The First Evidence of a Metallicity Cliff in the Formation of Super-Earths

Planet formation models predict that below a certain protoplanetary disk metallicity, the surface density of solid material is too low to form planets via core accretion. While previous studies have found a weak correlation between the occurrence rates of small planets (< 4R⊕) and stellar metallicity, so far no studies have probed below the metallicity limit beyond which planet formation is predicted to be suppressed. Here, we constructed a large catalog of ∼110,000 metal-poor stars observed by TESS with spectroscopically derived metallicities, and systematically probed planet formation within the metal-poor regime ([Fe/H] ≤ -0.5) for the first time. Extrapolating known higher-metallicity trends for small, short-period planets predicts the discovery of ∼ 68 super-Earths accounting for search completeness; however, we detect none. As a result, we have placed the most stringent upper limit on super-Earth occurrence rates around metal-poor stars to date, a 5.4 σ deviation from the prediction of metallicity trends derived with Kepler and K2. We find a host star metallicity cliff for super-Earths that could indicate the threshold below which planets are unable to grow beyond an Earth-mass at short orbital periods.