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

S. Dong,Jiwei Xie,Ji-lin Zhou,Zheng Zheng,A. Luo

Published 2017 in Proceedings of the National Academy of Sciences of the United States of America

ABSTRACT

Significance Hot Jupiters are Jupiter-size planets at £1/10 of the Sun–Earth distance, and even though they were the first exoplanet population discovered around sun-like stars, their origins still remain elusive. Using data from NASA’s Kepler satellite and China’s Large Sky Area Multi-Object Fiber Spectroscopic Telescope, we discover a population of close-in Neptune-size planets (called “Hoptunes”) that share key similarities with hot Jupiters. Like hot Jupiters, Hoptunes prefer to reside around stars with higher metal abundance than the Sun. Nearly half of the Kepler planets are discovered in systems with multiple transiting planets, but both hot Jupiters and Hoptunes are preferentially found in single-transiting planet systems. The “kinship” between hot Jupiters and Hoptunes suggests likely common origins and offers fresh clues into the formation of these exotic close-in planets. 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 Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) Data Release 4 (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” compared with 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 one is a population of Neptune-size planets (2R⊕≲Rp≲6R⊕), dubbed “Hoptunes.” Also like hot Jupiters, Hoptunes occur more frequently in systems with single-transiting planets although 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 6R⊕≲Rp≲10R⊕), and this “hot-Saturn valley” represents approximately an order-of-magnitude decrease in planet frequency compared with hot Jupiters and Hoptunes. The empirical “kinship” between Hoptunes and hot Jupiters suggests likely common processes (migration and/or formation) responsible for their existence.

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