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Planet Formation: Key Mechanisms and Global Models

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Demographics of Exoplanetary Systems

Part of the book series: Astrophysics and Space Science Library ((ASSL,volume 466))

Abstract

Models of planet formation are built on underlying physical processes. In order to make sense of the origin of the planets we must first understand the origin of their building blocks. This review comes in two parts. The first part presents a detailed description of six key mechanisms of planet formation:

  • The structure and evolution of protoplanetary disks

  • The formation of planetesimals

  • Accretion of protoplanets

  • Orbital migration of growing planets

  • Gas accretion and giant planet migration

  • Resonance trapping during planet migration.

While this is not a comprehensive list, it includes processes for which our understanding has changed in recent years or for which key uncertainties remain. The second part of this review shows how global models are built out of planet formation processes. We present global models to explain different populations of known planetary systems, including close-in small/low-mass planets (i.e., super-Earths), giant exoplanets, and the Solar System’s planets. We discuss the different sources of water on rocky exoplanets, and use cosmochemical measurements to constrain the origin of Earth’s water. We point out the successes and failings of different models and how they may be falsified. Finally, we lay out a path for the future trajectory of planet formation studies.

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Notes

  1. 1.

    The terms “terrestrial” and “rocky” planet are interchangeable: the Solar System community generally uses the term terrestrial and the exoplanet community uses rocky We use both terminologies in this review to represent planets with solid surfaces that are dominated (by mass) by rock and iron

  2. 2.

    The Alfvén speed is defined as the ratio between the magnetic field intensity B and \(\sqrt{\mu _0\rho }\), where \(\mu _0\) is the permeability of vacuum and \(\rho \) is the total mass density of the charged plasma particles. Apart from relativity effects, the Alfvén speed is the phase speed of the Alfvén wave, which is a magnetohydrodynamic wave in which ions oscillate in response to a restoring force provided by an effective tension on the magnetic field lines.

  3. 3.

    We define a hot Jupiter as a giant planet within 0.1 au from its star and warm/cold Jupiters as those that are beyond 0.5 au. Very few giant planets fall between 0.1 and 0.5 au so the exact values of these boundaries are not important. Debiased observations suggest that hot Jupiters are about 1/10th or less as abundant as warm/cold Jupiters (Fernandes et al. 2019; Wittenmyer et al. 2016, 2020).

  4. 4.

    This idea forms the basis of a model of super-Earth formation sometimes called inside-out planet formation (Boley and Ford 2013; Chatterjee et al. 2014, 2015; Hu et al. 2016, 2018). That model invokes the direct formation of super-Earths from pebbles at pressure bumps in the inner disk.

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Acknowledgements

We are grateful to the Agence Nationale pour la Recherche, who sponsored our research from 2014-2018 (project MOJO).

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Raymond, S.N., Morbidelli, A. (2022). Planet Formation: Key Mechanisms and Global Models. In: Biazzo, K., Bozza, V., Mancini, L., Sozzetti, A. (eds) Demographics of Exoplanetary Systems. Astrophysics and Space Science Library, vol 466. Springer, Cham. https://doi.org/10.1007/978-3-030-88124-5_1

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