Metallicity regulates planet formation across all masses

TL;DR

The paper addresses how the total heavy-element content of planet-building disks, quantified by the heavy-element mass fraction $Z$, governs planet occurrence, multiplicity, and mass across the full planetary spectrum. It estimates $Z$ from host-star abundances using a stoichiometric model with $Z_igodot=1.25\%$ and analyzes RV-detected planets alongside non-host control samples to identify metallicity-planet formation trends. The findings show that higher $Z$ environments preferentially form planets across all masses, with stronger signals for more massive planets and higher planetary multiplicity, consistent with core accretion and challenging a dominant gravitational-instability interpretation for the bulk of planets. The study also demonstrates the robustness of $Z$ as a disk-metallicity proxy across data sets and uncertainty thresholds, and highlights the nuanced difference between $Z$ and [Fe/H] as tracers. Overall, metallicity—via $Z$—shapes the entire planetary mass spectrum and system architectures, supporting a predominantly core-accretion–driven formation pathway with limited GI influence.

Abstract

The role of stellar metallicity in shaping planetary systems is central to our understanding of planet formation. While the core accretion paradigm is widely accepted as the dominant mechanism for forming low- and intermediate-mass planets, the origin of the most massive planets remains debated, with gravitational instability often invoked to explain their existence. In this study, we analyze the dependence of planet formation on metallicity using the total heavy-element mass fraction (Z), which is a proxy for the composition of the protoplanetary disk inferred from stellar photospheres. We show that even the most massive planets form preferentially in metal-rich environments. Z correlates not only with the presence of planets, but also with planetary system multiplicity and total planetary mass. The most massive planets are found in the most metal-rich environments, and, in agreement with core-accretion theory, only the upper end of the planetary mass distribution shows a clear positive correlation with metallicity. These findings suggest that the chemical enrichment of protoplanetary disks plays a central role in shaping the full spectrum of planetary masses.

Figures (5)

• Figure 1: Distribution of stars in the Kiel diagram. Grey points represent the non-host sample from Hypatia, blue points indicate the HARPS non-host stars, and red points correspond to the planet-hosting stars from our sample.
• Figure 2: Difference between primordial and present-day $Z$ as a function of stellar age (crosses) for the sample of 30 FGK stars from Adibekyan-24. The red error bars denote the uncertainties in the $Z$ differences.
• Figure 3: $Z$ distributions of planet-host and non-host stars. Left: Distributions based on individual planet detections, with hosts grouped by the mass of any detected planet in the system. Right: Same as left, but hosts are grouped according to the mass of the most massive planet in each system. The vertical dashed lines indicate the mean $Z$ of each distribution. The non-host sample is shown in gray for comparison.
• Figure 4: Relationship between $Z$ and planetary mass across system architectures. Left: sub-Neptune- and Neptune-mass planets. Center: Jupiter-mass planets. Right: Super-Jupiter-mass planets. Black points correspond to multiplanetary systems, while orange points indicate single-planet systems. Smoothed marginal distributions via Kernel density estimates (KDEs) of $Z$ and $M_{\mathrm{p}}$ are shown for both populations.
• Figure 5: Summed mass fraction of heavy elements as a function of planetary mass. Left:$Z$ versus the mass of individual planets in the system. Center:$Z$ versus the mass of the most massive planet in each system. Right:$Z$ versus the total planetary mass in the system. The solid black line shows the best-fit OLS regression to the full sample. Green dashed lines show separate OLS fits to the Jupiter-mass and super-Jupiter-mass regimes. The horizontal dotted blue line indicates the mean $Z$ of stars without detected planets, with the shaded region showing the $\pm1\sigma$ interval.