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How Jupiter pulls the strings The giant planets formed much earlier in the life of the Solar System than the terrestrial ones, taking just a few million years to coalesce from the protoplanetary disk. They were also quite mobile, on timescales in the order of 100,000 years. Simulations of the early Solar System now show how an inward migration of Jupiter followed by an outward migration could have produced a truncated planetesimal disk from which terrestrial planets formed over the next 30–50 million years. The terrestrial planets stopped accreting much later, and their characteristics, including Mars's small mass, are best reproduced by starting from a planetesimal disk that has an outer edge at around one astronomical unit from the Sun. Jupiter and Saturn formed in a few million years (ref. 1 ) from a gas-dominated protoplanetary disk, and were susceptible to gas-driven migration of their orbits on timescales of only ∼100,000 years (ref. 2 ). Hydrodynamic simulations show that these giant planets can undergo a two-stage, inward-then-outward, migration 3 , 4 , 5 . The terrestrial planets finished accreting much later 6 , and their characteristics, including Mars' small mass, are best reproduced by starting from a planetesimal disk with an outer edge at about one astronomical unit from the Sun 7 , 8 (1  au is the Earth–Sun distance). Here we report simulations of the early Solar System that show how the inward migration of Jupiter to 1.5  au , and its subsequent outward migration, lead to a planetesimal disk truncated at 1  au ; the terrestrial planets then form from this disk over the next 30–50 million years, with an Earth/Mars mass ratio consistent with observations. Scattering by Jupiter initially empties but then repopulates the asteroid belt, with inner-belt bodies originating between 1 and 3  au and outer-belt bodies originating between and beyond the giant planets. This explains the significant compositional differences across the asteroid belt. The key aspect missing from previous models of terrestrial planet formation is the substantial radial migration of the giant planets, which suggests that their behaviour is more similar to that inferred for extrasolar planets than previously thought.

A Fermi gas of atoms with resonant interactions is predicted to obey universal hydrodynamics, in which the shear viscosity and other transport coefficients are universal functions of the density and temperature. At low temperatures, the viscosity has a universal quantum scale ħ n, where n is the density and ħ is Planck's constant h divided by 2π, whereas at high temperatures the natural scale is pT³/ħ², where pT is the thermal momentum. We used breathing mode damping to measure the shear viscosity at low temperature. At high temperature T, we used anisotropic expansion of the cloud to find the viscosity, which exhibits precise T³/² scaling. In both experiments, universal hydrodynamic equations including friction and heating were used to extract the viscosity. We estimate the ratio of the shear viscosity to the entropy density and compare it with that of a perfect fluid.

Stable-isotope variations exist among inner solar system solids, planets, and asteroids, but their importance is not understood. We report correlated, mass-independent variations of titanium-46 and titanium-50 in bulk analyses of these materials. Because titanium-46 and titanium-50 have different nucleosynthetic origins, this correlation suggests that the presolar dust inherited from the protosolar molecular cloud was well mixed when the oldest solar system solids formed, but requires a subsequent process imparting isotopic variability at the planetary scale. We infer that thermal processing of molecular cloud material, probably associated with volatile-element depletions in the inner solar system, resulted in selective destruction of thermally unstable, isotopically anomalous presolar components, producing residual isotopic heterogeneity. This implies that terrestrial planets accreted from thermally processed solids with nonsolar isotopic compositions.

A drop in the ocean Earth's bulk composition is similar to that of a group of oxygen-poor meteorites called enstatite chondrites, thought to have formed in the early solar nebula. This leads to the suggestion that proto-Earth was dry, and that volatiles including water were delivered by asteroid and comet impacts. The deuterium-to-hydrogen (D/H) ratios measured in six Oort cloud comets are much higher than on Earth, however, apparently ruling out a dominant role for such bodies. Now the Herschel Space Telescope has been used to determine the D/H ratio in the Kuiper belt comet 103P/Hartley 2. The ratio is Earth-like, suggesting that this population of comets may have contributed to Earth's ocean waters. For decades, the source of Earth's volatiles, especially water with a deuterium-to-hydrogen ratio (D/H) of (1.558 ± 0.001) × 10 −4 , has been a subject of debate. The similarity of Earth’s bulk composition to that of meteorites known as enstatite chondrites 1 suggests a dry proto-Earth 2 with subsequent delivery of volatiles 3 by local accretion 4 or impacts of asteroids or comets 5 , 6 . Previous measurements in six comets from the Oort cloud yielded a mean D/H ratio of (2.96 ± 0.25) × 10 −4 . The D/H value in carbonaceous chondrites, (1.4 ± 0.1) × 10 −4 , together with dynamical simulations, led to models in which asteroids were the main source of Earth's water 7 , with ≤10 per cent being delivered by comets. Here we report that the D/H ratio in the Jupiter-family comet 103P/Hartley 2, which originated in the Kuiper belt, is (1.61 ± 0.24) × 10 −4 . This result substantially expands the reservoir of Earth ocean-like water to include some comets, and is consistent with the emerging picture of a complex dynamical evolution of the early Solar System 8 , 9 .

Complex organic compounds, including many important to life on Earth, are commonly found in meteoritic and cometary samples, though their origins remain a mystery. We examined whether such molecules could be produced within the solar nebula by tracking the dynamical evolution of ice grains in the nebula and recording the environments to which they were exposed. We found that icy grains originating in the outer disk, where temperatures were less than 30 kelvin, experienced ultraviolet irradiation exposures and thermal warming similar to that which has been shown to produce complex organics in laboratory experiments. These results imply that organic compounds are natural by-products of proto planetary disk evolution and should be important ingredients in the formation of all planetary systems, including our own.

Under certain circumstances, three or more interacting particles may form bound states. Although the general few-body problem is not analytically solvable, the so-called Efimov trimers appear for a system of three particles with resonant two-body interactions. The binding energies of these trimers are predicted to be universally connected to each other, independent of the microscopic details of the interaction. By exploiting a Feshbach resonance to widely tune the interactions between trapped ultracold lithium atoms, we find evidence for two universally connected Efimov trimers and their associated four-body bound states. A total of 11 precisely determined three- and four-body features are found in the inelastic-loss spectrum. Their relative locations on either side of the resonance agree well with universal theory, whereas a systematic deviation from universality is found when comparing features across the resonance.

When a planetary tidal disk—like Saturn's rings—spreads beyond the Roche radius (inside which planetary tides prevent aggregation), satellites form and migrate away. Here, we show that most regular satellites in the solar system probably formed in this way. According to our analytical model, when the spreading is slow, a retinue of satellites appear with masses increasing with distance to the Roche radius, in excellent agreement with Saturn's, Uranus', and Neptune's satellite systems. This suggests that Uranus and Neptune used to have massive rings that disappeared to give birth to most of their regular satellites. When the spreading is fast, only one large satellite forms, as was the case for Pluto and Earth. This conceptually bridges the gap between terrestrial and giant planet systems.

Solar System giants A collection of three papers in this issue, tackling seemingly unrelated planetary phenomena, marks a notable unification of Solar System dynamics. The three problems covered are the hard-to-explain orbits of giant planets, the evolution of the orbits of Jupiter's Trojan asteroids, and the cause of the ‘Late Heavy Bombardment’ that peppered the Moon with meteors, comets and asteroids some 700 million years after the planets were formed. Key to all these events, on this new model, was a rapid migration of the giant planets (Saturn, Jupiter, Neptune and Uranus) after a long period of stability within the Solar System. The petrology record on the Moon suggests that a cataclysmic spike in the cratering rate occurred ∼700 million years after the planets formed 1 ; this event is known as the Late Heavy Bombardment (LHB). Planetary formation theories cannot naturally account for an intense period of planetesimal bombardment so late in Solar System history 2 . Several models have been proposed to explain a late impact spike 3 , 4 , 5 , 6 , but none of them has been set within a self-consistent framework of Solar System evolution. Here we propose that the LHB was triggered by the rapid migration of the giant planets, which occurred after a long quiescent period. During this burst of migration, the planetesimal disk outside the orbits of the planets was destabilized, causing a sudden massive delivery of planetesimals to the inner Solar System. The asteroid belt was also strongly perturbed, with these objects supplying a significant fraction of the LHB impactors in accordance with recent geochemical evidence 7 , 8 . Our model not only naturally explains the LHB, but also reproduces the observational constraints of the outer Solar System 9 .

Core formation should have stripped the terrestrial, lunar, and martian mantles of highly siderophile elements (HSEs). Instead, each world has disparate, yet elevated HSE abundances. Late accretion may offer a solution, provided that ≥0.5% Earth masses of broadly chondritic planetesimals reach Earth's mantle and that approximately 10 and approximately 1200 times less mass goes to Mars and the Moon, respectively. We show that leftover planetesimal populations dominated by massive projectiles can explain these additions, with our inferred size distribution matching those derived from the inner asteroid belt, ancient martian impact basins, and planetary accretion models. The largest late terrestrial impactors, at 2500 to 3000 kilometers in diameter, potentially modified Earth's obliquity by approximately 10°, whereas those for the Moon, at approximately 250 to 300 kilometers, may have delivered water to its mantle.

Solar System giants A collection of three papers in this issue, tackling seemingly unrelated planetary phenomena, marks a notable unification of Solar System dynamics. The three problems covered are the hard-to-explain orbits of giant planets, the evolution of the orbits of Jupiter's Trojan asteroids, and the cause of the ‘Late Heavy Bombardment’ that peppered the Moon with meteors, comets and asteroids some 700 million years after the planets were formed. Key to all these events, on this new model, was a rapid migration of the giant planets (Saturn, Jupiter, Neptune and Uranus) after a long period of stability within the Solar System. Planetary formation theories 1 , 2 suggest that the giant planets formed on circular and coplanar orbits. The eccentricities of Jupiter, Saturn and Uranus, however, reach values of 6 per cent, 9 per cent and 8 per cent, respectively. In addition, the inclinations of the orbital planes of Saturn, Uranus and Neptune take maximum values of ∼2 degrees with respect to the mean orbital plane of Jupiter. Existing models for the excitation of the eccentricity of extrasolar giant planets 3 , 4 , 5 have not been successfully applied to the Solar System. Here we show that a planetary system with initial quasi-circular, coplanar orbits would have evolved to the current orbital configuration, provided that Jupiter and Saturn crossed their 1:2 orbital resonance. We show that this resonance crossing could have occurred as the giant planets migrated owing to their interaction with a disk of planetesimals 6 , 7 . Our model reproduces all the important characteristics of the giant planets' orbits, namely their final semimajor axes, eccentricities and mutual inclinations.