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Abstract Formation models in which terrestrial bodies grow via the pairwise accretion of planetesimals have been reasonably successful at reproducing the general properties of the Solar System, including small-body populations. However, planetesimal accretion has not yet been fully explored in the context of the wide variety of recently discovered extrasolar planetary systems, in particular those that host short-period terrestrial planets. In this work, we use directN-body simulations to explore and understand the growth of planetary embryos from planetesimals in disks extending down to ≃1 day orbital periods. We show that planetesimal accretion becomes nearly 100% efficient at short orbital periods, leading to embryo masses that are much larger than the classical isolation mass. For rocky bodies, the physical size of the object begins to occupy a significant fraction of its Hill sphere toward the inner edge of the disk. In this regime, most close encounters result in collisions, rather than scattering, and the system does not develop a bimodal population of dynamically hot planetesimals and dynamically cold oligarchs, as is seen in previous studies. The highly efficient accretion seen at short orbital periods implies that systems of tightly packed inner planets should be almost completely devoid of any residual small bodies. We demonstrate the robustness of our results to assumptions about the initial disk model, and we also investigate the effects that our simplified collision model has on the emergence of this non-oligarchic growth mode in a planet-forming disk.
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Understanding Planetesimal Accretion at Short Orbital Periods
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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- Award ID(s):
- 2006752
- PAR ID:
- 10287553
- Date Published:
- Journal Name:
- Bulletin of the American Astronomical Society
- Volume:
- 53
- Issue:
- 5
- ISSN:
- 2330-9458
- Page Range / eLocation ID:
- 206.01
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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.more » « less
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