Several properties of the Solar System, including the wide radial spacing of the giant planets, can be explained if planets radially migrated by exchanging orbital energy and momentum with outer disk ...planetesimals. Neptune's planetesimal-driven migration, in particular, has a strong advocate in the dynamical structure of the Kuiper belt. A dynamical instability is thought to have occurred during the early stages with Jupiter having close encounters with a Neptune-class planet. As a result of the encounters, Jupiter acquired its current orbital eccentricity and jumped inward by a fraction of an astronomical unit, as required for the survival of the terrestrial planets and from asteroid belt constraints. Planetary encounters also contributed to capture of Jupiter Trojans and irregular satellites of the giant planets. Here we discuss the dynamical evolution of the early Solar System with an eye to determining how models of planetary migration/instability can be constrained from its present architecture. Specifically, we review arguments suggesting that the Solar System may have originally contained a third ice giant on a resonant orbit between Saturn and Uranus. This hypothesized planet was presumably ejected into interstellar space during the instability. The Kuiper belt kernel and other dynamical structures in the trans-Neptunian region may provide evidence for the ejected planet. We favor the early version of the instability where Neptune migrated into the outer planetesimal disk within a few tens of millions of years after the dispersal of the protosolar nebula. If so, the planetary migration/instability was not the cause of the Late Heavy Bombardment. Mercury's orbit may have been excited during the instability.
ABSTRACT Much of the dynamical structure of the Kuiper Belt can be explained if Neptune migrated over several AU, and/or if Neptune was scattered to an eccentric orbit during planetary instability. ...An outstanding problem with the existing formation models is that the distribution of orbital inclinations they predicted is narrower than the one inferred from observations. Here we perform numerical simulations of Kuiper Belt formation starting from an initial state with Neptune at AU and a dynamically cold outer disk extending from beyond to 30 AU. Neptune's orbit is migrated into the disk on an e-folding timescale 1 ≤ τ ≤ 100 Myr. A small fraction (∼10−3) of the disk planetesimals become implanted into the Kuiper belt in the simulations. By analyzing the orbital distribution of the implanted bodies in different cases we find that the inclination constraint implies that Myr and AU. The models with Myr do not satisfy the inclination constraint, because there is not enough time for various dynamical processes to raise inclinations. The slow migration of Neptune is consistent with other Kuiper Belt constraints, and with recently developed models of planetary instability/migration. Neptune's eccentricity and inclination are never large in these models ( , ), as required to avoid excessive orbital excitation in the >40 AU region, where the Cold Classicals presumably formed.
ABSTRACT The Kuiper Belt is a population of icy bodies beyond the orbit of Neptune. A particularly puzzling and up-to-now unexplained feature of the Kuiper Belt is the so-called "kernel," a ...concentration of orbits with semimajor axes a 44 AU, eccentricities e ∼ 0.05, and inclinations . Here we show that the Kuiper Belt kernel can be explained if Neptune's otherwise smooth migration was interrupted by a discontinuous change of Neptune's semimajor axis when Neptune reached 28 AU. Before the discontinuity happened, planetesimals located at ∼40 AU were swept into Neptune's 2:1 resonance, and were carried with the migrating resonance outwards. The 2:1 resonance was at 44 AU when Neptune reached 28 AU. If Neptune's semimajor axis changed by fraction of AU at this point, perhaps because Neptune was scattered off of another planet, the 2:1 population would have been released at 44 AU, and would remain there to this day. We show that the orbital distribution of bodies produced in this model provides a good match to the orbital properties of the kernel. If Neptune migration was conveniently slow after the jump, the sweeping 2:1 resonance would deplete the population of bodies at 45-47 AU, thus contributing to the paucity of the low-inclination orbits in this region. Special provisions, probably related to inefficiencies in the accretional growth of sizable objects, are still needed to explain why only a few low-inclination bodies have been so far detected beyond 47 AU.
Studies of solar system formation suggest that the solar system's giant planets formed and migrated in the protoplanetary disk to reach the resonant orbits with all planets inside ~15 AU from the ...Sun. After the gas disk's dispersal, Uranus and Neptune were likely scattered by the gas giants, and approached their current orbits while dispersing the transplanetary disk of planetesimals, whose remains survived to this time in the region known as the Kuiper Belt. Here we performed N-body integrations of the scattering phase between giant planets in an attempt to determine which initial states are plausible. We found that the dynamical simulations starting with a resonant system of four giant planets have a low success rate in matching the present orbits of giant planets and various other constraints (e.g., survival of the terrestrial planets). The dynamical evolution is typically too violent, if Jupiter and Saturn start in the 3:2 resonance, and leads to final systems with fewer than four planets. Several initial states stand out in that they show a relatively large likelihood of success in matching the constraints. Some of the statistically best results were obtained when assuming that the solar system initially had five giant planets and one ice giant, with the mass comparable to that of Uranus and Neptune, and which was ejected to interstellar space by Jupiter. This possibility appears to be conceivable in view of the recent discovery of a large number of free-floating planets in interstellar space, which indicates that planet ejection should be common.
Abstract
The dynamical structure of the Kuiper Belt can be used as a clue to the formation and evolution of the solar system, planetary systems in general, and Neptune’s early orbital history in ...particular. The problem is best addressed by forward modeling where different initial conditions and Neptune’s orbital evolutions are tested, and the model predictions are compared to orbits of known Kuiper Belt objects (KBOs). It has previously been established that Neptune radially migrated, by gravitationally interacting with an outer disk of planetesimals, from the original radial distance
r
≲ 25 au to its current orbit at 30 au. Here we show that the migration models with a very low orbital eccentricity of Neptune (
e
N
≲ 0.03) do not explain KBOs with semimajor axes 50 <
a
< 60 au, perihelion distances
q
> 35 au, and inclinations
i
< 10°. If
e
N
≲ 0.03 at all times, the Kozai cycles control the implantation process and the orbits with
q
> 35 au end up having, due to the angular momentum’s
z
-component conservation,
i
> 10°. Better results are obtained when Neptune’s eccentricity is excited to
e
N
≃ 0.1 and subsequently damped by dynamical friction. The low-
e
and low-
i
orbits at 50–60 au are produced in this model when KBOs are lifted from the scattered disk by secular cycles—mainly the apsidal resonance
ν
8
—near various mean motion resonances. These results give support to a (mild) dynamical instability that presumably excited the orbits of giant planets during Neptune’s early migration.
Origin and Evolution of Long-period Comets Vokrouhlický, David; Nesvorný, David; Dones, Luke
The Astronomical journal,
05/2019, Letnik:
157, Številka:
5
Journal Article
Recenzirano
Odprti dostop
We develop an evolutionary model of the long-period comet (LPC) population, starting from their birthplace in a massive trans-Neptunian disk that was dispersed by migrating giant planets. Most comets ...that remain bound to the solar system are stored in the Oort cloud. Galactic tides and passing stars make some of these bodies evolve into observable comets in the inner solar system. Our approach models each step in a full-fledged numerical framework. Subsequent analysis consists of applying plausible fading models and computing the original orbits to compare with observations. Our results match the observed semimajor axis distribution of LPCs when Whipple's power-law fading scheme with an exponent is adopted. The cumulative perihelion (q) distribution is well fit by a linear increase plus a weak quadratic term. Beyond q = 15 au, however, the population increases steeply, and the isotropy of LPC orbital planes breaks. We find tentative evidence from the perihelion distribution of LPCs that the returning comets are depleted in supervolatiles and become active due to water ice sublimation for q ≤ 3 au. Using an independent calibration of the population of the initial disk, our predicted LPC flux is smaller than observations suggest by a factor of 2. Current data only characterize comets from the outer Oort cloud (semimajor axes 104 au). A true boost in understanding the Oort cloud's structure should result from future surveys when they detect LPCs with perihelia beyond 15 au. Our results provide observational predictions of what can be expected from these new data.
ABSTRACT In this work, we investigate the evolution of a primordial belt of asteroids, represented by a large number of massless test particles, under the gravitational effect of migrating Jovian ...planets in the framework of the jumping-Jupiter model. We perform several simulations considering test particles distributed in the Main Belt, as well as in the Hilda and Trojan groups. The simulations start with Jupiter and Saturn locked in the mutual 3:2 mean motion resonance plus three Neptune-mass planets in a compact orbital configuration. Mutual planetary interactions during migration led one of the Neptunes to be ejected in less than 10 Myr of evolution, causing Jupiter to jump by about 0.3 AU in semimajor axis. This introduces a large-scale instability in the studied populations of small bodies. After the migration phase, the simulations are extended over 4 Gyr, and we compare the final orbital structure of the simulated test particles to the current Main Belt of asteroids with absolute magnitude H < 9.7. The results indicate that, in order to reproduce the present Main Belt, the primordial belt should have had a distribution peaked at ∼10° in inclination and at ∼0.1 in eccentricity. We discuss the implications of this for the Grand Tack model. The results also indicate that neither primordial Hildas, nor Trojans, survive the instability, confirming the idea that such populations must have been implanted from other sources. In particular, we address the possibility of implantation of Hildas and Trojans from the Main Belt population, but find that this contribution should be minor.
Several properties of the solar system, including the wide radial spacing and orbital eccentricities of giant planets, can be explained if the early solar system evolved through a dynamical ...instability followed by migration of planets in the planetesimal disk. We found that the dynamical evolution is typically too violent, if Jupiter and Saturn start in the 3:2 resonance, leading to ejection of at least one ice giant from the solar system. Planet ejection can be avoided if the mass of the transplanetary disk of planetesimals was large, but we found that a massive disk would lead to excessive dynamical damping, and to smooth migration that violates consttaints from the survival of the terrestrial planets. The best results were obtained when the ejected planet was placed into the external 3:2 or 4:3 resonance with Saturn and M sub(disk) Asymptotically = to 20 M sub(Earth). The case with six giant planets shows interesting dynamics but does offer significant advantages relative to the five-planet case.
ABSTRACT The Kuiper Belt is a population of icy bodies beyond the orbit of Neptune. The complex orbital structure of the Kuiper Belt, including several categories of objects inside and outside of ...resonances with Neptune, emerged as a result of Neptune's migration into an outer planetesimal disk. An outstanding problem with the existing migration models is that they invariably predict excessively large resonant populations, while observations show that the non-resonant orbits are in fact common (e.g., the main belt population is 2-4 times larger than Plutinos in the 3:2 resonance). Here we show that this problem can be resolved if it is assumed that Neptune's migration was grainy, as expected from scattering encounters of Neptune with massive planetesimals. The grainy migration acts to destabilize resonant bodies with large libration amplitudes, a fraction of which ends up on stable non-resonant orbits. Thus, the non-resonant-to-resonant ratio obtained with the grainy migration is higher, up to ∼10 times higher for the range of parameters investigated here, than in a model with smooth migration. In addition, the grainy migration leads to a narrower distribution of the libration amplitudes in the 3:2 resonance. The best fit to observations is obtained when it is assumed that the outer planetesimal disk below 30 au contained 1000-4000 Plutos. We estimate that the combined mass of Pluto-class objects in the original disk represented 10%-40% of the estimated disk mass ( ). This constraint can be used to better understand the accretion processes in the outer solar system.
ABSTRACT We explore the past evolution of Saturn's moons using direct numerical integrations. We find that the past Tethys-Dione 3:2 orbital resonance predicted in standard models likely did not ...occur, implying that the system is less evolved than previously thought. On the other hand, the orbital inclinations of Tethys, Dione, and Rhea suggest that the system did cross the Dione-Rhea 5:3 resonance, which is closely followed by a Tethys-Dione secular resonance. A clear implication is that either the moons are significantly younger than the planet or their tidal evolution must be extremely slow (Q > 80,000). As an extremely slow evolving system is incompatible with intense tidal heating of Enceladus, we conclude that the moons interior to Titan are not primordial, and we present a plausible scenario for the system's recent formation. We propose that the midsized moons re-accreted from a disk about 100 Myr ago, during which time Titan acquired its significant orbital eccentricity. We speculate that this disk has formed through orbital instability and massive collisions involving the previous generation of Saturn's midsized moons. We identify the solar evection resonance perturbing a pair of midsized moons as the most likely trigger of such an instability. This scenario implies that most craters on the moons interior to Titan must have been formed by planetocentric impactors.
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