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The formation of super-Earths, the most abundant planets in the Galaxy, remains elusive. These planets have masses that typically exceed that of the Earth by a factor of a few, appear to be predominantly rocky, although often surrounded by H/He atmospheres, and frequently occur in multiples. Moreover, planets that encircle the same star tend to have similar masses and radii, whereas those belonging to different systems exhibit remarkable overall diversity. Here we advance a theoretical picture for rocky planet formation that satisfies the aforementioned constraints: building upon recent work, which has demonstrated that planetesimals can form rapidly at discrete locations in the disk, we propose that super-Earths originate inside rings of silicate-rich planetesimals at approximately ~1 au. Within the context of this picture, we show that planets grow primarily through pairwise collisions among rocky planetesimals until they achieve terminal masses that are regulated by isolation and orbital migration. We quantify our model with numerical simulations and demonstrate that our synthetic planetary systems bear a close resemblance to compact, multi-resonant progenitors of the observed population of short-period extrasolar planets.A planetary origin model that forms exoplanets from a narrow ring of silicate material at a stellocentric distance of 1 au is able to explain the physical properties of super-Earths and reproduce the ‘peas in a pod’ pattern of uniformity within planetary architecture.

The observed census of resonant extrasolar planets spans a tantalizing display of orbital architectures, ranging from familiar 2:1 and 3:2 mean-motion commensurabilities to nearly coorbital configurations characterized by period ratios close to unity. While mean-motion resonances are widely recognized as signposts of convergent disk-driven migration, the process through which the most compact systems are established remains puzzling, since resonance capture must repeatedly fail at a series of first-order commensurabilities before finally succeeding at a high resonant index. Motivated by this discrepancy, here, we develop an analytic theory that fuses the stability-based resonance capture criterion with the conventional paradigm of active accretion disks and the standard model of type-I migration. Within this framework, we derive an expression for the stellocentric radius of resonance capture, rc, and show that it depends only on the product of the disk viscosity parameter, α, and the opacity-contributing small-grain mass fraction, fμ. Applying this formalism to Kepler-36—the most compact known resonant system with a 7:6 period ratio—we find that resonance locking could not have been established near the disk’s inner edge. Instead, capture must have occurred at rc ≈ 1−4 au, implying orbital decay of the planetary pair by approximately an order of magnitude. Viewed in this light, compact resonant architectures provide the clearest evidence for long-range migration among sub-Jovian planets. Moreover, the emerging picture is fully consistent with formation models in which super-Earths accrete within localized rings of planetesimals at orbital distances comparable to those that gave rise to the terrestrial planets of the solar system.

For the origin of the radially concentrated solar system’s terrestrial planets, planet formation from a ring of solids at about 1 au from the Sun with convergent/suppressed type I migration is preferred. On the other hand, many super-Earths and sub-Neptunes are found in the close-in region with orbital periods of 10–100 days, so that planet formation from rings in the 1 au region would require some degree of inward migration. One way to realize these different formation scenarios is to use different gas disk models. In this study, we investigate whether different scenarios can be realized within a single framework. We consider a disk model that evolves via disk winds and develops a density peak, and study planet formation and orbital evolution using N-body simulations. Planets with masses less than an Earth mass formed from a low-mass ring resembling the solar system do not migrate inward even in the evolving disk and remain near 1 au orbits, maintaining a high radial mass concentration. On the other hand, planets with masses greater than an Earth mass formed from a massive ring slowly migrate inward above the outward migration region. As a result, the innermost planet can move to an orbit of about 10 days. The simulation results also reproduce the characteristics (e.g., mass distribution, eccentricity, orbital separation) of the solar system and super-Earth/sub-Neptune systems. Our model predicts that Earths and sub-Earths formed by migration from rings near the 1 au region are less abundant in the close-in region.

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.

Gravitational interactions after the Moon-forming event suggest that the current lunar inclination is the result of collisionless encounters of planetesimals with the early Moon–Earth system. The Moon's inclination explained Modelling of the impact process that produced the Moon predicts that the lunar material disaggregated to form a circumplanetary disk, and that lunar accretion subsequently placed the Moon in a near-equatorial orbit. However, the models predict a modern inclination at least an order of magnitude smaller than the present-day value. Kaveh Pahlevan and Alessandro Morbidelli now show that the modern lunar inclination can be reproduced naturally through collisionless encounters of the early Moon with a small quantity of mass, carried by a few bodies, consistent with constraints and models of late accretion. They also find that the modern lunar orbit provides a sensitive record of gravitational interactions with Earth-crossing planetesimals. The Moon is generally thought to have formed from the debris ejected by the impact of a planet-sized object with the proto-Earth towards the end of planetary accretion 1 , 2 . Models of the impact process predict that the lunar material was disaggregated into a circumplanetary disk and that lunar accretion subsequently placed the Moon in a near-equatorial orbit 3 , 4 , 5 , 6 . Forward integration of the lunar orbit from this initial state predicts a modern inclination at least an order of magnitude smaller than the lunar value—a long-standing discrepancy known as the lunar inclination problem 7 , 8 , 9 . Here we show that the modern lunar orbit provides a sensitive record of gravitational interactions with Earth-crossing planetesimals that were not yet accreted at the time of the Moon-forming event. The currently observed lunar orbit can naturally be reproduced via interaction with a small quantity of mass (corresponding to 0.0075–0.015 Earth masses eventually accreted to the Earth) carried by a few bodies, consistent with the constraints and models of late accretion 10 , 11 . Although the encounter process has a stochastic element, the observed value of the lunar inclination is among the most likely outcomes for a wide range of parameters. The excitation of the lunar orbit is most readily reproduced via collisionless encounters of planetesimals with the Earth–Moon system with strong dissipation of tidal energy on the early Earth. This mechanism obviates the need for previously proposed (but idealized) excitation mechanisms 12 , 13 , places the Moon-forming event in the context of the formation of Earth, and constrains the pristineness of the dynamical state of the Earth–Moon system.

Mercury is notoriously difficult to form in Solar System simulations, due to its small mass and iron-rich composition. Smooth particle hydrodynamics simulations of collisions have found that a Mercury-like body could be formed by one or multiple giant impacts, but due to the chaotic nature of collisions, it is difficult to create a scenario where such impacts will take place. Recent work has found more success forming Mercury analogues by adding additional embryos near Mercury’s orbit. In this work, we aim to form Mercury by simulating the formation of the Solar System in the presence of the giant planets Jupiter and Saturn. We test out the effect of an inner disk of embryos added on to the commonly used narrow annulus of initial material. We form Mercury analogues with core-mass fractions (CMFs) > 0.4 in ∼10% of our simulations, and twice that number of Mercury analogues form during the formation process but are unstable and do not last to the end of the simulations. Mercury analogues form at similar rates for both disks with and without an inner component, and most of our Mercury analogues have lower CMFs than that of Mercury, ∼0.7, due to significant accretion of debris material. We suggest that a more in-depth understanding of the fraction of debris mass that is lost to collisional grinding is necessary to understand Mercury’s formation, or some additional mechanism is required to stop this debris from accreting.

One of the key goals of the Rosetta mission was to understand how, where and when comets formed in our solar system. There are two major hypotheses for the origin of comets, both pre-Rosetta: (1) hierarchical accretion of dust and ice grains in the Solar Nebula and (2) the growth of pebbles, which are then brought together by streaming instabilities in the Solar Nebula to form larger bodies. Rosetta provided a wealth of new information on comet nuclei and confirmed many past ideas on comets, e.g., high volatile content, lack of aqueous alteration of grains, and the low bulk density of the nucleus. Rosetta also provided new data on the nature of cometary activity, the active geology on the nucleus surface and the interior structure and bulk density of the nucleus. Supporters of the above-mentioned origin hypotheses each find confirmation of their ideas in the Rosetta results. But the question of which hypothesis is preferred, or if there are other, better hypotheses that could be invoked, could not be answered. Theoretical studies suggest that comet nuclei were collisionally processed in the Primordial Disk though it is not clear that the nuclei we see today display the effects of that process. Both theoretical and observational studies suggest that the major end-states for cometary nuclei are dynamical ejection, random disruption and disintegration, and/or evolution of nuclei to inactive, asteroidal-appearing objects. Rosetta has provided us with many new insights that will help to guide future cometary missions, observations, experiments and theoretical investigations that will lead to answers to the fundamental questions with regard to cometary origin.

Mean-motion resonances are expected to frequently arise at the inner edges of protoplanetary disks, where planet–disk interactions facilitate large-scale orbital convergence. Under certain conditions, however, the same dissipative forces that promote resonant capture can drive resonant librations overstable, ultimately breaking commensurabilities. Here, we examine the onset of overstability near disk torque reversals and show that it can be subdued when the transition is sufficiently sharp. Adopting the dissipative circular restricted three-body problem as a paradigm, we present a WKB-style analysis that reduces the resonant dynamics to a damped, driven harmonic oscillator. Within this framework, we obtain an effective frictional term that is proportional to the local migration-rate gradient, parameterized by a dimensionless coefficient β that encodes the steepness of the local torque reversal. Our analytical theory predicts that overstability is quenched once β ≳ τa/τe, where τa and τe denote the characteristic disk-driven evolution timescales of semimajor axis and eccentricity. We verify and refine our analytic results with direct N-body integrations. Simple estimates based on conventional type-I scalings suggest that the competition between overstability and its mitigation at disk inner edges is a borderline outcome that is sensitive to the detailed structure of planet–disk interactions.

Earth and the Moon are shown here to have indistinguishable oxygen isotope ratios, with a difference in Δ'¹⁷ O 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 Δ'¹⁷ O within approximately 1 per mil of the terrestrial value, limiting possible sources for this late addition of mass to the Earth-Moon system.

By comparing asteroid detections and a near-Earth-object model the deficit of objects near the Sun is shown to arise from the breakup of most asteroids, especially low-albedo ones, at distances of a few tens of solar radii from the Sun. 'Lost asteroids' explained by a brush with the Sun Most models of near-Earth asteroid distribution predict asteroids on orbits that closely approach the Sun. However, the observed population of asteroids near the Sun is much smaller than predicted and shows an unexpected bias towards high albedos. Mikael Granvik et al . report a quantitative comparison of asteroid detections and a near-Earth-object model of interactions. They conclude that the deficit of low-albedo objects arises from the super-catastrophic disruption of a substantial fraction of asteroids when they achieve perihelion distances of a few tens of solar radii. Although both bright and dark asteroids are eventually disrupted, there is a preference for the elimination of low-albedo asteroids farther from the Sun, which explains the apparent excess of high-albedo near-Earth objects and suggests that low-albedo asteroids break more easily as a result of thermal effects. Most near-Earth objects came from the asteroid belt and drifted via non-gravitational thermal forces into resonant escape routes that, in turn, pushed them onto planet-crossing orbits 1 , 2 , 3 . Models predict that numerous asteroids should be found on orbits that closely approach the Sun, but few have been seen. In addition, even though the near-Earth-object population in general is an even mix of low-albedo (less than ten per cent of incident radiation is reflected) and high-albedo (more than ten per cent of incident radiation is reflected) asteroids, the characterized asteroids near the Sun typically have high albedos 4 . Here we report a quantitative comparison of actual asteroid detections and a near-Earth-object model (which accounts for observational selection effects). We conclude that the deficit of low-albedo objects near the Sun arises from the super-catastrophic breakup (that is, almost complete disintegration) of a substantial fraction of asteroids when they achieve perihelion distances of a few tens of solar radii. The distance at which destruction occurs is greater for smaller asteroids, and their temperatures during perihelion passages are too low for evaporation to explain their disappearance. Although both bright and dark (high- and low-albedo) asteroids eventually break up, we find that low-albedo asteroids are more likely to be destroyed farther from the Sun, which explains the apparent excess of high-albedo near-Earth objects and suggests that low-albedo asteroids break up more easily as a result of thermal effects.