![Left panel: Toomre diagram for the stars in our sample, color-coded by the TD/D value. Right panel: cumulative distribution function (CDF) of the TD/D values for stars with planetary systems classified into different categories. A system with at least one planet pair that is identified as plausible 1st-order (2nd-order) resonance by HS24 is labeled “w/ 1st MMR” (“w/ 2nd MMR”). The category, “w/ 1st MMR & Pin < 3 d”, corresponds approximately to the stars with plausibly 1st-order resonant and tidal decay timescale τe < 20 Myr in HS24. “USP” is the sample of planetary systems hosting ultra-short-period planets as identified in Schmidt et al. (2024).-- [astro-ph.EP]](https://i0.wp.com/astrobiology.com/wp-content/uploads/2026/08/Are-Near.png?fit=1200%2C678&ssl=1)
[astro-ph.EP] Recent studies have claimed that Kepler multi-planet systems hosting near-resonant planet pairs—particularly those near second-order mean-motion resonances (MMRs)—exhibit smaller stellar velocity dispersions than the general population of Kepler planet hosts. Interpreting velocity dispersion as an age indicator, these works concluded that near-resonant systems are systematically younger.
We revisit this claim, but we explicitly account for contamination by thick disk stars, which are kinematically hotter and follow a different age-velocity dispersion relation (AVR) than thin disk stars.
Using the kinematic criterion to separate thin and thick disk stars, we show that systems classified as having plausible second-order resonant pairs are preferentially hosted by brighter, closer stars and are therefore less contaminated by thick disk stars than the overall sample.
After applying a cut to remove probable thick disk contaminants (TD/D<0.1), the vertical velocity dispersion of near-resonant systems becomes statistically indistinguishable from that of the overall Kepler multi-planet sample.
We conclude that the apparent kinematic youth of near-resonant systems in Kepler may not be due to a genuine age difference, but rather arises from observational selection effects linked to host star properties and planet detectability. We also comment on the kinematic ages of ultra-short-period planets (USPs).
Wei Zhu, Qingru Hu
Comments: 7 pages, 5 figures, accepted. For a related issue, see our companion paper (Q. Hu et al.) appearing on the same day on arXiv
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2608.12786 [astro-ph.EP] (or arXiv:2608.12786v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2608.12786
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Related DOI:
https://doi.org/10.1088/1674-4527/ae9846
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Submission history
From: Qingru Hu
[v1] Thu, 13 Aug 2026 03:47:37 UTC (125 KB)
https://arxiv.org/abs/2608.12786
Astrobiology, Astrogeology, Stellar cartography, exoplanet,
