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The origin of Triton's retrograde orbit remains challenging. Today it is generally accepted that Triton was somehow captured by Neptune when the latter experienced a phase of planetesimal-driven migration. The most likely capture scenario invokes the trapping of one of the members of a binary planetesimal during a close encounter with Neptune (Agnor and Hamilton, 2006). Although viable, this scenario cannot explain the survival of Nereid on an orbit exterior to Triton's, given the extremely eccentric post-capture orbit of the major satellite (Nogueira et al. 2011). Here we propose an alternative scenario in which Triton was originally a regular satellite of Neptune on an equatorial orbit. In this scenario, Neptune experienced at least two collisions with planetary embryos during its formation, leading eventually to its current obliquity; because of these impulsive resets of Neptune's obliquity, Triton may have acquired an orbit with a large inclination with respect to Neptune's equator, a retrograde orbit being a possibility. If we impose the constraints that Neptune final rotation rate and obliquity end near its current values we estimate the final probability of Triton getting a retrograde orbit at roughly 5% to 10%. After the phase of collisions of planetary embryos with Neptune, we apply a simple tidal model to follow the evolution of Triton's retrograde orbit over 4.5 Gy and check its final compatibility with the current Triton-Neptune distance. Our simulations consider Neptune, two collisions with Earth-mass embryos, an equatorial debris disk generated by each collision and temporarily enhancing Neptune's J2, a Triton-sized satellite and the Sun. We tested different Sun–Neptune distances and different timescales for the decay of the equatorial debris disks and show that these two parameters have little influence on the outcome. We also checked that satellites that ended their tidal evolution near Triton's current orbit seldom had their apoapsis greater than Nereid's periapsis, making this scenario compatible with the preservation of Nereid and of other Neptune's irregular satellites. • Due to its retrograde orbit, Triton is considered an object captured by Neptune. • Triton may have been originally a regular satellite of Neptune. • Triton's orbit may have been turned retrograde by collisions of embryos on Neptune. [ABSTRACT FROM AUTHOR]

We study planetary migration in a gas-free disk of planetesimals. In the case of our Solar System we show that Neptune could have had either a damped migration, limited to a few AUs, or a forced migration up to the disk's edge, depending on the disk's mass density. We also study the possibility of runaway migration of isolated planets in very massive disk, which might be relevant for extra-solar systems. We investigate the problem of the mass depletion of the Kuiper belt in the light of planetary migration and conclude that the belt lost its pristine mass well before that Neptune reached its current position. Therefore, Neptune effectively hit the outer edge of the proto-planetary disk. We also investigate the dynamics of massive planetary embryos embedded in the planetesimal disk. We conclude that the elimination of Earth-mass or Mars-mass embryos originally placed outside the initial location of Neptune also requires the existence of a disk edge near 30 AU. Keywords: Kuiper belt Resonance; Solar System formation

• Simulations with an excited Neptune overexcite the KB cold population. • A self graviting planetesimal disk helps objects to return to an unexcited mode. • The self-gravity is effective just for a cold population with Pluto-sized objects. • Without self gravity, we need very specific secular configurations for Neptune. The link between the dynamical evolution of the giant planets and the Kuiper Belt orbital structure can provide clues and insight about the dynamical history of the Solar System. The classical region of the Kuiper Belt has two populations (the cold and hot populations) with completely different physical and dynamical properties. These properties have been explained in the framework of a sub-set of the simulations of the Nice Model , in which Neptune remained on a low-eccentricity orbit (Neptune's eccentricity is never larger than 0.1) throughout the giant planet instability (Nesvorný 2015a,b). However, recent simulations (Gomes et al., 2018) have showed that the remaining Nice model simulations, in which Neptune temporarily acquires a large-eccentricity orbit (larger than 0.1), are also consistent with the preservation of the cold population (inclination smaller than 4°), if the latter formed in situ. However, the resulting a cold population showed in many of the simulations eccentricities larger than those observed for the real population. The purpose of this work is to discuss the dynamical effects on the Kuiper belt region due to an excited Neptune phase. We focus on a short period of time, of about six hundred thousand years, which is characterized by Neptune's large eccentricity and smooth migration with a slow precession of Neptune's perihelion. This phase was observed during a full simulation of the Nice Model (Gomes et al., 2018) just after the last jump of Neptune's orbit due to an encounter with another planet. We show that if self-gravity is considered in the disk, the precession rate of the particles longitude of perihelion ϖ is slowed down, which in turn speeds up the cycle of ϖ N − ϖ (the subscript N referring to Neptune), associated to the particles eccentricity evolution. This, combined with the effect of mutual scattering among the bodies, which spreads all orbital elements, allows some objects to return to low eccentricities. However, we show that if the cold population originally had a small total mass, this effect is negligible. Thus, we conclude that the only possibilities to keep at low eccentricity some cold-population objects during a high-eccentricity phase of Neptune are that (i) either Neptune's precession was rapid, as suggested by Batygin et al. (2011) or (ii) Neptune's slow precession phase was long enough to allow some particles to experience a full secular cycle of ϖ − ϖ N. [ABSTRACT FROM AUTHOR]

The Classical Kuiper Belt is populated by a group of objects with low inclination orbits, reddish colors and usually belonging to a binary system. This so called Cold Classical Kuiper Belt is considered to have been formed in situ from primordial ice pebbles that coagulated into planetesimals hundreds of kilometers in diameter. According to this scenario, the accretion of pebbles into large planetesimals would have occurred through the streaming instability mechanism that would be effective in the primordial Solar System disk of gas and solids. Nevertheless other objects with the same color characteristics as those found in the Cold Classical Kuiper Belt can be encountered also past the 2:1 mean motion resonance with Neptune as scattered or detached objects. Here I propose a mechanism that can account for both the cold Classical Kuiper Belt objects and other reddish objects outside the Classical Kuiper Belt. According to the proposed scenario, reddish objects were primordially in the outer portion of the planetesimal disk which was however truncated somewhere below ~42 au. In this manner the cold Classical Kuiper Belt and its scattered / detached counterpart were respectively transported outwards by a short range or slightly scattered to their present locations. Resonant objects were also formed by the same process. This mechanism is aimed at explaining the distribution of all objects that share the same color characteristics as coming from a common origin in the outer borders of the primordial planetesimal disk. According to the scenario here proposed the Cold Classical Kuiper Belt would have been formed ~4 au inside its present location with a total mass 20 − 100 times as large as its present value. • The Cold Classical Kuiper Belt is usually believed to be formed in situ. • I propose a scenario by which the CCKB was transported a short distance from the planetesimal disk. • This scenario can account for the implantation of reddish objects as detached and resonant TNOs. • In this scenario the cold KB objects would be formed with a mass 20 to 100 times as large as the present CCKB. [ABSTRACT FROM AUTHOR]

The planetary migration of the Solar System giant planets in the framework of the Nice model (Tsiganis, K., Gomes, R., Morbidelli, A., Levison, H.F. [2005]. Nature 435,459–461; Morbidelli, A., Levison, H.F., Tsiganis, K., Gomes, R. [2005]. Nature 435, 462–465; Gomes, R., Levison, H.F., Tsiganis, K., Morbidelli, A. [2005]. Nature 435, 466–469) creates a dynamical mechanism which can be used to explain the distribution of objects currently observed in the Kuiper belt (e.g., Levison, H.F., Morbidelli, A., Vanlaerhoven, C., Gomes, R., Tsiganis, K. [2008]. Icarus 196, 258–273). Through this mechanism the planetesimals within the disk, heliocentric distance ranging from beyond Neptune’s orbit to approximately 34 AU, are delivered to the belt after a temporary eccentric phase of Uranus and Neptune’s orbits. We reproduced the mechanism proposed by Levison et al. to implant bodies into the Kuiper belt. The capture of Pluto into the external 3:2 mean motion resonance with Neptune is associated with this gravitational scattering model. We verified the existence of several close encounters between the ice giants and the planetesimals during their outward radial migration, then we believe that the analysis of the dynamical history of the plutonian satellites during this kind of migration is important, and would provide some constrains about their place of formation – within the primordial planetesimal disk or in situ . We performed N -body simulations and recorded the trajectories of the planetesimals during close approaches with Uranus and Neptune. Close encounters with Neptune are the most common, reaching approximately 1200 in total. A Pluto similarly sized body assumed the hyperbolic trajectories of the former primordial planetesimal with respect to those giant planets. We assumed the current mutual orbital configuration and sizes for Pluto’s satellites, then we found that the rate of destruction of systems similar to that of Pluto with closest approaches to Uranus or Neptune <0.10 AU is 40%, i.e. these close approaches can lead to ejections of satellites or to changes in the satellites eccentricities at least 1 order of magnitude larger than the currently observed. However, we also found that the number of closest approaches which the minimum separation to Uranus or Neptune <0.10 AU is negligible, reaching 6%. In the other 60% of close encounter histories with closest approaches >0.10 AU, none of the systems have been destroyed. The latter sample concentrates 94% of closest approaches with the ice giants. Recall that throughout the early history of the Solar System giant impacts were common (McKinnon, W.B. [1989]. Astrophys. J. 344, L41–L44; Stern, A. [1991]. Icarus 90; Canup, R.M. [2005]. Science 307, 546–550). Also, impacts capable of forming a binary like Pluto-Charon can occur possibly prior to 0.5–1 Gyr (Kenyon, S.J., Bromley, B.C. [2014]. Astron. J. 147, 8), and small satellites such as Nix and Hydra can grow in debris from the giant impact (e.g., Canup, R.M. [2011]. Astron. J. 141, 35). Thus, we conclude that if Pluto and its satellites were emplaced into the KB from lower heliocentric orbits, then the Pluto system could survive the encounters that may have happened for emplacement of the Plutinos through the mechanism proposed by Levison et al. [ABSTRACT FROM AUTHOR]

The Dwarf Planet Ceres revealed the presence of ammonia and other unique properties compared to other asteroids in the main belt which suggests that it was not formed in situ. We model the early dynamical evolution of the outer Solar System to study possible dynamical mechanisms to implant a Ceres-sized planetesimal in the asteroid belt from the trans-Saturnian region. We calculate that the fraction of the population of Ceres-sized planetesimals that are captured in the asteroid belt is in the range of 2. 8 × 1 0 − 5 to 1. 2 × 1 0 − 3 depending on the initial location in the outer planetesimal disk. The captured bodies have a 70% probability to have a semimajor axis between 2.5 and 3 au, a 33% probability to have an eccentricity smaller than 0.2 and a 45% probability to have an orbital inclination smaller than 10°. Assuming the existence of 3,600 Ceres-size planetesimals in the inner part of the trans-Saturnian disk, consistent with the estimate of Nesvorný and Vokrouhlický (2016) for the trans-Neptunian disk, our estimated capture probability and a final 80% depletion of the asteroid belt during the subsequent giant planet instability, lead to capture ∼ 1 Ceres in the asteroid belt, with a probability of 15%, 34%, and 51% to be located in the inner, middle and outer belt respectively. • We model the early evolution of Solar System to investigate the origin of Ceres. • Our model can lead to capture of one Ceres in the Asteroid Belt. • The probability of Ceres is in the inner belt is 15%. • The probability of Ceres is in the middle belt is 34%. • The probability of Ceres is in the outer belt is 51%. [ABSTRACT FROM AUTHOR]

The dynamical structure of the Solar System can be explained by a period of orbital instability experienced by the giant planets. While a late instability was originally proposed to explain the Late Heavy Bombardment, recent work favors an early instability. Here we model the early dynamical evolution of the outer Solar System to self-consistently constrain the most likely timing of the instability. We first simulate the dynamical sculpting of the primordial outer planetesimal disk during the accretion of Uranus and Neptune from migrating planetary embryos during the gas disk phase, and determine the separation between Neptune and the inner edge of the planetesimal disk. We performed simulations with a range of (inward and outward) migration histories for Jupiter. We find that, unless Jupiter migrated inwards by 10 AU or more, the instability almost certainly happened within 100 Myr of the start of Solar System formation. There are two distinct possible instability triggers. The first is an instability that is triggered by the planets themselves, with no appreciable influence from the planetesimal disk. About half of the planetary systems that we consider have a self-triggered instability. Of those, the median instability time is ∼ 4Myr. Among self-stable systems – where the planets are locked in a resonant chain that remains stable in the absence of a planetesimal's disk– our self-consistently sculpted planetesimal disks nonetheless trigger a giant planet instability with a median instability time of 37–62 Myr for a reasonable range of migration histories of Jupiter. The simulations that give the latest instability times are those that invoked long-range inward migration of Jupiter from 15 AU or beyond; however these simulations over-excited the inclinations of Kuiper belt objects and are inconsistent with the present-day Solar System. We conclude on dynamical grounds that the giant planet instability is likely to have occurred early in Solar System history. • The dynamical structure of Solar System can be explained by giant planet instability. • The early evolution of giant planets is used to get the timing of the instability. • The giant planet instability is likely to have occurred early in Solar System. [ABSTRACT FROM AUTHOR]