Stellar chemistry and planet size: Insights from GALAH DR4

¹ School of Physics and Astronomy, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv 6997801, Israel
² Department of Geophysics, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv 6997801, Israel
³ Institut für Astrophysik, Universität Zürich, Winterthurerstr. 190, 8057 Zurich, Switzerland
⁴ Astrophysics Group, Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, UK

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Received: 22 July 2025
Accepted: 28 November 2025

Abstract

The well-known correlation between stellar metallicity and the presence of planets is strongest for giant planets and weaker for smaller planets, suggesting that detailed elemental patterns beyond [Fe/H] may be relevant. Using abundances from the fourth data release of the GALAH spectroscopic survey, we analyzed 104 host stars with 141 confirmed transiting planets. We divided the planets at r_p = 2.6 R_⊕, the theoretical threshold radius above which planets are unlikely to be pure-water worlds. We find that large-planet hosts are enriched by ~0.2 dex in iron and show a possible excess of highly volatile elements (C, N, and O), though these measurements are affected by observational limitations, whereas small-planet hosts exhibit an enhanced contribution of the classical rock-forming elements (Mg, Si, Ca, and Ti) relative to iron; this corresponds to a modest [Rock/Fe] offset of 0.06 dex, which is statistically significant, with a p value of 10⁻⁴. These offsets remain significant for alternative radius cuts. A matched control sample of non-planet-host stars shows only weak and mostly statistically insignificant similar trends, confirming that the stronger chemical signatures are linked to the planetary characteristics. As our study relies on transiting planets, it mainly probes short-period systems (P < 100 days). These results refine the planet–metallicity relation, highlighting the role of the relative balance between iron, volatiles, and rock-forming elements in planet formation.