# Absence of a metallicity effect for ultra-short-period planets

## TL;DR

This study investigates whether ultra-short-period planets are the remnants of hot Jupiters by analyzing stellar metallicities, finding that USP planets are not predominantly associated with metal-rich stars, unlike hot Jupiters.

## Contribution

The paper provides observational evidence that USP planets are unlikely to be evaporated hot Jupiters, challenging previous hypotheses about their origins.

## Key findings

- USP host stars have different metallicity distributions from hot Jupiter hosts.
- USP planets' host stars have similar metallicities to stars with short-period 2-4 R⊕ planets.
- USP planets are likely not the cores of formerly gaseous hot Jupiters.

## Abstract

Ultra-short-period (USP) planets are a newly recognized class of planets with periods shorter than one day and radii smaller than about 2 Earth radii. 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. We find the metallicity distributions of USP-planet and hot-Jupiter hosts to be significantly different ($p = 3\times 10^{-4}$), 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 stars with USP planets is indistinguishable from that of stars with short-period planets with sizes between 2--4~$R_\oplus$. Thus it remains possible that the USP planets are the solid cores of formerly gaseous planets smaller than Neptune.