首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 15 毫秒
1.
The resonance theory is discussed with respect to the Solar System with a view to show that every triad of successive planets in the Solar System follows Laplace's resonance relation. With rings now known to exist around three of the four major planets, scientists have begun to speculate about the possible existence of ring structure and one or two small planets going around the Sun itself. It is also believed that the ring systems may exist around the planets Neptune and Mars. In this paper an attempt is made to provide a basis to these beliefs using Laplace's resonance relation. The triads of successive innermost objects (rings and/or satellites) in the satellite — systems of Jupiter, Saturn and Uranus are also shown to follow Laplace's resonance relation.  相似文献   

2.
Numerous studies in the past few years have analyzed possible effects of planetary migration on the small bodies of the Solar System (mainly asteroids and KBOs), with the double aim of explaining certain dynamical structures in these systems, as well as placing limits on the magnitude of the radial migration of the planets. Here we undertake a similar aim, only this time concentrating on the dynamical stability of planetary satellites in a migration scenario. However, different from previous works, the strongest perturbations on satellite systems are not due to the secular variation of the semimajor axes of the planets, but from the planetesimals themselves. These perturbations result from close approaches between the planetesimals and satellites.We present results of several numerical simulations of the dynamical evolution of real and fictitious satellite systems around the outer planets, under the effects of multiple passages of a population of planetesimals representing the large-body component of a residual rocky disk. Assuming that this component dominated the total mass of the disk, our results show that the present systems of satellites of Uranus and Neptune do not seem to be compatible with a planetary migration larger than even one quarter that suggested by previous studies, unless these bodies were originated during the late stage of evaporation of the planetesimal disk. For larger variations of the semimajor axes of the planets, most of the satellites would either be ejected from the system or suffer mutual collisions due to excitation in their eccentricities. For the systems of Jupiter and Saturn, these perturbations are not so severe, and even large migrations do not introduce large instabilities.Nevertheless, even a small number of 1000-km planetesimals in the region may introduce significant excitation in the eccentricities and inclinations of satellites. Adequate values of this component may help explain the present dynamical distribution of distant satellites, including the highly peculiar orbit of Nereid.  相似文献   

3.
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.  相似文献   

4.
The core-accretion mechanism for gas giant formation may be too slow to create all observed gas giant planets during reasonable gas disk lifetimes, but it has yet to be firmly established that the disk instability model can produce permanent bound gaseous protoplanets under realistic conditions. Based on our recent simulations of gravitational instabilities in disks around young stars, we suggest that, even if instabilities due to disk self-gravity do not produce gaseous protoplanets directly, they may create persistent dense rings that are conducive to accelerated growth of gas giants through core accretion. The rings occur at and near the boundary between stable and unstable regions of the disk and appear to be produced by resonances with discrete spiral modes on the unstable side.  相似文献   

5.
Giant planets in circumstellar disks can migrate inward from their initial (formation) positions at several AUs. Inward radial migration of the planet is caused by torques between the planet and the disk; outward radial migration of the planet is caused by torques between the planet and the spinning star, and by torques due to Roche lobe overflow and consequent mass loss from the planet. We present self-consistent numerical considerations of the problem of migrating giant planets by summing torques on planets for various physical parameters of the disk and of planets. We find that Jupiter-mass planets can stably arrive and survive at small heliocentric distances, thus reproducing observed properties of some of the recently discovered extra-solar planets. The range of fates of massive planets is broad, and some perish by losing all their mass onto the central star during Roche lobe overflow, while others survive for the lifetime of the central star. Surviving planets cluster into two groups when examined in terms of final mass and final heliocentric distance: those which have lost mass and those which have not. Some of the observed extrasolar planets fall into each of these two exclusive classes. We also find that there is an inner boundary for planets' final heliocentric distances, caused by tidal torques with the central star. Planets in small orbits are shown to be stable against atmospheric loss.  相似文献   

6.
Highly noncircular outer stellar disks have been detected in two SA0 (unbarred) galaxies by comparing the spectroscopic data on the rotation of stars and the photometric data on the shape and orientation of isophotes. In NGC 502, the oval distortion of the disk is manifested in the shape of the inner and outer elliptical rings occupying wide radial zones between the bulge and the disk and at the outer disk edge; such a structure can be a consequence of the so-called “dry minor merger,” multiple cannibalization of gas-free satellites. In NGC 5485, the stellar kinematics is absolutely unrelated to the orientation of isophotes in the disk region, and for this galaxy the conclusion about its global triaxial structure is unavoidable.  相似文献   

7.
The development of principles, systems, and instruments enable the detection of exoplanets with 6–8 Earth masses or less. The launches of specialized satellites, such as CoRoT (2006) and Kepler (2009), into orbits around the Earth have enabled the discovery of new exoplanetary systems. These missions are searching for relatively low-mass planets by observing their transits over the disks of their parent stars. At the same time, supporting studies of exoplanets using ground-based facilities (that measure Keplerian components of radial velocities) are in progress. The properties of at least two objects discovered by different methods, Kepler-22 and GJ 1214b, suggested that there was another class of celestial bodies among the known types of extrasolar planets: planetans, or oceanic planets. The structure of Kepler-22 and GJ 1214b suggest that they can be these oceanic planets. In this paper, we consider to what extent this statement is valid. The consideration of exoplanet Gl 581g as an oceanic planet is more feasible. Some specific features of the physical nature of these unusual planets are presented.  相似文献   

8.
In this paper main implication of basic properties detected in the satellite systems of Jupiter, Saturn and Uranus, and presented by the author in an earlier contribution (Barricelli, 1971b) are investigated. The similarity between the primary periods in the three systems, their apparent relation to the axial rotation periods of the three planets and other features suggesting that collisions with the planetary surfaces may have played a role in the evolution of the three satellite systems are interpreted by assuming that in each case a satellite of unusually large size was originally disintegrated at the Roche limit of its primary. The disintegration of large satellites and their fusion with the respective planets is assumed to be a normal feature in the latest stage of planetary growth and the main cause of axial rotation in the respective planets.These assumptions make it possible to give a selfconsistent interpretation of the similarity between the axial rotation periods of the three planets and their relation to the primary periods (as defined by Barricelli, 1971b) in the three systems.Similar assumptions when applied to the Earth-Moon system make it possible to understand why the Moon, in its closest approach to the Earth is found to have been almost exactly at the Roche limit (Gerstenkorn, 1955; MacDonald, 1964), a coincidence which is too good to be accidental. According to this interpretation our Moon is a portion (representing about one third) of our original satellite, which survived its approach to the Roche limit and the ensuing fusion process with the Earth. It can be shown (see text) that under certain conditions this could leave a residual satellite with a stationary distance from the Earth (which in retrospect would be identified as its lowest distance from the Earth) at the Roche limit.The only other case in which we have observational evidence of parts of a satellite surviving its fusion process at the Roche limit is represented by the rings of Saturn and possibly the small innermost satellite Janus which seems to have been feeding on the rings.  相似文献   

9.
Statistical analysis of the available data on the sizes and inertial parameters for all hitherto known satellites of the Solar system’s planets is performed. Analytical approximations are derived for the size distribution of satellites. Empirical relations are obtained to approximately estimate the inertial parameters of a satellite from its size. These relations can be used in statistical studies of the possibility of manifestations of various nonstandard rotational modes of planetary satellites. In particular, the probability of the “Amalthea effect” (the presence of two centers of synchronous resonance in the phase space of rotational motion) is shown to be much higher for minor (with diameters smaller than 100 km) satellites moving in close-to-circular orbits than for other satellites.  相似文献   

10.
《Planetary and Space Science》2006,54(9-10):1014-1023
Faint rings of micrometre-sized dust particles embrace many planets in the Solar system. As a rule, they are replenished by ejecta from embedded atmosphereless moons. On a number of occasions, the ejecta are generated by hypervelocity meteoroid impacts into the moons. Small ejecta fragments are then swiftly shifted into rings by an array of non-gravitational forces, e.g. radiation pressure or plasma drag. A significant fraction of ejecta mass, however, is contained in relatively big, multi-micrometre fragments which are subject to gravity only. Having escaped from the satellite, they stay close to its orbit and form a belt around planet. This belt is itself a source of ring dust through collisional disruption of its particles. Here the contributions of belts to the respective rings are estimated for selected satellites of Jupiter and Saturn. The belts under review could supply substantially more dust to rings than the direct ejecta from satellites and should be taken into account when estimating ring dust budgets. The belts are very difficult to observe, however, and some of them remain a theoretical proposition. We find an appealing evidence for the belts due to Amalthea and Thebe around Jupiter, and for the belt due to Enceladus around Saturn.  相似文献   

11.
We found a new empirical fonmula for the distance of the n-th satellite in the Jovian, Saturnian and Uranian systems, an = B1 × Bn, with just two constants b1 and B for each system. The difference between the observed distances and the values calculated according to this formula is generally less than 10%. We take the view that the satellites were formed from the accretion of planetesimals in the gas-planetesimal disk surrounding the planet, that the main component of the disk was gas so that the effect of gas drag would be very important in the above process. Our theoretical analysis shows that one type of radial perturbation in the disk will lead to instability and hence the formation of gaseous rings with enhanced density. Within these rings, the planetesimals stick together to form the satellites, and it is the form of the distribution of the rings that leads to the distance law.  相似文献   

12.
In this work, we study the stability of hypothetical satellites of extrasolar planets. Through numerical simulations of the restricted elliptic three-body problem we found the borders of the stable regions around the secondary body. From the empirical results, we derived analytical expressions of the critical semimajor axis beyond which the satellites would not remain stable. The expressions are given as a function of the eccentricities of the planet, e P, and of the satellite, e sat. In the case of prograde satellites, the critical semimajor axis, in the units of Hill's radius, is given by a E≈ 0.4895   (1.0000 − 1.0305 e P− 0.2738 e sat). In the case of retrograde satellites, it is given by a E≈ 0.9309  (1.0000 − 1.0764 e P− 0.9812 e sat). We also computed the satellite stability region ( a E) for a set of extrasolar planets. The results indicate that extrasolar planets in the habitable zone could harbour the Earth-like satellites.  相似文献   

13.
The stability of a self‐gravitating infinitesimally thin gaseous disk rotating around a central mass is studied. Our global linear analysis concerns marginal stability, i.e. it yields the critical temperature for the onset of instability for any given ratio of the disk mass to the central mass. Both axisymmetric and low‐m nonaxisymmetric excitations are analysed. When the fractional disk mass increases, the symmetry character of the instability changes from rings (m = 0) to one‐armed trailing spirals (m = 1). The distribution of the surface density along the spiral arms is not uniform, but describes a sequence of maxima that might be identified with forming planets. The number of the mass concentrations decreases with increasing fractional disk mass. We also obtain solutions in the form of global nonaxisymmetric vortices, which are, however, never excited.  相似文献   

14.
Within the framework of the nebular theory of the origin of the solar system, conservation laws are applied to the condensation of a ring shaped cloud of orbiting particles. The final configuration is assumed to be a point-like planet in a circular orbit around the Sun. On this ground, it is possible to relate the masses of the planets with the interplanetary distances. This relation is confirmed satisfactorily by the observed masses and orbital radii of several planets and satellites of the solar system.  相似文献   

15.
日冕是太阳大气活动的关键区域,是日地空间天气的源头.受观测限制,对日冕低层大气等离子体结构和磁场状态的研究非常欠缺,国际上对于可见光波段日冕低层大气的亮度分层研究很少.利用丽江日冕仪YOGIS(Yunnan Green-line Imaging System)的日冕绿线(FeⅩⅣ5303?)观测资料,对内日冕区域(1.03R☉-1.25R☉,R☉表示太阳半径)亮结构及其中冕环进行了有效的强度衰减分析.对亮结构的强度在太阳径向高度上进行了指数衰减拟合,比较这些拟合结果发现所得到的静态内冕环的衰减指数在一固定值附近.然后将比较明显的冕环提取出来,通过对不同高度的绿线强度进行指数拟合,得出的衰减指数与亮结构中也比较相近,这对进一步研究日冕中的各项物理参数演化提供了参考.  相似文献   

16.
The theory discussed in the present paper is a solar nebula-type theory which assumes the initial existence of a big disk-shaped gas cloud in rotational motion around the Sun. At the outer edge of the gas cloud there is a steady loss of angular momentum, which is mainly caused by the diffusion induced by turbulence and shock waves. This leads to the formation of a doughnutshaped gas ring at the edge of the cloud, outside of which there is plasma in a state of partial corotation. The gas ring is then slowly shifted towards the Sun, whereby the grains of solid matter within the gas cloud are also transported and collected within the gas torus. During the contraction process the following two situations arise: First, due to the small amount of friction, the angular momentum of the inner part of the ring rapidly exceeds that of the outer part. Second, the angle between the orbits of the inner and outer part of the gas ring increases gradually. When, during contraction, a certain distance is covered, the gas ring turns over, i.e. there is a sudden interchange of the inner and outer parts of the gas ring, where two adjacent rings of solid matter (jet streams) are formed. Immediately after the turn-over process the speed of contraction is at first drastically reduced, but then the gas ring is shifted once more towards the Sun. This process is then repeated periodically. The planets originate from the outer rings of solid matter, which contain much more matter than their adjacent inner rings. The inclination between the inner and outer rings is roughly 5°. In particular, Mercury, the Moon, Titan as well as Triton result from the innermost rings of matter. Having gone through the formation process, most of the planets acquire a rotating gas disk out of which the regular satellites are also created by the same periodic contraction process (hetegonic principle). This theory is the first that can explain all noteworthy facts about our planetary system and the satellite systems in a qualitative yet conclusive way.  相似文献   

17.
T.C. Van Flandern 《Icarus》1981,47(3):480-486
The recent evidence that many minor planets may have satellites, together with recently iscovered physical, chemical, and lightcurve similarities between minor planets and comets, lead naturally to the question, “Might comets have satellites also?” This paper explores several puzzling features of comets which do not fit easily into conventional cometary models, but which can be satisfactorily explained if it is assumed that comets have a full range of gravitationally bound masses, from dust size to the size of the nucleus, in orbit around the principal nucleus. This discussion also implies a higher probability of destruction of a spacecraft near a comet than is usually assumed.  相似文献   

18.
A number of extrasolar planets have been detected in close orbits around nearby stars. It is probable that these planets did not form in these orbits but migrated from their formation locations beyond the ice line. Orbital migration mechanisms involving angular momentum transfer through tidal interactions between the planets and circumstellar gas-dust disks or by gravitational interaction with a residual planetesimal disk together with several means of halting inward migration have been identified. These offer plausible schemes to explain the orbits of observed extrasolar giant planets and giant planets within the Solar System. Recent advances in numerical integration methods and in the power of computer workstations have allowed these techniques to be applied to modelling directly the mechanisms and consequences of orbital migration in the Solar System. There is now potential for these techniques also to be applied to modelling the consequences of the orbital migration of planets in the observed exoplanetary systems. In particular the detailed investigation of the stability of terrestrial planets in the habitable zone of these systems and the formation of terrestrial planets after the dissipation of the gas disk is now possible. The stability of terrestrial planets in the habitable zone of selected exoplanetary systems has been established and the possibility of the accretion of terrestrial planets in these systems is being investigated by the author in collaboration with Barrie W. Jones (Open University), and with John Chambers (NASA-Ames) and Mark Bailey of Armagh Observatory, using numerical integration. The direct simulation of orbital migration by planetesimal scattering must probably await faster hardware and/or more efficient algorithms. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

19.
Gas giant planets have been detected in orbit around an increasing number of nearby stars. Two theories have been advanced for the formation of such planets: core accretion and disk instability. Core accretion, the generally accepted mechanism, requires several million years or more to form a gas giant planet in a protoplanetary disk like the solar nebula. Disk instability, on the other hand, can form a gas giant protoplanet in a few hundred years. However, disk instability has previously been thought to be important only in relatively massive disks. New three-dimensional, "locally isothermal," hydrodynamical models without velocity damping show that a disk instability can form Jupiter-mass clumps, even in a disk with a mass (0.091 M middle dot in circle within 20 AU) low enough to be in the range inferred for the solar nebula. The clumps form with initially eccentric orbits, and their survival will depend on their ability to contract to higher densities before they can be tidally disrupted at successive periastrons. Because the disk mass in these models is comparable to that apparently required for the core accretion mechanism to operate, the models imply that disk instability could obviate the core accretion mechanism in the solar nebula and elsewhere.  相似文献   

20.
The Voyager spacecraft discovered that small moons orbit within all four observed ring systems coincident with the discovery of narrow and dusty rings around Jupiter, Saturn, Uranus and Neptune. These moons can provide the source for new rings if they are catastrophically disrupted by a comet or large meteoroid impact. This hypothesis for ring origins provides a natural mechanism for the ongoing creation of planetary rings. While it relieves somewhat the problem of explaining the continued existence of rings with apparently short evolutionary lifetimes, it raises the problem of explaining the continued existence of small moons, and the coexistence of moons and rings at comparable locations within the Roche zones of the giant planets. This problem has been studied in some detail recently, and the present work is a review of our current understanding of the processes in satellite disruption that pertain to the creation of planetary rings and the collisional cascade of circumplanetary bodies. Significant progress has been made. Narrow rings are produced by disruption of small moons in numerical simulations, and a self-consistent model of the collisional cascade can explain present-day moon populations. Absolute timescales and initial moon populations remain uncertain due to our poor knowledge of the impactor population and uncertainties in the strength of planetary satellites. More pressing are the qualitative issues that remain to be resolved including the nature of reaccretion of the debris and the origin of Saturn's rings.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号