, to no observed magnetization, the planets and moons in our solar system offer magnetic clues to their interior structure and evolution. Here we review numerical dynamo simulations for planets other than Earth. For the terrestrial planets and satellites, we discuss specific magnetic field oddities that dynamo models attempt to explain. For the giant planets, we discuss both non-magnetic and magnetic convection models and their ability to reproduce observations of surface zonal flows and magnetic field morphology. Future improvements to numerical models and new missions to collect planetary magnetic data will continue to improve our understanding of the magnetic field generation process inside planets.     

The Delivery of Water During Terrestrial Planet Formation

The planetary building blocks that formed in the terrestrial planet region were likely very dry, yet water is comparatively abundant on Earth. Here we review the various mechanisms proposed for the origin of water on the terrestrial planets. Various in-situ mechanisms have been suggested, which allow for the incorporation of water into the local planetesimals in the terrestrial planet region or into the planets themselves from local sources, although all of those mechanisms have difficulties. Comets have also been proposed as a source, although there may be problems fitting isotopic constraints, and the delivery efficiency is very low, such that it may be difficult to deliver even a single Earth ocean of water this way. The most promising route for water delivery is the accretion of material from beyond the snow line, similar to carbonaceous chondrites, that is scattered into the terrestrial planet region as the planets are growing. Two main scenarios are discussed in detail. First is the classical scenario in which the giant planets begin roughly in their final locations and the disk of planetesimals and embryos in the terrestrial planet region extends all the way into the outer asteroid belt region. Second is the Grand Tack scenario, where early inward and outward migration of the giant planets implants material from beyond the snow line into the asteroid belt and terrestrial planet region, where it can be accreted by the growing planets. Sufficient water is delivered to the terrestrial planets in both scenarios. While the Grand Tack scenario provides a better fit to most constraints, namely the small mass of Mars, planets may form too fast in the nominal case discussed here. This discrepancy may be reduced as a wider range of initial conditions is explored. Finally, we discuss several more recent models that may have important implications for water delivery to the terrestrial planets.     

Vesta and Ceres: Crossing the History of the Solar System

The evolution of the Solar System can be schematically divided into three different phases: the Solar Nebula, the Primordial Solar System and the Modern Solar System. These three periods were characterized by very different conditions, both from the point of view of the physical conditions and from that of the processes there were acting through them. Across the Solar Nebula phase, planetesimals and planetary embryos were forming and differentiating due to the decay of short-lived radionuclides. At the same time, giant planets formed their cores and accreted the nebular gas to reach their present masses. After the gas dispersal, the Primordial Solar System began its evolution. In the inner Solar System, planetary embryos formed the terrestrial planets and, in combination with the gravitational perturbations of the giant planets, depleted the residual population of planetesimals. In the outer Solar System, giant planets underwent a violent, chaotic phase of orbital rearrangement which caused the Late Heavy Bombardment. Then the rapid and fierce evolution of the young Solar System left place to the more regular secular evolution of the Modern Solar System. Vesta, through its connection with HED meteorites, and plausibly Ceres too were between the first bodies to form in the history of the Solar System. Here we discuss the timescale of their formation and evolution and how they would have been affected by their passage through the different phases of the history of the Solar System, in order to draw a reference framework to interpret the data that Dawn mission will supply on them.     

Magnetic Fields of the Outer Planets

The rapidly rotating giant planets of the outer solar system all possess strong dynamo-driven magnetic fields that carve a large cavity in the flowing magnetized solar wind. Each planet brings a unique facet to the study of planetary magnetism. Jupiter possesses the largest planetary magnetic moment, 1.55×10²⁰ Tm³, 2×10⁴ times larger than the terrestrial magnetic moment whose axis of symmetry is offset about 10° from the rotation axis, a tilt angle very similar to that of the Earth. Saturn has a dipole magnetic moment of 4.6×10¹⁸ Tm³ or 600 times that of the Earth, but unlike the Earth and Jupiter, the tilt of this magnetic moment is less than 1° to the rotation axis. The other two gas giants, Uranus and Neptune, have unusual magnetic fields as well, not only because of their tilts but also because of the harmonic content of their internal fields. Uranus has two anomalous tilts, of its rotation axis and of its dipole axis. Unlike the other planets, the rotation axis of Uranus is tilted 97.5° to the normal to its orbital plane. Its magnetic dipole moment of 3.9×10¹⁷ Tm³ is about 50 times the terrestrial moment with a tilt angle of close to 60° to the rotation axis of the planet. In contrast, Neptune with a more normal obliquity has a magnetic moment of 2.2×10¹⁷ Tm³ or slightly over 25 times the terrestrial moment. The tilt angle of this moment is 47°, smaller than that of Uranus but much larger than those of the Earth, Jupiter and Saturn. These two planets have such high harmonic content in their fields that the single flyby of Voyager was unable to resolve the higher degree coefficients accurately. The four gas giants have no apparent surface features that reflect the motion of the deep interior, so the magnetic field has been used to attempt to provide this information. This approach works very well at Jupiter where there is a significant tilt of the dipole and a long baseline of magnetic field measurements (Pioneer 10 to Galileo). The rotation rate is 870.536° per day corresponding to a (System III) period of 9 h 55 min 26.704 s. At Saturn, it has been much more difficult to determine the equivalent rotation period. The most probable rotation period of the interior is close to 10 h 33 min, but at this writing, the number is still uncertain. For Uranus and Neptune, the magnetic field is better suited for the determination of the planetary rotation period but the baseline is too short. While it is possible that the smaller planetary bodies of the outer solar system, too, have magnetic fields or once had, but the current missions to Vesta, Ceres and Pluto do not include magnetic measurements.     

Neutral Atmospheres of the Giant Planets: An Overview of Composition Measurements

Measurements of the chemical composition of the giant planets provide clues of their formation and evolution processes. According to the currently accepted nucleation model, giant planets formed from the initial accretion of an icy core and the capture of the protosolar gas, mosly composed of hydrogen and helium. In the case of Jupiter and Saturn (the gaseous giants), this gaseous component dominates the composition of the planet, while for Uranus and Neptune (the icy giants) it is only a small fraction of the total mass. The measurement of elemental and isotopic ratios in the giant planets provides key diagnostics of this model, as it implies an enrichment in heavy elements (as well as deuterium) with respect to the cosmic composition. Neutral atmospheric constituents in the giant planets have three possible sources: (1) internal (fromthe bulk composition of the planet), (2) photochemical (fromthe photolysis ofmethane) and(3) external (from meteoritic impacts, of local or interplanetary origin). This paper reviews our present knowledge about the atmospheric composition in the giant planets, and their elemental and istopic composition. Measurements concerning key parameters, like C/H, D/H or rare gases in Jupiter, are analysed in detail. The conclusion addresses open questions and observations to be performed in the future.     

Future Space Missions to Search for Terrestrial Planets

Since the first exoplanet was discovered more than 10 years ago, this field has developed rapidly. Currently we know of more than 200 external systems of stars and planets, apart from our own, but what has become the ‘holy grail’ of exoplanetary research is still eluding us. Here we are, of course, referring to solar systems like our own, containing a number of terrestrial or ‘rocky’ planets orbiting within the so-called ‘habitable zone’, and with giant planets such as Jupiter and Saturn—which mostly or totally consist of elements as H or He—at distance much further out from the central star. No such system have to date been discovered around anything resembling our Sun, albeit because of observational biases. Nevertheless, in order to develop the emerging science discipline of Comparative Planetology, we will have to utilize new techniques that will enable us to search for, and then study in detail such systems in an un-biased fashion. This paper describes the emerging techniques and space missions that will allow, finally, the investigation of planets capable of hosting life as we know it.     

The Interior Structure, Composition, and Evolution of Giant Planets

We discuss our current understanding of the interior structure and thermal evolution of giant planets. This includes the gas giants, such as Jupiter and Saturn, that are primarily composed of hydrogen and helium, as well as the “ice giants,” such as Uranus and Neptune, which are primarily composed of elements heavier than H/He. The effect of different hydrogen equations of state (including new first-principles computations) on Jupiter’s core mass and heavy element distribution is detailed. This variety of the hydrogen equations of state translate into an uncertainty in Jupiter’s core mass of 18M _⊕ . For Uranus and Neptune we find deep envelope metallicities up to 0.95, perhaps indicating the existence of an eroded core, as also supported by their low luminosity. We discuss the results of simple cooling models of our solar system’s planets, and show that more complex thermal evolution models may be necessary to understand their cooling history. We review how measurements of the masses and radii of the nearly 50 transiting extrasolar giant planets are changing our understanding of giant planets. In particular a fraction of these planets appear to be larger than can be accommodated by standard models of planetary contraction. We review the proposed explanations for the radii of these planets. We also discuss very young giant planets, which are being directly imaged with ground- and space-based telescopes.