Exoplanets in ancient stellar populations: occurrence constraints and hot-Jupiter candidates in the Galactic halo

The Galactic halo preserves a record of the Milky Way’s earliest assembly and contains both in situ stars and stars accreted from dwarf galaxies. Possible planets around these stars, therefore, probe formation in ancient, metal-poor environments, including systems of extragalactic origin. We present a search for short-period transiting planets around kinematically selected halo dwarfs using Gaia DR3 and TESS, focusing on planets with periods of 1 \, d< P < 10 d. We identify two hot-Jupiter (HJ) candidates, one in the in situ and one in the accreted halo, although the latter is highly grazing and excluded from the occurrence analysis. The accreted halo planet candidate, if confirmed, would orbit the most metal-poor HJ host known ([Fe/H] ≈ -1). Using injection–recovery tests and automated vetting, we constrain occurrence in the full halo, in situ, and accreted samples. In the HJ regime (8\, R_⊕< Rₚ < 22\, R_⊕, 1\, d < P < 10 d), the non-grazing candidate implies an overall halo occurrence rate of 0.13+0.12-0.07 per cent if planetary, while the absence of confirmed detections gives a corresponding 1σ upper limit of < 0.14 per cent. For the in situ halo, we infer 0.17+0.17-0.10 per cent (or < 0.19 per cent assuming no detections), while for the accreted halo we derive an upper limit of < 0.56 per cent. These rates lie well below the corresponding short-period giant-planet occurrence measured in the Galactic disc. A forward model assuming Kepler-like occurrence also predicts 10 ± 3 detections compared with at most one observed. We find no significant occurrence difference between the in situ and accreted halo populations, strengthening the evidence that close-in giant planets are rare across the old, metal-poor halo.