[Paper Review] The New Generation Planetary Population Synthesis (NGPPS). IV. Planetary systems around low-mass stars

Remo Burn, Martin Schlecker+6 more|arXiv (Cornell University)|May 10, 2021

Stellar, planetary, and galactic studiesPhysics and Astronomy3 references10 citations

TL;DR

This study uses the Generation III Bern planetary population synthesis model to simulate planet formation around low-mass stars (0.1–1.0 M⊙), incorporating planetary migration, N-body interactions, and atmospheric evolution. It finds that temperate, Earth-sized planets are most frequent around early M dwarfs (0.3–0.5 M⊙), with peak occurrence driven by disk mass and migration dynamics, while giant planets form only above 0.5 M⊙ due to embryo ejection and core accretion pathways in ultra-late M dwarfs require suppressed type I migration.

ABSTRACT

Context. Previous theoretical works on planet formation around low-mass stars have often been limited to large planets and individual systems. As current surveys routinely detect planets down to terrestrial size in these systems, models have shifted toward a more holistic approach that reflects their diverse architectures. Aims. Here, we investigate planet formation around low-mass stars and identify differences in the statistical distribution of modeled planets. We compare the synthetic planet populations to observed exoplanets and we discuss the identified trends. Methods. We used the Generation III Bern global model of planet formation and evolution to calculate synthetic populations, while varying the central star from Solar-like stars to ultra-late M dwarfs. This model includes planetary migration, N-body interactions between embryos, accretion of planetesimals and gas, and the long-term contraction and loss of the gaseous atmospheres. Results. We find that temperate, Earth-sized planets are most frequent around early M dwarfs (0.3 M⊙–0.5 M⊙) and that they are more rare for Solar-type stars and late M dwarfs. The planetary mass distribution does not linearly scale with the disk mass. The reason behind this is attributed to the emergence of giant planets for M⋆ ≥ 0.5 M⊙, which leads to the ejection of smaller planets. Given a linear scaling of the disk mass with stellar mass, the formation of Earth-like planets is limited by the available amount of solids for ultra-late M dwarfs. For M⋆ ≥ 0.3 M⊙, however, there is sufficient mass in the majority of systems, leading to a similar amount of Exo-Earths going from M to G dwarfs. In contrast, the number of super-Earths and larger planets increases monotonically with stellar mass. We further identify a regime of disk parameters that reproduces observed M-dwarf systems such as TRAPPIST-1. However, giant planets around late M dwarfs, such as GJ 3512b, only form when type I migration is substantially reduced. Conclusions. We are able to quantify the stellar mass dependence of multi-planet systems using global simulations of planet formation and evolution. The results fare well in comparison to current observational data and predict trends that can be tested with future observations.

Motivation & Objective

• To understand the statistical distribution of planetary systems around low-mass stars, particularly the frequency and architecture of temperate, Earth-sized planets.
• To investigate how stellar mass influences planetary population outcomes, including giant planet formation and embryo ejection.
• To test whether observed systems like TRAPPIST-1 and GJ 3512b can be reproduced under realistic disk and migration conditions.
• To quantify the role of disk mass scaling, migration suppression, and initial embryo placement in shaping planetary system diversity.

Proposed method

• Employed the Generation III Bern global model of planet formation and evolution, simulating ~1000 synthetic multi-planet systems across five stellar masses (0.1–1.0 M⊙).
• Used 50 planetary embryos per system, with initial disk mass scaled linearly with stellar mass and fixed disk lifetime.
• Incorporated type I and type II planetary migration, N-body interactions between embryos, planetesimal and gas accretion, and long-term atmospheric contraction and loss.
• Maintained constant physical disk boundaries in orbital period and fixed inner disk edge (0.06 au) to isolate stellar mass effects.
• Varied initial conditions including solid mass content (30–50 M⊕) and embryo spacing in mutual Hill radii to test system formation pathways.
• Compared synthetic populations to observed exoplanet data, focusing on period ratios, eccentricities, resonant configurations, and planetary compositions.

Experimental results

Research questions

• RQ1 What is the stellar mass dependence of temperate, Earth-sized planet occurrence around low-mass stars?
• RQ2 How do planetary migration and embryo ejection shape the mass distribution and architecture of multi-planet systems?
• RQ3 What disk parameters reproduce the TRAPPIST-1 system, and what conditions are required for giant planets around ultra-late M dwarfs?
• RQ4 How does the efficiency of solid accretion scale with stellar mass, given competing processes like ejection and giant planet formation?
• RQ5 What role does initial embryo spacing and disk mass content play in forming compact, resonant systems like TRAPPIST-1?

Key findings

• Temperate, Earth-sized planets are most frequent around early M dwarfs (0.3–0.5 M⊙), with a peak occurrence rate that declines for both higher (G-type) and lower (ultra-late M) stellar masses.
• The planetary mass function does not scale linearly with stellar mass due to increased ejection of embryos by growing giant planets in systems with M⋆ ≥ 0.5 M⊙.
• Giant planets form only for M⋆ ≥ 0.5 M⊙; no giant planets form in simulations with stars below this threshold.
• TRAPPIST-1-like systems are best reproduced with an initial solid mass content of 30–50 M⊕ and an inner disk edge at or inside 0.06 au.
• A high occurrence of mean-motion resonances (up to ∼10% more pairs within 300 d in ultra-late M dwarfs) is driven by migration, though this exceeds observed resonance rates.
• Rocky compositions dominate in the innermost 0.1 au of low-mass stars due to enhanced accretion of rocky planetesimals, especially when embryos are initially placed near the water iceline.