Exoplanets & Exomoons

Planetary Population Synthesis and the Emergence of Four Classes of Planetary System Architectures

By Keith Cowing
Status Report
December 11, 2023
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Planetary Population Synthesis and the Emergence of Four Classes of Planetary System Architectures
One example of formation tracks for a system that belongs to Class I. The solid grey lines show the tracks of all protoplanets. The ones that remain through the end (5 Gyr) are additionally marked with colour symbols at the end of the track, according to their composition. Green (blue) full circles show rocky (icy) bodies without H/He, while green (blue) empty circles show rocky (icy) bodies with a H/He envelope, and red full circles are gas-rich bodies (more than 50 % H/He by mass). Black stars and empty circles show target and impactor of giant impacts, with dotted black lines connecting the impact partners. The analytical isolation (violet, Eq. (1)), Goldreich (orange, Eq. (12)), saturation (green, Eq. (9)), equality (cyan, Eq. (4)), and growth (pink, Eq. (2)) mass scales are shown. The latter is only shown where it is lower than isolation mass. In Class I systems, the masses in the inner system are governed by the (mean) Goldreich’s mass MGold, i.e., via the final giant impact phase. The saturation mass is evaluated at two different times. — astro-ph.EP

Planetary population synthesis is a tool to understand the physics of planetary system formation. It builds on a model that includes a multitude of physical processes. The outcome can be statistically compared with exoplanet observations.

Here, we review the population synthesis method and then use one population to explore how different planetary system architectures emerge and which conditions lead to their formation. The systems can be classified into four main architectures: Class I of near-in situ compositionally ordered terrestrial and ice planets, Class II of migrated sub-Neptunes, Class III of mixed low-mass and giant planets, broadly similar to the Solar System, and Class IV of dynamically active giants without inner low-mass planets.

These four classes exhibit distinct typical formation pathways and are characterised by certain mass scales. Class I systems form from the local accretion of planetesimals followed by a giant impact phase, and the final planet masses correspond to the ‘Goldreich mass’ . Class II systems form when planets reach the “equality mass” (equal accretion and migration timescales) before the dispersal of the gas disc, but not large enough to allow for rapid gas accretion. Giant planets form when the `equality mass’ allows for rapid gas accretion while the planet are migrating, i.e. when the critical core mass is reached.

The main discriminant of the four classes is the initial mass of solids in the disc, with contributions from the lifetime and mass of the gas disc. The breakdown into classes allows to better understand which physical processes are dominant. Comparison with observations reveals certain differences to the actual population, pointing at limitation of theoretical understanding. For example, the overrepresentation of synthetic super Earths and sub-Neptunes in Class I causes these planets to be found at lower metallicities than in observations.

Alexandre Emsenhuber, Christoph Mordasini, Remo Burn

Comments: Invited review accepted for publication in EPJ+, Focus Point on Environmental and Multiplicity Effects on Planet Formation by guest editors G. Lodato and C.F. Manara
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2303.00012 [astro-ph.EP] (or arXiv:2303.00012v1 [astro-ph.EP] for this version)
Journal reference: Eur. Phys. J. Plus (2023) 138:181
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Submission history
From: Alexandre Emsenhuber
[v1] Tue, 28 Feb 2023 19:00:01 UTC (4,407 KB)


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