Where in the Milky Way Do Exoplanets Preferentially Form? (2501.11660v1)

Abstract: Exoplanets are detected around stars of different ages and birthplaces within the Galaxy. The aim of this work is to infer the Galactic birth radii ($r_\text{birth}$) of stars and, consequently, their planets, with the ultimate goal of studying the Galactic aspects of exoplanet formation. We used photometric, spectroscopic, and astrometric data to estimate the stellar ages of two samples of stars hosting planets and, for comparison, a sample of stars without detected planets. The $r_\text{birth}$ of exoplanets were inferred by projecting stars back to their birth positions based on their estimated age and metallicity [Fe/H]. We find that stars hosting planets have higher [Fe/H], are younger, and have smaller $r_\text{birth}$ compared to stars without detected planets. In particular, stars hosting high-mass planets show higher [Fe/H], are younger, and have smaller $r_\text{birth}$ than stars hosting low-mass planets. We show that the formation efficiency of planets, calculated as the relative frequency of planetary systems, decreases with the galactocentric distance, which relationship is stronger for high-mass planets than for low-mass planets. Additionally, we find that (i) the formation efficiency of high-mass planets increases with time and encompasses a larger galactocentric distance over time; (ii) the formation efficiency of low-mass planets shows a slight increase between the ages of 4 and 8 Gyr and also encompasses a larger galactocentric distance over time; and (iii) stars without detected planets appear to form at larger galactocentric distances over time. We conclude that the formation of exoplanets throughout the Galaxy follows the Galactic chemical evolution, for which our results are in agreement with the observed negative interstellar medium (ISM) metallicity gradient and its enrichment and flattening with time at any radius.

• The paper finds that stars hosting planets tend to originate from smaller galactic birth radii and possess higher metallicities and younger ages than non-hosting stars.
• High-mass planets are particularly correlated with higher stellar metallicity, and their formation efficiency increases over time, extending to larger galactic distances as the galaxy evolves.
• These findings support the core accretion model, highlighting metallicity's role and suggesting future exoplanet surveys should target metal-rich, younger stellar populations in specific galactic regions.

Overview of Exoplanetary Formation in the Galactic Context

The paper "Where in the Milky Way do exoplanets preferentially form?" by Teixeira et al. investigates the galactic aspects of exoplanet formation by determining the Galactic birth radii of stars and their planets. By leveraging photometric, spectroscopic, and astrometric data, the paper aims to shed light on how the location and properties of star formation within the Milky Way influence the formation of planetary systems.

The authors utilize the SWEET-Cat and HARPS-GTO databases to compile samples of stars with and without detected planets, focusing specifically on stars in the solar neighborhood. They combine this with stellar parameters—effective temperature, surface gravity, metallicity ([Fe/H]), and distance estimates derived from parallax measurements. Importantly, the methodology hinges on the projection of stars back to their hypothesized origin points in the Galaxy based on their age and metallicity characteristics.

Key Findings

1. Metallicity and Formation Efficiency: The paper confirms that stars hosting planets generally possess higher metallicities compared to single stars, which aligns with the core accretion model predictions. High-mass planetary host stars, in particular, are found to have distinctly higher metallicity values compared to those hosting low-mass planets.
2. Age Distributions: Planet-hosting stars are observed to be younger on average than those without detected planets. The paper delineates a statistical distinction in age, especially within the SWEET-Cat sample, indicating that high-mass planetary systems tend to be younger.
3. Galactic Birth Radii: Stars hosting planets are characterized by smaller Galactic birth radii than their non-hosting counterparts. Implicit in this finding is the concentration of planetary formation efficiency toward the inner regions of the Galaxy, where higher metallicity environments prevail.
4. Planetary Mass Influence: High-mass planets are not only correlated with stars of higher metallicity but also display a stronger dependency on metallicity for their formation efficiency.
5. Temporal Evolution: The formation efficiency of high-mass planetary systems increases over time, progressively extending to larger galactocentric distances, which reflects the enrichment and flattening of the interstellar medium's metallicity gradient over time.

Theoretical and Practical Implications

The statistical analysis undertaken suggests a dynamic and ongoing process of planet formation that is meaningfully tied to the Milky Way's chemical evolution. This relationship underscores the importance of metallicity as a critical factor in planet formation, particularly for massive exoplanets.

From a theoretical perspective, the correlation between galactic location, stellar age, and metallicity provides further validation to the core accretion model's assumptions concerning the dependency of planet formation on elemental abundances. The evidence for planetary formation efficiency varying spatially and temporally along the Galactic disc is aligned with broader models of galactic chemical evolution, which predict these variations.

Practically, these insights inform the targeting strategies of future exoplanet surveys. Regions of the Galaxy that are metal-rich and composed of younger stellar populations may yield higher detection rates for both terrestrial and gaseous giants, specifically at intermediate galactocentric distances over time.

Future Directions

The implications of this research await further extension with forthcoming data, such as that anticipated from the PLATO mission, which promises to enhance both the size and precision of stellar age measurements. Such advancements could refine the planetary formation models by providing more detailed star formation histories and metallicity gradients across different regions of the Galaxy.

By continuously integrating new observations, the framework developed by Teixeira et al. could be expanded to encompass broader empirical datasets, offering greater clarity on the interplay between galactic dynamics and planetary system formation.