Gravity-sensitive Spectral Indices in Ultracool Dwarfs: Investigating Correlations with Metallicity and Planet Occurrence Using SpeX and Fire Observations

Abstract We present a near-infrared spectroscopic analysis (0.9–2.4 μ m) of gravity indices for 56 ultracool dwarfs (M5.5–L0), including exoplanet hosts SPECULOOS-2, SPECULOOS-3, and LHS 3154. Our dataset includes 59 spectra from the SpeX and FIRE spectrographs. We also discuss literature results for TRAPPIST-1. Using gravity-sensitive spectral indices, including FeH absorption (0.99, 1.20, and 1.55 μ m), the VO band at 1.06 μ m, the H -band continuum, and alkali lines such as K I (1.17 and 1.25 μ m), we investigate correlations between gravity classification, stellar metallicity, and the presence of close-in transiting planets. All four planet-hosting stars exhibit intermediate-gravity spectral signatures, despite indicators of field age. However, a volume-corrected logistic regression reveals no significant association between gravity class and planet occurrence. Among individual indices, we find FeH z to be the most promising tracer of planet-hosting status. We tentatively identify a correlation between FeH z (0.99 μ m) and planet presence at the 2 σ level, though the result may reflect observational biases, including transit probability, small-number statistics, and detection sensitivity. More robustly, we find a significant anticorrelation between FeH z and [Fe/H] (3.3 σ ). A Kruskal–Wallis test shows no significant [Fe/H] difference across gravity classes, suggesting the observed FeH z –[Fe/H] trend is not driven by bulk metallicity differences. We propose this anticorrelation may reflect the interplay between age, gravity, and composition: higher-metallicity objects may be systematically younger and have lower gravities, suppressing FeH absorption. While our results only hint at a link between gravity-related characteristics and planet occurrence among late-M dwarfs, they underscore the need for caution when using spectral diagnostics to infer the properties of planet-hosting ultracool dwarfs.