Models of Jupiter and Saturn after Galileo mission

New models of Jupiter are based on observational data provided by the Galileo spaceprobe, which considerably improved previously existing estimates of the helium abundance in the atmosphere of Jupiter. These data yield for Jupiter’s atmosphere 20% of the solar oxygen abundance and do not agree with the results of the analysis of the collision of comet Shoemaker-Levy 9 with Jupiter (10 times the solar value). Therefore, both the models of Jupiter with water-depleted and water-enriched atmosphere are considered. By analogy with Jupiter, trial models of Saturn with a water-depleted external envelope are also developed. The molecular-metallic phase transition pressure of hydrogen P_m was taken to be 1.5, 2 and 3 Mbar. Since Saturn’s internal molecular envelope is noticeably enriched in the IR-component (its weight concentration, 0.25–0.30, being by a factor of 3–4 higher than in Jupiter), the phase transition pressure in Saturn can be lower than in Jupiter. In the constructed models, the IR-core masses are 3–3.5 M_⊕ for Jupiter and 3–5.5 M_⊕ for Saturn. Jupiter’s and Saturn’s IR-cores can be considered embryos onto which the accretion of the gas occurred during the formation of the planets. The mass of the hydrogen–helium component dispersed in the zone of planetary formation constitutes ≈2–5 planetary masses for Jupiter and ≈11–14 planetary masses for Saturn.     

Planets in the asteroid belt

Abstract— The main asteroid belt has lost >99.9% of its solid mass since the time at which the planets were forming, according to models for the protoplanetary nebula. Here we show that the primordial asteroid belt could have been cleared efficiently if much of the original mass accreted to form planetsized bodies, which were capable of perturbing one another into unstable orbits. We provide results from 25 N‐body integrations of up to 200 planets in the asteroid belt, with individual masses in the range 0.017–0.33 Earth masses. In the simulations, these bodies undergo repeated close encounters which scatter one another into unstable resonances with the giant planets, leading to collision with the Sun or ejection from the solar system. In response, the giant planets' orbits migrate radially and become more circular. This reduces the size of the main‐belt resonances and the clearing rate, although clearing continues. If ～3 Earth masses of material was removed from the belt this way, Jupiter and Saturn would initially have had orbital eccentricities almost twice their current values. Such orbits would have made Jupiter and Saturn 10–100x more effective at clearing material from the belt than they are on their current orbits. The time required to remove 90% of the initial mass from the belt depends sensitively on the giant planets' orbits, and weakly on the masses of the asteroidal planets. 18 of the 25 simulations end with no planets left in the belt, and the clearing takes up to several hundred million years. Typically, the last one or two asteroidal planets are removed by interactions with planets in the terrestrial region     

On the stability of Earth-like planets in multi-planet systems

We present a continuation of our numerical study on planetary systems with similar characteristics to the Solar System. This time we examine the influence of three giant planets on the motion of terrestrial-like planets in the habitable zone (HZ). Using the Jupiter–Saturn–Uranus configuration we create similar fictitious systems by varying Saturn’s semi-major axis from 8 to 11 AU and increasing its mass by factors of 2–30. The analysis of the different systems shows the following interesting results: (i) Using the masses of the Solar System for the three giant planets, our study indicates a maximum eccentricity (max-e) of nearly 0.3 for a test-planet placed at the position of Venus. Such a high eccentricity was already found in our previous study of Jupiter–Saturn systems. Perturbations associated with the secular frequency g ₅ are again responsible for this high eccentricity. (ii) An increase of the Saturn-mass causes stronger perturbations around the position of the Earth and in the outer HZ. The latter is certainly due to gravitational interaction between Saturn and Uranus. (iii) The Saturn-mass increased by a factor 5 or higher indicates high eccentricities for a test-planet placed at the position of Mars. So that a crossing of the Earth’ orbit might occur in some cases. Furthermore, we present the maximum eccentricity of a test-planet placed in the Earth’ orbit for all positions (from 8 to 11 AU) and masses (increased up to a factor of 30) of Saturn. It can be seen that already a double-mass Saturn moving in its actual orbit causes an increase of the eccentricity up to 0.2 of a test-planet placed at Earth’s position. A more massive Saturn orbiting the Sun outside the 5:2 mean motion resonance (a _S  ≥9.7 AU) increases the eccentricity of a test-planet up to 0.4.     

The chemical atmospheric composition of the giant planets

For a long time it was believed that the atmospheres of the giant planets, dominated by molecular hydrogen and helium, were similar in composition to the primordial nebula from which they formed. However, this image has strongly evolved over the past twenty years, due to new developments of ground-based infrared spectroscopy, coupled with the success of the Voyager space mission.Significant differences were measured in the abundances of helium, deuterium and carbon of the four giant planets. The variations in the C/H and D/H ratios have given support to the "nucleation" formation scenario, in which the four giant planets first accreted a nucleus of about ten terrestrial masses, big enough to bind gravitationally the surrounding gaseous nebula; the helium depletion in Saturn has been interpreted as a differentiation effect in Saturn's interior; the apparent helium excess in Neptune, coupled with the recent unexpected detection of CO and HCN in this planet, might imply the presence of molecular nitrogen. In the case of Jupiter and Saturn, disequilibrium species have been detected (CO, PH₃, GeH₄, AsH₃), which are tracers of vertical dynamical motions.In the future, significant progress in our knowledge of the Jovian composition, including the noble gases, should be obtained with the mass spectrometer of the Galileo probe. The ISO mission is expected to provide new far-infrared spectroscopic data which should lead to the detection of new minor species and a better determination of the D/H ratio.     

Forming Jupiter, Saturn, Uranus and Neptune in few million years by core accretion

Giant planet formation process is still not completely understood. The current most accepted paradigm, the core instability model, explains several observed properties of the Solar System’s giant planets but, to date, has faced difficulties to account for a formation time shorter than the observational estimates of protoplanetary disks’ lifetimes, especially for the cases of Uranus and Neptune. In the context of this model, and considering a recently proposed primordial Solar System orbital structure, we performed numerical calculations of giant planet formation. Our results show that if accreted planetesimals follow a size distribution in which most of the mass lies in 30-100 m sized bodies, Jupiter, Saturn, Uranus and Neptune may have formed according to the nucleated instability scenario. The formation of each planet occurs within the time constraints and they end up with core masses in good agreement with present estimations.     

Formation of the giant planets

Observational constraints on interior models of the giant planets indicate that these planets were all much hotter when they formed and they all have rock and/or ice cores of ten to thirty earth masses. These cores are probably soluble in the envelopes above, especially in Jupiter and Saturn, and are therefore likely to be primordial. They persist despite the continual upward mixing by thermally driven convection throughout the age of the solar system, because of the inefficiency of double-diffusive convection. Thus, these planets most probably formed by the hydrodynamic collapse of a gaseous envelope onto a core rather than by direct instability of the gaseous solar nebula. Recent calculations by Mizuno (1980, Prog. Theor. Phys.64, 544) show that this formation mechanism may explain the similarity of giant planet core masses. Problems remain however, and no current model is entirely satisfactory in explaining the properties of the giant planets and simultaneously satisfying the terrestrial planet constraints. Satellite systematics and protoplanetary disk nebulae are also discussed and related to formation conditions.     

Accretion disks around Jupiter and Saturn at the stage of regular satellite formation

Modern models of the formation of the regular satellites of giant planets, constructed with consideration for their structure and composition suggest that this process lasted for a considerable period of time (0.1–1 Myr) and developed in gas-dust circumplanetary disks at the final stage of giant planet formation. The parameters of protosatellite disks (e.g., the radial distribution of surface density and temperature) serve as important initial conditions for such models. Therefore, the development of protosatellite disk models that take into account currently known cosmochemical and physical restrictions remains a pressing problem. It is this problem that is solved in the paper. New models of the accretion disks of Jupiter and Saturn were constructed with consideration for the disk heating by viscous dissipation of turbulent motions, by accretion of material from the surrounding region of the solar nebula, and by radiation from the central planets. The influence of a set of input model parameters (the total rate of mass infall onto the disk, the turbulent viscosity and opacity of disk material, and the centrifugal radius of the disk) on thermal conditions in the accretion disks was studied. The dependence of opacity on temperature and the abundance and size of solid particles present in the disk was taken into account. Those constructed models that satisfy the existing constraints limit the probable values of input parameters (primarily rates of mass infall onto the disks of Jupiter and Saturn at the stage of regular satellite formation and, to a lesser extent, the disk opacities). Constraints on the location of the regions of formation of the major satellites of Jupiter and Saturn are suggested based on the constructed models and simple analytical estimates concerning the formation of satellites in the accretion disks. It is shown that Callisto and Titan could hardly be formed at significantly greater distances from their planets.     

The formation of a gas giant planet in a viscously evolved protoplanetary disk within the core accretion model

The gas giant planets’ formation processes in a viscously evolved protoplanetary disk are studied in the context of the core accretion model. In this paper, we follow the entire formation process of the core accretion model (the three stages). We find that the gas giant planets’ final masses and formation regions have strong dependence on the molecular cloud core’s properties (angular velocity \(\omega \) and mass \(M _{c d}\)) and the \(\alpha _{ \mathit{min} }\) parameter. We find and build the relationship between gas giant planets’ properties and molecular cloud core’s properties. In contrast to the previous works, we find that the formation process can be finished within the protoplanetary disk’s lifetime (4×10⁶ yr) in our disk model. This is because the mass influx produced by the molecular cloud core can provide enough material to the protoplanetary disk. We also find that the gas giant planets’ final masses increase generally with the viscosity coefficient \(\alpha \). This is because most of the gas giant planet’s mass is captured during the rapid gas accretion phase (the third stage of the core accretion model), and furthermore the accretion of gas in this phase is dominated by the “gap limiting case”. And our numerical results can also be compared with the observed data of exoplanet systems.     

Core formation in giant gaseous protoplanets

Sedimentation rates of silicate grains in gas giant protoplanets formed by disk instability are calculated for protoplanetary masses between 1 MSaturn to 10 MJupiter. Giant protoplanets with masses of 5 MJupiter or larger are found to be too hot for grain sedimentation to form a silicate core. Smaller protoplanets are cold enough to allow grain settling and core formation. Grain sedimentation and core formation occur in the low mass protoplanets because of their slow contraction rate and low internal temperature. It is predicted that massive giant planets will not have cores, while smaller planets will have small rocky cores whose masses depend on the planetary mass, the amount of solids within the body, and the disk environment. The protoplanets are found to be too hot to allow the existence of icy grains, and therefore the cores are predicted not to contain any ices. It is suggested that the atmospheres of low mass giant planets are depleted in refractory elements compared with the atmospheres of more massive planets. These predictions provide a test of the disk instability model of gas giant planet formation. The core masses of Jupiter and Saturn were found to be ∼0.25 M_⊕ and ∼0.5 M_⊕, respectively. The core masses of Jupiter and Saturn can be substantially larger if planetesimal accretion is included. The final core mass will depend on planetesimal size, the time at which planetesimals are formed, and the size distribution of the material added to the protoplanet. Jupiter's core mass can vary from 2 to 12 M_⊕. Saturn's core mass is found to be ∼8 M_⊕.     

The Structure of Jupiter, Saturn, and Exoplanets: Key Questions for High-Pressure Experiments

We give an overview of our current understanding of the structure of gas giant planets, from Jupiter and Saturn to extrasolar giant planets. We focus on addressing what high-pressure laboratory experiments on hydrogen and helium can help to elucidate about the structure of these planets.     

Models of the giant planets

Models of the giant planets were constructed based on the assumption that the hydrogen to helium ratio is solar in these planets. This assumption, together with arguments about the condensation sequence in the primitive solar nebula, yields models with a central core of rock and possibly ice surrounded by an envelope of hydrogen, helium, methane, ammonia, and water. These last three volatiles may be individually enhanced due to condensation at the period of core formation. Jupiter was found to have a core of about 40 earth masses and a water enhancement in the atmosphere of about 7.5 times the solar value. Saturn was found to have a core of 20 earth masses and a water enhancement in the atmosphere of about 25 times the solar value. Rock plus ice constitute 75–85% of the mass of Uranus and Neptune. Temperatures in the interiors of these planets are probably above the melting points, if there is an adiabatic relation throughout the interiors. Some aspects of the sensitivities of these results to uncertainties in rotational flattening are discussed.     

The atmospheric composition and structure of Jupiter and Saturn from ISO observations: a preliminary review

Infrared spectra of Jupiter and Saturn have been recorded with the two spectrometers of the Infrared Space Observatory (ISO) in 1995–1998, in the 2.3–180 μm range. Both the grating modes (R=150–2000) and the Fabry-Pérot modes (R=8000–30,000) of the two instruments were used. The main results of these observations are (1) the detection of water vapour in the deep troposphere of Saturn; (2) the detection of new hydrocarbons (CH₃C₂H, C₄H₂, C₆H₆, CH₃) in Saturn’s stratosphere; (3) the detection of water vapour and carbon dioxide in the stratospheres of Jupiter and Saturn; (4) a new determination of the D/H ratio from the detection of HD rotational lines. The origin of the external oxygen source on Jupiter and Saturn (also found in the other giant planets and Titan in comparable amounts) may be either interplanetary (micrometeoritic flux) or local (rings and/or satellites). The D/H determination in Jupiter, comparable to Saturn’s result, is in agreement with the recent measurement by the Galileo probe (Mahaffy, P.R., Donahue, T.M., Atreya, S.K., Owen, T.C., Niemann, H.B., 1998. Galileo probe measurements of D/H and ³He/⁴He in Jupiters atmosphere. Space Science Rev. 84 251–263); the D/H values on Uranus and Neptune are significantly higher, as expected from current models of planetary formation.     

Comparative evolution of Jupiter and Saturn

We present evolutionary sequences for Jupiter and Saturn, based on new non-gray model atmospheres, which take into account the evolution of the solar luminosity and partitioning of dense components to deeper layers. The results are used to set limits on the extent to which possible interior phase separation of hydrogen and helium may have progressed in the two planets. When combined with static models constrained by the gravity field, our evolutionary calculations constrain the helium mass fraction in Jupiter to be between 0.20 and 0.27, relative to total hydrogen and helium. This is consistent with the Galileo determination. The helium mass fraction in Saturn’s atmosphere lies between 0.11 and 0.21, higher than the Voyager determination. Based on the discrepancy between the Galileo and Voyager results for Jupiter, and our models, we predict that revised observational results for Saturn will yield a higher atmospheric helium mass fraction relative to the Voyager value.     

Young Jupiters are faint: new models of the early evolution of giant planets

Here we show preliminary calculations of the cooling and contraction of a 2 M_J planet. These calculations, which are being extended to 1–10 M_J, differ from other published “cooling tracks” in that they include a core accretion‐gas capture formation scenario, the leading theory for the formation of gas giant planets.We find that the initial post‐accretionary intrinsic luminosity of the planet is ∼3 times less than previously published models which use arbitrary initial conditions. These differences last a few tens of millions of years. Young giant planets are intrinsically fainter than has been previously appreciated. We also discuss how uncertainties in atmospheric chemistry and the duration of the formation time of giant planets lead to challenges in deriving planetary physical properties from comparison with tabulated model values. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Formation of the regular satellite systems and rings of the major planets

In this paper, we apply the ideas presented by one of us (Prentice, 1978a, b) for the development of the proto-solar cloud into a system of Laplacian rings to the development of the protoplanetary clouds which ultimately led to Jupiter, Saturn and Uranus. We show that if one accepts this scenario — especially the idea of supersonic turbulence in the proto-planetary clouds — one can satisfactorily explain, on the basis of fixing a single adjustable parameter, both the geometric precession of the orbital radii of the regular satellite systems of these three planets and the chemical composition and mass distribution of these satellites. We suggest that thermal stirring in the proto-planetary cloud in the vicinity of the surface of the planet may be responsible for the smaller masses of some of the inner satellites as well as for the formation of the rocky rings of Uranus. The icy rings of Saturn are suggested to be the product of condensation processes in a continuous gaseous disc within the Roche limit of the planet.     

Formation of terrestrial planets in disks with different surface density profiles

We present the results of an extensive study of the final stage of terrestrial planet formation in disks with different surface density profiles and for different orbital configurations of Jupiter and Saturn. We carried out simulations in the context of the classical model with disk surface densities proportional to \({r^{-0.5}}, {r^{-1}}\) and \({r^{-1.5}}\), and also using partially depleted, non-uniform disks as in the recent model of Mars formation by Izidoro et al. (Astrophys J 782:31, 2014). The purpose of our study is to determine how the final assembly of planets and their physical properties are affected by the total mass of the disk and its radial profile. Because as a result of the interactions of giant planets with the protoplanetary disk, secular resonances will also play important roles in the orbital assembly and properties of the final terrestrial planets, we will study the effect of these resonances as well. In that respect, we divide this study into two parts. When using a partially depleted disk (Part 1), we are particularly interested in examining the effect of secular resonances on the formation of Mars and orbital stability of terrestrial planets. When using the disk in the classical model (Part 2), our goal is to determine trends that may exist between the disk surface density profile and the final properties of terrestrial planets. In the context of the depleted disk model, results of our study show that in general, the \(\nu _5\) resonance does not have a significant effect on the dynamics of planetesimals and planetary embryos, and the final orbits of terrestrial planets. However, \(\nu _6\) and \(\nu _{16}\) resonances play important roles in clearing their affecting areas. While these resonances do not alter the orbits of Mars and other terrestrial planets, they strongly deplete the region of the asteroid belt ensuring that no additional mass will be scattered into the accretion zone of Mars so that it can maintain its mass and orbital stability. In the context of the classical model, the effects of these resonances are stronger in disks with less steep surface density profiles. Our results indicate that when considering the classical model (Part 2), the final planetary systems do not seem to show a trend between the disk surface density profile and the mean number of the final planets, their masses, time of formation, and distances to the central star. Some small correlations were observed where, for instance, in disks with steeper surface density profiles, the final planets were drier, or their water contents decreased when Saturn was added to the simulations. However, in general, the final orbital and physical properties of terrestrial planets seem to vary from one system to another and depend on the mass of the disk, the spatial distribution of protoplanetary bodies (i.e., disk surface density profile), and the initial orbital configuration of giant planets. We present results of our simulations and discuss their implications for the formation of Mars and other terrestrial planets, as well as the physical properties of these objects such as their masses and water contents.     

The potential for Earth-mass planet formation around brown dwarfs

Recent observations point to the presence of structured dust grains in the discs surrounding young brown dwarfs, thus implying that the first stages of planet formation take place also in the substellar regime. Here, we investigate the potential for planet formation around brown dwarfs and very low-mass stars according to the sequential core accretion model of planet formation. We find that, for a brown dwarf mass 0.05 M_⊙, our models predict a maximum planetary mass of  ∼5   M_⊕  , orbiting with semimajor axis ∼ 1 au. However, we note that the predictions for the mass–semimajor axis distribution are strongly dependent upon the models chosen for the disc surface density profiles and the assumed distribution of disc masses. In particular, if brown dwarf disc masses are of the order of a few Jupiter masses, Earth-mass planets might be relatively frequent, while if typical disc masses are only a fraction of Jupiter mass, we predict that planet formation would be extremely rare in the substellar regime. As the observational constraints on disc profiles, mass dependencies and their distributions are poor in the brown dwarf regime, we advise caution in validating theoretical models only on stars similar to the Sun and emphasize the need for observational data on planetary systems around a wide range of stellar masses. We also find that, unlike the situation around solar-like stars, Type II migration is totally absent from the planet formation process around brown dwarfs, suggesting that any future observations of planets around brown dwarfs would provide a direct measure of the role of other types of migration.     

Habitable zones of host stars during the post-MS phase

A star will become brighter and brighter with stellar evolution, and the distance of its habitable zone will become larger and larger. Some planets outside the habitable zone of a host star during the main sequence phase may enter the habitable zone of the host star during other evolutionary phases. A terrestrial planet within the habitable zone of its host star is generally thought to be suitable for the existence of life. Furthermore, a rocky moon around a giant planet may be also suitable for life to survive, provided that the planet–moon system is within the habitable zone of its host star. Using Eggleton’s code and the boundary flux of the habitable zone, we calculate the habitable zone of our Solar system after the main sequence phase. It is found that Mars’ orbit and Jupiter’s orbit will enter the habitable zone of the Solar system during the subgiant branch phase and the red giant branch phase, respectively. And the orbit of Saturn will enter the habitable zone of Solar during the He-burning phase for about 137 million years. Life is unlikely at any time on Saturn, as it is a giant gaseous planet. However, Titan, the rocky moon of Saturn, may be suitable for biological evolution and become another Earth during that time. For low-mass stars, there are similar habitable zones during the He-burning phase as our Solar, because there are similar core masses and luminosities for these stars during that phase.     

The international outer planets watch atmospheres node database of giant-planet images

The Atmospheres Node of the International Outer Planets Watch (IOPW, formerly known as International Jupiter Watch; Russell et al., 1990) intends to encourage and coordinate the imaging observations and study of the atmospheres of the Giant Planets. The main activity of the atmospheres node is to provide an interaction between the professional and amateur astronomical communities maintaining a large database of images of the giant planets (primarily Jupiter and Saturn but with increasing contributions of Uranus and Neptune too). The observational datasets of Jupiter and Saturn correspond to images obtained in the visible range (300 nm-1 μm), during the last decade, most of them performed by amateur observers. We here describe the organization and structure of the database as posted on the Internet and in particular the PVOL software (Planetary Virtual Observatory Laboratory) designed to manage the site in the spirit of the Virtual Observatory projects. We also describe with examples the important role of the amateur-professional collaboration in the study of the atmospheres of Jupiter and Saturn in an epoch of large telescopes and spacecraft observations of both planets.     

A comparison of the atmospheres of Jupiter and Saturn: deep atmospheric composition, cloud structure, vertical mixing, and origin.

We present our current understanding of the composition, vertical mixing, cloud structure and the origin of the atmospheres of Jupiter and Saturn. Available observations point to a much more vigorous vertical mixing in Saturn's middle-upper atmosphere than in Jupiter's. The nearly cloud-free nature of the Galileo probe entry site, a 5-micron hotspot, is consistent with the depletion of condensable volatiles to great depths, which is attributed to local meteorology. Somewhat similar depletion of water may be present in the 5-micron bright regions of Saturn also. The supersolar abundances of heavy elements, particularly C and S in Jupiter's atmosphere and C in Saturn's, as well as the progressive increase of C from Jupiter to Saturn and beyond, tend to support the icy planetesimal model of the formation of the giant planets and their atmospheres. However, much work remains to be done, especially in the area of laboratory studies, including identification of possible new microwave absorbers, and modelling, in order to resolve the controversy surrounding the large discrepancy between Jupiter's global ammonia abundance, hence the nitrogen elemental ratio, derived from the earth-based microwave observations and that inferred from the analysis of the Galileo probe-orbiter radio attenuation data for the hotspot. We look forward to the observations from Cassini-Huygens spacecraft which are expected to result not only in a rich harvest of information for Saturn, but a better understanding of the formation of the giant planets and their atmospheres when these data are combined with those that exist for Jupiter.