Insights into Planet Formation from the Ages, Masses, and Elemental Abundances of Host Stars

TL;DR

This work leverages the LAMOST–Kepler–Gaia census with $oldsymbol{ extalpha}$-enhanced stellar evolution models to derive precise ages and elemental abundances for nearly 19{,}000 dwarfs and subgiants, including 392 planetary systems. Using a Bayesian framework, the authors link host star properties to planetary characteristics, finding that about 86% of young planet hosts reside on the metal-rich branch of the age–[Fe/H] relation and that a substantial fraction likely originated in the inner Galactic disk before migrating outward. They show that, after controlling for age and mass, planet hosts tend to be more metal-rich than non-hosts in the high-[Fe/H] regime, while metal-poor hosts show a deficit relative to non-hosts, implying that [Fe/H] is important but not exclusively determinant in metal-poor environments. The analysis also reveals systematic depletion of volatile elements, especially C, in planet hosts, and finds only weak or nuanced correlations between host metallicity or [alpha/Fe] and planet radius, with [alpha/Fe] correlating more with small-planet formation. Overall, the study highlights metal-rich environments and Galactic birth conditions as key factors in planet formation, while volatile chemistry imprints and migration histories add further context to planetary system architectures.

Abstract

How planetary systems form and evolve is a key question in astronomy. Revealing how host star properties, such as elemental abundances, age, and mass, differ from those of non-host stars, and how they correlate with planetary characteristics such as radius, provides new insights into the formation and evolutionary pathways of planetary systems. We determine precise ages for 18890 dwarfs and subgiants from the LAMOST-Kepler-Gaia sample with a mean age uncertainty of about 15 percent (median about 10 percent). Within the framework of Galactic chemical evolution, we find that about 86 percent of planet-hosting stars younger than 8 Gyr occupy the upper branch ([Fe/H] > -0.2) of the characteristic V-shaped age-metallicity relation of the Galactic disk. Based on guiding radii (Rg), we further infer that about 19 percent of these young hosts likely originated in the inner disk and subsequently migrated to the solar neighborhood. Among stars older than 10 Gyr, host stars tend to be more metal-rich, with nearly 59 percent having [Fe/H] > -0.2. This suggests that both young and old planet-hosting stars preferentially form in relatively metal-rich environments. However, for host stars with [Fe/H] < -0.2, we find that their metallicities are on average lower by about 0.16 dex compared to non-host stars of similar age and mass, indicating that [Fe/H] is unlikely to be the dominant factor governing planet formation in metal-poor environments. We also identify a systematic depletion of volatile elements, especially carbon, in planet hosts. Moreover, host star [Fe/H] exhibits a weak correlation with planet radius, while [alpha/Fe] primarily support the formation of small planets.

Figures (9)

• Figure 1: Kiel diagram of all LAMOST-Kepler-Gaia sample stars. Dwarfs, subgiants, and giants are shown in blue, orange, and red, respectively, based on classification with the evolstate algorithm. The black open circles indicate the location of our final sample of 18890 stars with well-constrained ages.
• Figure 2: Age precision and distribution of 18990 sample stars. (a) Two-dimensional histogram of stellar age versus age uncertainty, with dashed lines indicating fractional age uncertainties of 10%, 20%, 30%, 40%, and 50%. The colorbar indicates the number of stars per bin in logarithmic scale. (b) Age histogram (gray bars) overlaid with a KDE-smoothed curve (red). Vertical dashed lines and arrows highlight the detected peaks in the age distribution.
• Figure 3: (a) Normalized age distributions $p(\tau\,|\,[\mathrm{Fe/H}])$ for single stars, binned in [Fe/H] and smoothed using Gaussian filtering. Each vertical slice is normalized by its peak value. The blue line marks the peak age in each bin. Colored points show planet-hosting stars, classified by guiding-center radius $R_g$: inner ($R_g < 7.2$ kpc), local ($7.2 \leq R_g \leq 9.2$ kpc), and outer ($R_g > 9.2$ kpc). (b) Same as (a), but for [$\alpha$/Fe] instead of [Fe/H].
• Figure 4: Comparison of [Fe/H] and [$\alpha$/Fe] differences between stars hosting planets and non-host stars with similar mass and age. (a) Scatter plot of differences in stellar age ($\Delta$Age) versus stellar mass ($\Delta$Mass) between host stars and their nearest non-host stars. (b) Distribution of host stars in the [Fe/H] versus [$\alpha$/Fe] plane, colored by the difference in [Fe/H] between hosts and matched non-hosts.
• Figure 5: Elemental differences between planet-hosting and single stars across stellar age and [Fe/H] bins. Each point represents the $p$-value from a two-sample Kolmogorov--Smirnov (KS) test comparing the abundance distributions of stars with and without planets. Points with $p < 0.05$ indicate statistically significant differences.