[Paper Review] Not the Birth Cluster: the Stellar Clustering that Shapes Planetary Systems is Generated by Galactic-Dynamical Perturbations

J. M. Diederik Kruijssen, Steven N. Longmore+5 more|arXiv (Cornell University)|Sep 13, 2021

Stellar, planetary, and galactic studies8 references4 citations

TL;DR

This paper demonstrates that the stellar phase space overdensities influencing planetary system architectures are not remnants of birth clusters, but long-lived galactic structures—such as kpc-scale streams and ripples—generated by bar, spiral arm, and satellite-induced galactic dynamical perturbations. The key result is that planetary systems in these overdensities, particularly those associated with the phase space spiral, show a 10× higher hot Jupiter-to-cold Jupiter ratio than field systems, indicating late-time galactic-scale perturbations shape planetary systems over Gyr timescales.

ABSTRACT

Recent work has demonstrated that exoplanetary system properties correlate strongly with ambient stellar clustering in six-dimensional stellar position-velocity phase space, quantified by dividing planetary systems into sub-samples with high or low phase space densities (`overdensity' and `field' systems, respectively). We investigate the physical origins of the phase space overdensities and, thereby, which environmental mechanisms may have impacted the planetary systems. We consider the galactic-scale kinematic structure of the Milky Way observed with Gaia and show that the overdensities correspond to the well-known, kpc-scale kinematic ripples and streams in the Galactic disk, which are thought to be generated by bar and spiral arm-driven resonances and satellite galaxy passages. We also find indications that the planet demographics may vary between individual phase space overdensities, which potentially have differing physical origins and histories. Planetary systems associated with the `phase space spiral' (a recent perturbation of the Galactic disk) have a hot Jupiter-to-cold Jupiter ratio that is 10 times higher than in field systems. Finally, the hot Jupiter-to-cold Jupiter ratio within overdensities may increase with host stellar age over Gyr timescales. Because the overdensities persist for several Gyr, we argue that late-time perturbations of planetary systems most likely explain these trends, although additional perturbations at birth may contribute too. This suggests that planetary system properties are not just affected by stellar clustering in their immediate surroundings, but by galaxy-scale processes throughout their evolution. We conclude by discussing the main open questions towards understanding the diversity of physical processes that together set planetary system architectures.

Motivation & Objective

• To determine the physical origin of stellar phase space overdensities that correlate with altered planetary system properties.
• To test whether these overdensities are relics of bound birth clusters or long-lived galactic structures.
• To assess whether galactic-scale dynamical perturbations, rather than early cluster interactions, are responsible for observed planetary system trends.
• To investigate whether planetary system properties, such as hot Jupiter frequency, evolve with host star age within overdensities.
• To evaluate the role of late-time environmental effects in shaping planetary system architectures beyond initial formation phases.

Proposed method

• Utilized Gaia astrometry to measure six-dimensional phase space densities (position and velocity) of exoplanet host stars.
• Mapped phase space overdensities to known kpc-scale kinematic structures in the Galactic disk, such as streams and ripples.
• Correlated planetary system properties—like hot Jupiter frequency and multiplicity—across different overdensities and field regions.
• Analyzed stellar ages from the literature to assess time-dependent trends in planetary demographics within overdensities.
• Compared observed overdensity kinematics and ages to theoretical models of bar, spiral arm, and satellite-induced perturbations.
• Used statistical analysis to test for correlations between planet demographics and overdensity-specific features, including age and morphology.

Experimental results

Research questions

• RQ1 Are the stellar phase space overdensities that correlate with planetary system properties remnants of bound birth clusters?
• RQ2 What galactic-scale dynamical processes generate the observed phase space overdensities in the Milky Way?
• RQ3 Do planetary system properties, such as hot Jupiter frequency, vary between different types of overdensities with distinct origins?
• RQ4 Does the hot Jupiter-to-cold Jupiter ratio in overdensities evolve with host star age, indicating ongoing environmental influence?
• RQ5 Can phase space overdensities be age-dated using planetary system demographics, and what does this imply for the timing of perturbations?

Key findings

• The phase space overdensities correlated with altered planetary system properties are not relics of bound birth clusters, but long-lived galactic structures formed by bar, spiral arm, and satellite-induced resonances.
• Overdensities correspond to kpc-scale kinematic streams and ripples in the Galactic disk, which persist for several billion years.
• Planetary systems in the 'phase space spiral' overdensity exhibit a hot Jupiter-to-cold Jupiter ratio 10 times higher than in field systems.
• The hot Jupiter-to-cold Jupiter ratio within overdensities increases with host stellar age over Gyr timescales, indicating ongoing environmental perturbations.
• The observed correlations are unlikely to be due to dynamical interactions in bound birth clusters, given the rarity of such clusters in the solar neighborhood.
• Late-time galactic-dynamical perturbations—such as stellar flybys and tidal forces—likely drive the observed planetary system trends, suggesting that planetary population synthesis models must include long-term environmental effects.