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A successful Solar system model must reproduce the four terrestrial planets. Here, we focus on (1) the likelihood of forming Mercury and the four terrestrial planets in the same system (a 4-P system); (2) the orbital properties and masses of each terrestrial planet; and (3) the timing of Earth's last giant impact and the mass accreted by our planet thereafter. Addressing these constraints, we performed 450 N -body simulations of terrestrial planet formation based on narrow protoplanetary discs with mass confined to 0.7–1.0 au. We identified 164 analogue systems, but only 24 systems contained Mercury analogues, and eight systems were 4-P ones. We found that narrow discs containing a small number of embryos with individual masses comparable to that of Mars and the giant planets on their current orbits yielded the best prospects for satisfying those constraints. However, serious shortcomings remain. The formation of Mercury analogues and 4-P systems was too inefficient (5 per cent and 2 per cent, respectively), and most Venus-to-Earth analogue mass ratios were incorrect. Mercury and Venus analogues also formed too close to each other (∼0.15–0.21 au) compared to reality (0.34 au). Similarly, the mutual distances between the Venus and Earth analogues were greater than those observed (0.34 versus 0.28 au). Furthermore, the Venus–Earth pair was not reproduced in orbital-mass space statistically. Overall, our results suggest serious problems with using narrow discs to explain the inner Solar system. In particular, the formation of Mercury remains an outstanding problem for terrestrial planet formation models. [ABSTRACT FROM AUTHOR]

ABSTRACT Recently, the first collisional family was identified in the trans-Neptunian belt (otherwise known as the Edgeworth-Kuiper belt), providing direct evidence of the importance of collisions between trans-Neptunian objects (TNOs). The family consists of the dwarf planet (136108) Haumea (formerly 2003 EL61), located at a semimajor axis, a, of ∼43 au, and at least 10 other ∼100-km-sized TNOs located in the region a= 42-44.5 au. In this work, we model the long-term orbital evolution (4 Gyr) of an ensemble of fragments (particles) representing hypothetical post-collision distributions at the time of the family's birth based on our limited current understanding of the family's creation and of asteroidal collision physics. We consider three distinct scenarios, in which the kinetic energy of dispersed particles was varied such that their mean ejection velocities ( veje) were of the order of 200, 300 and 400 m s−1, respectively. Each simulation considered resulted in collisional families that reproduced that currently observed, despite the variation in the initial conditions modelled. The results suggest that 60-75 per cent of the fragments created in the collision will remain in the trans-Neptunian belt, even after 4 Gyr of dynamical evolution. The surviving particles were typically concentrated in wide regions of orbital element space centred on the initial impact location, with their orbits spread across a region spanning Δ a∼ 6-12 au, Δ e∼ 0.1-0.15 and Δ i∼ 7°-10°, with the exact range covered being proportional to veje used in the model. Most of the survivors populated the so-called classical and detached regions of the trans-Neptunian belt, whilst a minor fraction either entered the scattered disc reservoir (<1 per cent) or were captured in Neptunian mean-motion resonances (<10 per cent). In addition, except for those fragments located near strong resonances (such as the 5:3 and 7:4), the great majority displayed negligible long-term orbital variation. This implies that the orbital distribution of the intrinsic Haumean family can be used to constrain the orbital conditions and physical nature of the collision that created the family, billions of years ago. Indeed, our results suggest that the formation of the Haumean collisional family most likely occurred after the bulk of Neptune's migration was complete, or even some time after the migration had completely ceased, although future work is needed to confirm this result. [ABSTRACT FROM AUTHOR]

Centaurs have orbits between Jupiter and Neptune and are thought to originate from the trans-Neptunian region. Observations of surface properties of Centaurs and comparison with those of trans-Neptunian objects (TNOs) would provide constraints on their origin and evolution. We analyzed imaging data of nine known Centaurs observed by the Hyper Suprime-Cam (HSC) installed on the Subaru Telescope with the g - and i -band filters. Using the data available in the public HSC data archive, as well as those obtained by the HSC Subaru Strategic Program (HSC-SSP) by the end of 2017 June, we obtained the g − i colors of the nine Centaurs. We compared them with those of known TNOs in the HSC-SSP data obtained by T. Terai et al. (2018, PASJ, 70, S40). We found that the color distribution of the nine Centaurs is similar to that of those TNOs with high orbital inclinations, but distinct from those TNOs with low orbital inclinations. We also examined correlations between the colors of these Centaurs and their orbital elements and absolute magnitude. The Centaurs' colors show a moderate positive correlation with semi-major axis, while no significant correlations between the color and other orbital elements or absolute magnitude were found for these Centaurs. On the other hand, recent studies on Centaurs with larger samples show interesting correlations between their color and absolute magnitude or orbital inclination. We discuss how our data fit in these previous studies, and also discuss implications of these results for their origin and evolution. [ABSTRACT FROM AUTHOR]