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We have calculated the number of dormant, nearly isotropic Oort cloud comets in the solar system by (i) combining orbital distribution models with statistical models of dormant comet discoveries by well-defined surveys and (ii) comparing the model results to observations of a population of dormant comets. Dynamical models that assume that comets are not destroyed predict that we should have discovered ∼100 times more dormant nearly isotropic comets than are actually seen. Thus, as comets evolve inward from the Oort cloud, the majority of them must physically disrupt. [ABSTRACT FROM AUTHOR]

The article discusses the orbital separation of planets. Planetary scientists are finding increasing evidence that the orbital separations between the giant planets increased substantially as a result of interactions between the planets and a disk of "planetesimals" that were left over after planet formation. The evidence comes from the Kuiper belt, a population of small bodies at the outer edge of today's Solar System that are the last remnants of this disk. Twenty years ago, computer simulations showed that planetary orbits should expand or contract to conserve energy and angular momentum while the planets eject the planetesimals left over from planet formation from their neighborhoods.

The giant planets of the Solar System formed on initially compact orbits, which transitioned to the current wider configuration by means of an orbital instability. The timing of that instability is poorly constrained. In this work, we use dynamical simulations to demonstrate that the instability implanted planetesimal fragments from the terrestrial planet region into the asteroid main belt. We use meteorite data to show that the implantation occurred >60 million years (Myr) after the Solar System began to form. Combining this constraint with a previous upper limit derived from JupiterÕs trojan asteroids, we conclude that the orbital instability occurred 60 to 100 Myr after the beginning of Solar System formation. The giant impact that formed the Moon occurred within this range, so it might be related to the giant planet instability. [ABSTRACT FROM AUTHOR]

Explains research which studied the structure and composition of the Kuiper belt, beyond Neptune. Research by Ward and Hahn in this issue; Insights into the structure and evolution of the Kuiper belt; Possible third population of bodies in the belt, in addition to the inner and classical belts; Object size in the belt; Structural constraints on the belt and its evolution; Mass depletion in the belt.

Describes the model constructed for the orbital and size distributions of the near-Earth asteroids (NEA). Definition of NEA; Reasons behind the complication observed in the distribution of NEA; Three intermediate sources for the NEA.

A quarter of known asteroids is associated with more than 100 distinct asteroid families, meaning that these asteroids originate as impact fragments from the family parent bodies. The determination of which asteroids of the remaining population are members of undiscovered families, or accreted as planetesimals from the protoplanetary disk, would constrain a critical phase of planetary formation by unveiling the unknown planetesimal size distribution. We discovered a 4-billion-year-old asteroid family extending across the entire inner part of the main belt whose members include most of the dark asteroids previously unlinked to families. This allows us to identify some original planetesimals, which are all larger than 35 kilometers, supporting the view of asteroids being born big. Their number matches the known distinct meteorite parent bodies. [ABSTRACT FROM AUTHOR]

Highly siderophile elements (HSEs) are strongly depleted in the bulk silicate Earth (BSE) but are present in near-chondritic relative abundances. The conventional explanation is that the HSEs were stripped from the mantle by the segregation of metal during core formation but were added back in near-chondritic proportions by late accretion, after core formation had ceased. Here we show that metal-silicate equilibration and segregation during Earth’s core formation actually increased HSE mantle concentrations because HSE partition coefficients are relatively low at the high pressures of core formation within Earth. The pervasive exsolution and segregation of iron sulfide liquid from silicate liquid (the “Hadean matte”) stripped magma oceans of HSEs during cooling and crystallization, before late accretion, and resulted in slightly suprachondritic palladium/iridium and ruthenium/iridium ratios. [ABSTRACT FROM AUTHOR]

Earth and the Moon are shown here to have indistinguishable oxygen isotope ratios, with a difference in Δ'17O of -1 ± 5 parts per million (2 standard error). On the basis of these data and our new planet formation simulations that include a realistic model for primordial oxygen isotopic reservoirs, our results favor vigorous mixing during the giant impact and therefore a high-energy, high-angular-momentum impact. The results indicate that the late veneer impactors had an average D'17O within approximately 1 per mil of the terrestrial value, limiting possible sources for this late addition of mass to the Earth-Moon system. [ABSTRACT FROM AUTHOR]

Examines the dynamical spreading of asteroid families by the Yarkovsky effect. Ideas on the orbital distributions of and production of asteroid families; Effect of interactions between drifting asteroid family members and resonances can also produce unique eccentricity and inclination changes; Definition of Yarkovsky effect as a thermal radiation force that causes objects to undergo semimajor axis drift.