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1. Understanding Planetesimal Accretion at Short Orbital Periods

   Wallace, Spencer (June 2021, Bulletin of the American Astronomical Society)

   null (Ed.)

   Using the tree-based N-body code ChaNGa, we explore the consequences of planetesimal-planetesimal accretion in the context of systems of tightly-packed inner planets (STIPs). Formation models for STIPs often begin with the assumption that the building blocks for the planets previously underwent a phase of oligarchic growth, in which a handful of large bodies develop and leave behind a dynamically hot population of residual planetesimals. We show that the conditions required for oligarchic growth become difficult to achieve close to the star, where the dynamical and orbital timescales are short. In this case, planetary embryos can form much larger than the typical isolation mass. We explore a range of physically motivated initial conditions for the starting distribution of planetesimals and find that a number of different outcomes for the resulting embryo and residual planetesimal populations are possible. In some cases, a transition region develops partway out in the disk, the location of which serves as an artifact of the original planetesimal distribution. 

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2. An In-Situ Formation Model for Systems of Tightly-Packed Inner Planets

   Wallace, S.; Quinn, T. (April 2022, Bulletin of the American Astronomical Society)

   Using high-resolution N-body simulations, we investigate the outcome of terrestrial planet formation at short (< 100 day) orbital periods under a migration-free model. The collisional and dynamical evolution of systems of nearly 106 self-interacting planetesimals are directly followed through the final planet assembly phase. This is done by first modeling the planetesimal evolution with the tree-based N-body code ChaNGa, and then passing the results to the hybrid-symplectic N-body code genga, once the particle count has dropped sufficiently. Previously, we showed that oligarchic growth fails to operate at arbitrarily short orbital periods. This leaves a distinct feature in the mass and orbital distribution of the planetary embryos. In this most recent work, we explore whether this boundary between oligarchic and non-oligarchic growth leaves any kind of imprint on the terrestrial planets that form. If so, this would provide an important clue to evaluate whether migration played a significant role in shaping the architecture systems of tightly-packed inner planets. 

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3. Early volatile depletion on planetesimals inferred from C–S systematics of iron meteorite parent bodies

   https://doi.org/10.1073/pnas.2026779118  

   Hirschmann, Marc M.; Bergin, Edwin A.; Blake, Geoff A.; Ciesla, Fred J.; Li, Jie (March 2021, Proceedings of the National Academy of Sciences)

   null (Ed.)

   During the formation of terrestrial planets, volatile loss may occur through nebular processing, planetesimal differentiation, and planetary accretion. We investigate iron meteorites as an archive of volatile loss during planetesimal processing. The carbon contents of the parent bodies of magmatic iron meteorites are reconstructed by thermodynamic modeling. Calculated solid/molten alloy partitioning of C increases greatly with liquid S concentration, and inferred parent body C concentrations range from 0.0004 to 0.11 wt%. Parent bodies fall into two compositional clusters characterized by cores with medium and low C/S. Both of these require significant planetesimal degassing, as metamorphic devolatilization on chondrite-like precursors is insufficient to account for their C depletions. Planetesimal core formation models, ranging from closed-system extraction to degassing of a wholly molten body, show that significant open-system silicate melting and volatile loss are required to match medium and low C/S parent body core compositions. Greater depletion in C relative to S is the hallmark of silicate degassing, indicating that parent body core compositions record processes that affect composite silicate/iron planetesimals. Degassing of bare cores stripped of their silicate mantles would deplete S with negligible C loss and could not account for inferred parent body core compositions. Devolatilization during small-body differentiation is thus a key process in shaping the volatile inventory of terrestrial planets derived from planetesimals and planetary embryos. 

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4. The Origin of Universality in the Inner Edges of Planetary Systems

   https://doi.org/10.3847/2041-8213/acdb5d  

   Batygin, Konstantin; Adams, Fred C.; Becker, Juliette (July 2023, The Astrophysical Journal Letters)

   Abstract The characteristic orbital period of the innermost objects within the galactic census of planetary and satellite systems appears to be nearly universal, withPon the order of a few days. This paper presents a theoretical framework that provides a simple explanation for this phenomenon. By considering the interplay between disk accretion, magnetic field generation by convective dynamos, and Kelvin–Helmholtz contraction, we derive an expression for the magnetospheric truncation radius in astrophysical disks and find that the corresponding orbital frequency is independent of the mass of the host body. Our analysis demonstrates that this characteristic frequency corresponds to a period ofP∼ 3 days although intrinsic variations in system parameters are expected to introduce a factor of a ∼2–3 spread in this result. Standard theory of orbital migration further suggests that planets should stabilize at an orbital period that exceeds disk truncation by a small margin. Cumulatively, our findings predict that the periods of close-in bodies should spanP∼ 2–12 days—a range that is consistent with observations. 

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5. Formation of rocky super-earths from a narrow ring of planetesimals

   https://doi.org/10.1038/s41550-022-01850-5  

   Batygin, Konstantin; Morbidelli, Alessandro (March 2023, Nature Astronomy)

   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. 

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