[Paper Review] Planet Formation Theory in the Era of ALMA and Kepler: from Pebbles to Exoplanets

Joanna Drążkowska, Bertram Bitsch+8 more|arXiv (Cornell University)|Mar 18, 2022

Stellar, planetary, and galactic studiesPhysics and Astronomy56 citations

TL;DR

This chapter reviews how ALMA and Kepler observations constrain dust evolution, pebble and planetesimal accretion, and the population synthesis of diverse planetary systems, linking disks to exoplanets.

ABSTRACT

Our understanding of planet formation has been rapidly evolving in recent years. The classical planet formation theory, developed when the only known planetary system was our own Solar System, has been revised to account for the observed diversity of the exoplanetary systems. At the same time, the increasing observational capabilities of the young stars and their surrounding disks bring new constraints on the planet formation process. In this chapter, we summarize the new information derived from the exoplanets population and the circumstellar disks observations. We present the new developments in planet formation theory, from dust evolution to the growth of planetary cores by accretion of planetesimals, pebbles, and gas. We review the state-of-the-art models for the formation of diverse planetary systems, including the population synthesis approach which is necessary to compare theoretical model outcomes to the exoplanet population. We emphasize that the planet formation process may not be spatially uniform in the disk and there are preferential locations for the formation of planetesimals and planets. Outside of these locations, a significant fraction of solids is not growing past the pebble-sizes. The reservoir of pebbles plays an important role in the growth of planetary cores in the pebble accretion process. The timescale of the emergence of massive planetary cores is an important aspect of the present models and it is likely that the cores within one disk form at different times. In addition, there is growing evidence that the first planetary cores start forming early, during the circumstellar disk buildup process.

Motivation & Objective

• Summarize how exoplanet demographics constrain planet formation theory.
• Consolidate observational constraints from circumstellar disks on early planet formation stages.
• Outline developments in core, pebble, and gas accretion models within a unified framework.

Proposed method

• Discuss constraints from exoplanet statistics (super-Earths, mini-Neptunes, giants) and disk observations (dust/gas masses, sizes, substructures).
• Describe the pebble and planetesimal accretion framework and radial redistribution of solids.
• Explain population synthesis as a tool to compare theory with exoplanet populations.

Experimental results

Research questions

• RQ1 What do exoplanet demographics imply for the efficiency and timescales of core and gas accretion?
• RQ2 How do disk substructures and dust evolution inform where and when planetesimals and planets form?
• RQ3 How can pebble accretion and planetesimal accretion be reconciled within a population synthesis framework to reproduce observed planetary architectures?

Key findings

• Exoplanet demographics show a dominant population of super-Earths and mini-Neptunes with occurrence near 140–200% for sun-like stars within 1 au (after bias corrections).
• Giant planets are relatively rare inside 1 au and most common at 1–10 au, with long-period giants detected up to 25 au and a turnover near 2–3 au in some surveys.
• Kepler multi-planet systems are typically regularly spaced with low mutual inclinations and are often not in resonant chains.
• Disk observations reveal substructures (gaps, rings, spirals) suggesting planet formation is underway early, even during or before disk assembly, with mass budgets showing substantial but uncertain solid/dust masses.
• Dust growth and radial drift are evident, with disks often smaller in dust than gas, and substructures potentially mitigating drift.
• Turbulence in disks is generally low to moderate (alpha roughly 10^-4 to 3×10^-3) and may vary with height and radius, affecting planet formation efficiency.