# LAMOST telescope reveals that Neptunian cousins of hot Jupiters are mostly single offspring of stars that are rich in heavy elements

## TL;DR

This study reveals that Neptunian-sized planets called Hoptunes, like hot Jupiters, are mainly found around metal-rich stars and are often single-planet systems, indicating shared formation or migration processes.

## Contribution

It identifies a new population of Neptune-sized planets called Hoptunes and demonstrates their similarities with hot Jupiters in terms of host star metallicity and system architecture.

## Key findings

- Hoptunes are more common around metal-rich stars.
- Hoptunes and hot Jupiters are separated by a 'hot-Saturn valley'.
- Both populations are often found as single-planet systems.

## Abstract

We discover a population of short-period, Neptune-size planets sharing key similarities with hot Jupiters: both populations are preferentially hosted by metal-rich stars, and both are preferentially found in Kepler systems with single transiting planets. We use accurate LAMOST DR4 stellar parameters for main-sequence stars to study the distributions of short-period 1d < P < 10d Kepler planets as a function of host star metallicity. The radius distribution of planets around metal-rich stars is more "puffed up" as compared to that around metal-poor hosts. In two period-radius regimes, planets preferentially reside around metal-rich stars, while there are hardly any planets around metal-poor stars. One is the well-known hot Jupiters, and the other is a population of Neptune-size planets (2 R_Earth <~ R_p <~ 6 R_Earth), dubbed as "Hoptunes". Also like hot Jupiters, Hoptunes occur more frequently in systems with single transiting planets though the fraction of Hoptunes occurring in multiples is larger than that of hot Jupiters. About 1% of solar-type stars host "Hoptunes", and the frequencies of Hoptunes and hot Jupiters increase with consistent trends as a function of [Fe/H]. In the planet radius distribution, hot Jupiters and Hoptunes are separated by a "valley" at approximately Saturn size (in the range of 6 R_Earth <~ R_p <~ 10 R_Earth), and this "hot-Saturn valley" represents approximately an order-of-magnitude decrease in planet frequency compared to hot Jupiters and Hoptunes. The empirical "kinship" between Hoptunes and hot Jupiters suggests likely common processes (migration and/or formation) responsible for their existence.