Outer Giant Planets Research Articles

The formation and habitability of terrestrial planets in the presence of close-in giant planets

‘Hot jupiters,’ giant planets with orbits very close to their parent stars, are thought to form farther away and migrate inward via interactions with a massive gas disk. If a giant planet forms and migrates quickly, the planetesimal population has time to re-generate in the lifetime of the disk and terrestrial planets may form [P.J. Armitage, A reduced efficiency of terrestrial planet formation following giant planet migration, Astrophys. J. 582 (2003) L47–L50]. We present results of simulations of terrestrial planet formation in the presence of hot/warm jupiters, broadly defined as having orbital radii ⩽0.5 AU. We show that terrestrial planets similar to those in the Solar System can form around stars with hot/warm jupiters, and can have water contents equal to or higher than the Earth's. For small orbital radii of hot jupiters (e.g., 0.15, 0.25 AU) potentially habitable planets can form, but for semi-major axes of 0.5 AU or greater their formation is suppressed. We show that the presence of an outer giant planet such as Jupiter does not enhance the water content of the terrestrial planets, but rather decreases their formation and water delivery timescales. We speculate that asteroid belts may exist interior to the terrestrial planets in systems with close-in giant planets.

Deep two-dimensional turbulence: An idealized model for atmospheric jets of the giant outer planets

The present paper proposes a new type of two–dimensional turbulence with a deep configuration as an idealization of the dynamics of the global circulations of the Jovian atmospheres under the constraint of the Taylor–Proudman theorem. A major difference of the deep two-dimensional turbulence from a standard shallow version is that the former reverses the sign of the planetary β-effect, leading to an opposite sign for the equatorial jet under the homogenization tendency of the potential vorticity. As a result, a westerly (prograde) equatorial jet is generated, which contrasts with the easterly (retrograde) equatorial jet obtained using the shallow configuration. The results suggest that the zonal flows of Jupiter and Saturn are probably of deep origin, whereas those of Uranus and Neptune are likely to be of shallow origin.

Radio and plasma waves at the outer planets

We review our present knowledge of plasma waves and non-thermal radio emissions at the outer planets. The review mainly concerns waves linked to electron dynamics. After a summary of the basics of radio and plasma wave modes as derived from the theory and from observations in the Earth's vicinity, we discuss the counterpart of these waves as observed in outer planets' magnetospheres. Plasma wave spectra display a remarkable overall similarity at all magnetized planets in spite of the very diverse magnetospheric configurations, emphasizing the importance of similar microphysics processes at all magnetized planets, in particular the outer giant planets. But the magnetospheres of the outer planets also give birth to a large diversity of radiosources, reflecting their complex structure (plasma reservoirs, gradients, regimes of f pe / f ce , etc.) and dynamics (locations and distributions of energetic electrons). We discuss in more details these radio components, including their generation mechanisms. Emphasis is put on the direct generation of the very intense high latitude radio emissions, but conversion processes also occur in low latitude regions. The Earth stands as a reference in our comparative approach, but the study of radio and plasma waves at the outer planets allows to apply the concepts developed at Earth in different plasma environments, broadening the regime of plasma parameters involved in the generation of these waves. One specific Jovian radio component is identified (made of sporadic bursts), whose study could bring significant conceptual advances in the physics of the generation process involved. Finally, radio waves, because they can freely propagate far from their source regions, are of utmost importance as they allow for remote sensing of magnetospheric plasmas.

Radio and plasma waves at the outer planets

We review our present knowledge of plasma waves and non-thermal radio emissions at the outer planets. The review mainly concerns waves linked to electron dynamics. After a summary of the basics of radio and plasma wave modes as derived from the theory and from observations in the Earth's vicinity, we discuss the counterpart of these waves as observed in outer planets' magnetospheres. Plasma wave spectra display a remarkable overall similarity at all magnetized planets in spite of the very diverse magnetospheric configurations, emphasizing the importance of similar microphysics processes at all magnetized planets, in particular the outer giant planets. But the magnetospheres of the outer planets also give birth to a large diversity of radiosources, reflecting their complex structure (plasma reservoirs, gradients, regimes of f pe/ f ce, etc.) and dynamics (locations and distributions of energetic electrons). We discuss in more details these radio components, including their generation mechanisms. Emphasis is put on the direct generation of the very intense high latitude radio emissions, but conversion processes also occur in low latitude regions. The Earth stands as a reference in our comparative approach, but the study of radio and plasma waves at the outer planets allows to apply the concepts developed at Earth in different plasma environments, broadening the regime of plasma parameters involved in the generation of these waves. One specific Jovian radio component is identified (made of sporadic bursts), whose study could bring significant conceptual advances in the physics of the generation process involved. Finally, radio waves, because they can freely propagate far from their source regions, are of utmost importance as they allow for remote sensing of magnetospheric plasmas.

Rapid Formation of Outer Giant Planets by Disk Instability

The formation mechanism of the ice giant planets is uncertain. A new hypothesis envisions rapid formation of several gaseous protoplanets in a marginally gravitationally unstable protoplanetary disk, and coagulation and settling of dust grains within the protoplanets to form rock and ice cores, followed by the loss of the outer disk gas and the gaseous envelopes of the protoplanets through photoevaporation driven by nearby OB stars. We demonstrate here that the first part of this new scenario for ice giant planet formation is feasible in a disk with a gas density similar to that believed to be necessary to form the giant planets. A three-dimensional gravitational hydrodynamics code, including a full treatment of thermodynamics and radiative transfer in the diffusion approximation, is used to show that a disk is likely to form two or more gravitationally bound clumps, with masses on the order of 2 Jupiter masses, between 20 and 30 AU from a solar-mass star. Such protoplanets could be massive enough to explain the production of Uranus and Neptune following photoevaporation of most of their gaseous envelopes. This scenario implies that planetary systems similar to our own could form even in seemingly hostile regions of high-mass star formation.

Oligarchic growth of giant planets

Runaway growth ends when the largest protoplanets dominate the dynamics of the planetesimal disk; the subsequent self-limiting accretion mode is referred to as “oligarchic growth.” Here, we begin by expanding on the existing analytic model of the oligarchic growth regime. From this, we derive global estimates of the planet formation rate throughout a protoplanetary disk. We find that a relatively high-mass protoplanetary disk (∼10 × minimum-mass) is required to produce giant planet core-sized bodies (∼10 M ⊕) within the lifetime of the nebular gas (≲10 million years). However, an implausibly massive disk is needed to produce even an Earth mass at the orbit of Uranus by 10 Myrs. Subsequent accretion without the dissipational effect of gas is even slower and less efficient. In the limit of noninteracting planetesimals, a reasonable-mass disk is unable to produce bodies the size of the Solar System’s two outer giant planets at their current locations on any timescale; if collisional damping of planetesimal random velocities is sufficiently effective, though, it may be possible for a Uranus/Neptune to form in situ in less than the age of the Solar System. We perform numerical simulations of oligarchic growth with gas and find that protoplanet growth rates agree reasonably well with the analytic model as long as protoplanet masses are well below their estimated final masses. However, accretion stalls earlier than predicted, so that the largest final protoplanet masses are smaller than those given by the model. Thus the oligarchic growth model, in the form developed here, appears to provide an upper limit for the efficiency of giant planet formation.

The Formation of Uranus and Neptune among Jupiter and Saturn

The outer giant planets, Uranus and Neptune, pose a challenge to theories of planet formation. They exist in a region of the Solar System where long dynamical timescales and a low primordial density of material would have conspired to make the formation of such large bodies ($\sim$ 15 and 17 times as massive as the Earth, respectively) very difficult. Previously, we proposed a model which addresses this problem: Instead of forming in the trans-Saturnian region, Uranus and Neptune underwent most of their growth among proto-Jupiter and -Saturn, were scattered outward when Jupiter acquired its massive gas envelope, and subsequently evolved toward their present orbits. We present the results of additional numerical simulations, which further demonstrate that the model readily produces analogues to our Solar System for a wide range of initial conditions. We also find that this mechanism may partly account for the high orbital inclinations observed in the Kuiper belt.

1999 KUIPER PRIZE LECTURE: Cometary Origin of the Biosphere

Most of the biosphere was brought on the primitive Earth by an intense bombardment of comets. This included the atmosphere, the seawater and those volatile carbon compounds needed for the emergence of life. Comets were thrown into the inner Solar System by the strong perturbation induced by the growth of the giant planets' cores. The bulk of the Earth's bombardment came from those comets that accreted in Jupiter's zone, where the original deuterium enrichment had been diminished by steam coming from the hot, inner parts of the Solar System. This steam had condensed into icy chunks before their accretion into larger cometary nuclei. In contrast, comets that accreted in the zones of the outer giant planets kept their interstellar isotopic enrichments. Those comets contributed to the Earth's bombardment for a small amount only; they were mostly ejected into the Oort cloud and are the major source of the long-period comets observed today. The short-period comets, which come from the Kuiper Belt, should also have the same interstellar enrichment. The deuterium enrichment of seawater, accurately predicted by the previous scenario, has become one of the best telltales for the cometary origin of our biosphere. This cometary origin may have far-reaching cosmological consequences, in particular for the origin of life in other planetary systems.

A Cosmochemical Determinism in the Formation of Earth-like Planets

The discovery of giant planets around other Sun-like stars has stimulated interest in the search for planets capable of supporting life. Here, it is proposed that the abundance of water in the progenitor nebula is a fundamental determinant in the formation of planetary systems like our own, i.e., including an inner, water-rich planet (Earth) and an outer giant planet (Jupiter). The amount of water is a sensitive function of the elemental ratio of carbon to oxygen in the nebula. The Sun appears to be unusual in this respect since the ratio in most nearby stars suggests they formed from relatively water-poor nebulae. Chemical evolution models suggest that the Galactic disk is evolving from an oxidizing, water-rich state toward a carbon-rich, water-poor condition. This may have implications for the appearance and distribution of habitable worlds.

A similarity model for the windy jovian thermocline

A new conceptual model for the observed banded wind structure of the giant outer planets is proposed, assuming a smoothly distributed potential vorticity (PV) atop a layer of deeply seated, latitudinally variable stratification. With a vanishing horizontal entropy contrast presumably enforced at the bottom of the flow layer by the underlying convection, the thermal-wind balance of observed cloud-top motions implies a mapping of constant potential temperature surfaces, nearly vertical at upper tropospheric levels, down to a deeper, flatter, but variably sloped “thermocline”. For a generally colder-poleward distribution of isentropes consistent with the strong equatorial jets of both Jupiter and Saturn, this kind of mapping would also imply a poleward decreasing static stability. The proposed temperature-stability distribution is just the reverse of the Earth’s troposphere, where the strongest latitudinal potential temperature gradients are at the bottom instead of the top, with static stability generally increasing toward the pole. The warmer–stabler correlation for Jupiter would be consistent, however, with a nearly monotonic distribution of PV from low to high latitudes, over planetary scales for which the planetary vorticity dominates the relative vorticity of the jets. In this way efficient PV mixing for the isentropically bounded thermocline can account for the dynamical maintenance of the cyclonic flanks of the equatorial jet. Over latitudinal intervals comparable to the internal deformation scale, however, the gradient of the absolute vorticity is dominated by the flow curvature, correlated for a well-mixed PV distribution with the local latitudinal gradient of the static stability, itself proportional for the assumed thermocline structure to the local potential temperature gradient and therefore the local departures in geostrophic velocity. The resulting correlation of velocities and vorticity gradients would imply an alternation of the flow over the internal deformation radius set by the vertical entropy contrast. The requisite size of the deep stability is itself roughly consistent with the diagnostic interpretation of the apparent dispersive character and vertical wavelength of observed temperature oscillations in Jupiter’s upper troposphere and stratosphere, as well as a plausible extrapolation of the deep sub-adiabatic lapse rate detected by the Galileo probe.

Magnetopause structure and the role of reconnection at the outer planets

In situ measurements have been obtained at the magnetopause boundary of all the giant outer planets. The Jovian magnetopause was probed by Pioneers 10 and 11, Voyagers 1 and 2, Ulysses, and most recently by Galileo. Saturn was visited by Pioneer 11 and Voyagers 1 and 2, and Uranus and Neptune were visited by Voyager 2. The observations at Jupiter show evidence for flux transfer event (FTE) structures and rotational discontinuities associated with magnetic reconnection with the interplanetary magnetic field (IMF), but in accord with previous studies we find no FTE signatures beyond Jupiter in the data sets we studied. At Saturn, one magnetopause encounter shows evidence for reconnection in the form of a rotational discontinuity with a finite magnetic field component through the boundary and significant plasma acceleration. At Uranus and Neptune the flank magnetopause boundary observed by Voyager 2 exhibits a complex structure (similar to that seen at Earth during high‐β conditions) for which unsteady reconnection appears to occur. Closely spaced multiple magnetopause crossings at Jupiter and Saturn are consistent with a boundary surface disturbance or wave, occurring with unsteady reconnection, and coincident with a jump in upstream solar wind dynamic pressure in the Jupiter case. Because of the transient, bursty nature of the reconnection signatures and their low (∼0.1 to 0.4 mV/m) convective electric fields compared to those of corotation at Jupiter and Saturn, the role of reconnection in driving the dynamics of these magnetospheres is thought to be minimal in the general case. Nevertheless, the unsteady reconnection observed is important to the properties of the boundary layers. At Uranus and Neptune, observations are limited (Voyager 2 only) but suggest that bursty reconnection at the flank magnetopause can only remove plasma (in the antisunward direction), and infrequent reconnection on the dayside would provide little opportunity for solar wind entry to energize the ionospheric plasma and supply the magnetosphere; these factors may contribute to producing the relatively empty magnetospheres that were seen.

The Morphogenesis of Bands and Zonal Winds in the Atmospheres on the Giant Outer Planets

The atmospheres of Jupiter, Saturn, Uranus, and Neptune were modeled as shallow layers of turbulent fluid overlying a smooth, spherical interior. With only the observed values of radius, rotation rate, average wind velocity, and mean layer thickness as model parameters, bands and jets spontaneously emerged from random initial conditions. The number, width, and amplitude of the jets, as well as the dominance of anticyclonic vortices, are in good agreement with observations for all four planets.

The Near-Infrared Spectrum of the Brown Dwarf Gliese 229B

A medium-resolution 1.0-2.5 μm spectrum of the brown dwarf Gliese 229B has been obtained using the facility spectrometer CGS4 on the United Kingdom Infrared Telescope. In addition to the broad spectral structure seen in earlier low-resolution observations, the new spectrum reveals a large number of absorption lines, many of which can be identified with water vapor. Water and methane are both shown to be strong absorbers in the near-infrared spectrum of the object. Several spectral features in Gl 229B that are attributable to methane match ones seen in reflection in the giant outer planets and, in particular, Titan.

Planetary and protostellar nuclear fission: implications for planetary change, stellar ignition and dark matter

The feasibility of thermal neutron fission and fast neutron fission in planetary and protostellar matter is calculated from nuclear reactor theory. Means for concentrating actinide elements and for separating actinide elements from reactor poisons are described. The implications of intermittent or interrupted planetary-scale nuclear fission breeder reactors are discussed in connection with observed changes in the giant outer planets and changes in the geomagnetic field. The con­cept that thermonuclear fusion reactions in stars are ignited by nuclear fission energy is disclosed. The suggestion is made that dark matter, inferred to exist in the Universe, might be accounted for, at least in part, by the presence of dark stars (not necessarily brown dwarfs) whose protostellar nuclear fission reactors failed to ignite thermonuclear fusion reactions.

TheVNIR-VIMS experiment for Craf/Cassini

We describe a multispectral camera in the wavelength range of (0.35 ÷ 1.050) μm. VNIR constitutes the visible channel of the VIMS (Visual near-Infrared Mapping Spectrometer) instrument designed for the Craf/Cassini NASA mission to the Hamburga asteroid and the outer giant planets, Jupiter and Saturn. Possible scientific investigations are analysed and a detailed technical description of the VNIR experiment is given.

An example of stable chaos in the Solar System

MANY planets have been shown to have chaotic instabilities in their orbital motions, but the long-term significance of this is not fully understood1. The eccentricity of Mercury, for example, changes by about 25% of its value over 40 times the Lyapunov time2,3(the e-folding time for divergence of nearby orbits), but the orbit of Pluto, in an integration lasting 50 Lyapunov times4, shows no significant change. Here we show that the orbit of the near-Jupiter asteroid 522 Helga is chaotic, with an unusually short Lyapunov time of 6,900 yr. We integrate its motion, including perturbations from the outer giant planets, over a period 1,000 times longer than this, and find no significant instability. Chaos in the orbit of 522 Helga is caused by a 7:12 resonance with the orbit of Jupiter, but the size of the chaotic region in phase space is small; stability is ensured because the eccentricity and precession of the orbit are such that it avoids close encounters with Jupiter. Asteroid orbits with larger proper eccentricity would, we suggest, be genuinely unstable, consistent with the sparse asteroid population near Helga. Although Helga is the first clear-cut example of a stable chaotic orbit, we argue that 'stable chaos' may be a rather common feature of Solar System dynamics.

Lorentz resonances and the vertical structure of dusty rings: Analytical and numerical results

Dust grains orbiting the giant outer planets carry a weak and nearly constant electrical charge. Through interaction with the planet's irregular magnetic field, the inclinations i and eccentricities e of these charged grains can be pumped up significantly. This occurs especially when a period of the Lorentz force matches the radial or out-of-plane epicyclic periods. Since orbiting particles sample the spatial structure of the planetary magnetic field at differing rates depending on their mean orbital radius a, these Lorentz resonances (LR) occur at specific orbital radii. Several of the strongest LR have been related to structural features of the Jovian dust ring system. In this paper we use a combination of analytical and numerical techniques in order to understand the nature of these resonances. A simple extension of the perturbation theory for the LRs yields the charge-to-mass ratio ( q/ m) corrections to the periods of the in-plane and out-of-plane motion, thus allowing for an accurate determination of the resonance locations. The analysis also indicates that the two modes of oscillation are weakly coupled. We then show that the mean orbital radius a, assumed constant in previous analytical developments, undergoes a small but significant resonant forcing as well, resulting in shifts in a which break the exact linear resonance condition. An analysis of the energy of the grain's orbit - for small values of eccentricity e and inclination i - shows that it can be decomposed into three parts: energies of vertical and horizontal oscillators, and the energy of the circular equatorial orbit from which these grains are perturbed by the Lorentz force. In a reference frame corotating with the planet where no electric field is present, the Lorentz force cannot do work but it can exert torque; hence it can transfer energy between the circular orbit and the horizontal and vertical oscillators. Numerical integrations in the resonance zones support this view of the dynamical interaction, and also confirm the existence of a small range of charge-to-mass ratios where the vertical and horizontal resonances overlap, thereby producing what appears to be chaotic motion. Resonance zones can be traversed as orbits evolve in semimajor axis, e.g., due to plasma drag. Using numerical simulations to study this passage through resonance, we find that the orbital elements a, e and i undergo large jumps, and we determine the size of these jumps as a function of ( q/ m) and drag evolution rates. We also ascertain the distribution of final orbital parameters, after passage, for a distribution of initial launch longitudes. In particular we show that the longitude of nodes, Ω, is uniformly distributed over the interval 0 < Ω < 2π. In this case, a cross-section of the Jovian halo would be predicted to have symmetry about the equatorial plane, as confirmed through analysis of the Voyager data by Showalter et al. (1987, Icarus, 69, 458–498). Using adiabatic invariants of the motion, we show that the large inclinations induced in passing through one resonance survive during the excursion to the next resonance encounter. Thus many features of the Jovian ring halo are explained by the Lorentz resonance mechanism.

Nuclear fission reactors as energy sources for the giant outer planets

Nuclear fission reactors as energy sources for the giant outer planets

The Magnetospheres of the Outer Planets

The past four years have witnessed the culmination of the initial reconnaissance of the giant outer planets and their magnetospheres. This includes more mature modeling of dynamics of and processes in the magnetospheres of Jupiter and Saturn, the first detailed reports on the fields and particles environment of Uranus, and the initial reports concerning the magnetosphere of Neptune, first sampled directly by Voyager 2 in August, 1989. Of the major bodies of the outer solar system, only the asteroids and the Pluto/Charon system remain unvisited, although some attention was focused on the latter during this quadrennium. We review work on and our understanding of the magnetospheres of all of the outer planets during the previous four years.

The Research and Development of Silicon-Germanium Thermoelements for Power Generation

ABSTRACTOn the 25th of August 1989, radio signals from the unmanned Voyager II spacecraft produced high resolution radio displays of the planet Neptune and its moon, Triton, on the monitors of Caltech's Jet Propulsion Laboratory. Thus ended the 12-year NASA Voyager I and II missions to the giant outer planets of our solar systems. Somewhat overlooked in the excitement over the impressive scientific findings from Voyager's epic planetary encounters was the power system aboard the spacecraft for data collection and signal transmission. This was the multi-hundred watt, radioisotope fueled thermoelectric generator using thermocouple modules made of Si-Ge alloys. The first section of this paper presents a historical review of the research program at the RCA Laboratories in Princeton, N.J. during the 4-year period required for the conception and development of Si-Ge thermoelements for power generation. This includes RCA's strategy in thermoelectric materials research from the viewpoint of device principles and requirements, scientific issues relating to the growth and thermoelectric characterization of Si-Ce alloys, and device feasibility studies. The performance of Si-Ge thermoelements in the power source of several space missions is also discussed. Finally, a perspective is presented on the research management of this remarkably successful program. The second section of the paper describes the results of more recent research on the thermoelectric properties of hot-pressed, sintered Si-Ge alloys and, in particular, the effects of grain size and additions of GaP. In the final section, consideration is given to future areas of research based, in part, on the results of the earlier studies of Si-Ge technology for thermoelectric power generation.