A Chemo-dynamical Search For Planet-candidate Hosts Of Possible Extragalactic Origin

by Keith CowingAugust 19, 2026August 19, 2026

Chemical abundance distributions of the Sample II planet-candidate host stars in the [Fe/H]–[Mg/Fe] plane. The grey-scale background shows the number density distribution of the LAMOST DR9 DD-Payne stellar sample. In panel (a), blue points denote disc stars, while red and green symbols represent accreted halo and in-situ halo stars, respectively. For halo stars, error bars indicate the uncertainties in [Fe/H] and [Mg/Fe]. Two halo stars with multiple LAMOST spectra are highlighted with open squares, with the number of available spectra annotated next to each source. For these multi-epoch objects, we adopt inverse-variance-weighted mean abundances and propagate the corresponding uncertainties. Panel (a) shows the chemical distribution of disc and halo stars relative to the LAMOST DR9 background. Panel (b) presents the same background distribution with the chemical separation boundaries overplotted. Yellow points indicate the Gaussian-mixture-model (GMM)–derived valley positions in the [Mg/Fe] distributions of the LAMOST DR9 sample, computed in bins of [Fe/H]. The yellow dashed line represents a linear fit to these valley points, and the yellow shaded region corresponds to the 95 per cent bootstrap confidence interval of the fit. We adopt this GMM-valley relation as an empirical boundary separating low-[Mg/Fe] accreted-halo candidates from high-[Mg/Fe] in-situ halo stars. For comparison, the purple dotted line shows the selection boundary of Nissen & Schuster (2010), while the purple dashed line indicates its metal-poor extrapolation. The Gaussian-mixture fits used to derive the valley locations are presented in Appendix Figure A2.-- [astro-ph.SR]

[astro-ph.SR] To date, all known exoplanetary systems have been identified around stars currently residing in the Milky Way, whereas planets formed in external galaxies remain largely unexplored. Such systems would offer a unique probe of planet formation in galactic environments distinct from the Milky Way.

We combine a literature-compiled sample of Kepler, K2, and TESS planet-candidate host stars with Gaia DR3 astrometry and radial velocities, and incorporate metallicities and [Mg/Fe] abundances from the LAMOST DR9 DD-Payne catalogue, to search for candidate accreted-halo planet hosts.

We identify 11 planet-candidate hosts with halo-like kinematics, five of which have reliable chemical abundance measurements. Among these, four systems exhibit low metallicities ([Fe/H]<-0.7) and low [Mg/Fe] ratios that are inconsistent with the canonical Milky Way thick-disc sequence, indicative of enrichment histories characteristic of accreted dwarf galaxies.

We further carry out a uniform false-positive assessment using Gaia RUWE, Gaia DR3 neighbourhood checks, odd–even transit-depth comparisons, secondary-eclipse searches, independent BLS period recovery, and comparison with ExoFOP and available follow-up information. This vetting identifies EPIC~211407755 and TIC~239541449 as the most plausible, although still unvalidated, planet-candidate systems.

TIC~293432942 is more likely associated with a blended or otherwise binary-related false positive, whereas TIC~184739529 remains a high-risk giant-companion candidate whose planetary nature is uncertain.

If confirmed, EPIC~211407755 and TIC~239541449 would suggest that planetary systems can form in dwarf-galaxy environments and subsequently survive accretion into the Milky Way.

Sun Tiancheng, Chen Xunzhou, Xiang Maosheng, Zhou Jianzhao, Lu Zixuan

Comments: 11 pages, 5 figures, accepted for publication in MNRAS
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2608.13895 [astro-ph.SR] (or arXiv:2608.13895v1 [astro-ph.SR] for this version)
https://doi.org/10.48550/arXiv.2608.13895
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Submission history
From: Tiancheng Sun
[v1] Fri, 14 Aug 2026 02:53:30 UTC (2,960 KB)
https://arxiv.org/abs/2608.13895

astrobiology, exoplanet, interstellar,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp... More by Keith Cowing

A Chemo-dynamical Search For Planet-candidate Hosts Of Possible Extragalactic Origin

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