Absence of a Metallicity Effect for Ultra-short-period Planets

Ultra-short-period (USP) planets are a newly recognized class of planets with periods shorter than one day, and radii smaller than about 2 R⊕. It has been proposed that USP planets are the solid cores of hot Jupiters that lost their gaseous envelopes due to photo-evaporation or Roche lobe overflow. We test this hypothesis by asking whether USP planets are associated with metal-rich stars, as has long been observed for hot Jupiters. In contrast, we find the metallicity distributions of USP and hot-Jupiter hosts to be significantly different (p = 2× 10−4), based on Keck spectroscopy of Kepler stars. Evidently the sample of USP planets is not dominated by the evaporated cores of hot Jupiters. The metallicity distribution of USP hosts is indistinguishable from that of short-period planets with sizes between 2-4 R⊕. Thus it remains possible that the USP planets are the solid cores of formerly gaseous sub-Neptune planets.