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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

Research paper by Subo Dong, Ji-Wei Xie, Ji-Lin Zhou, Zheng Zheng, Ali Luo

Indexed on: 29 Dec '17Published on: 28 Dec '17Published in: PNAS



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 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 <mml:math><mml:mrow><mml:mo stretchy="false">(</mml:mo><mml:mrow><mml:mrow><mml:mpadded width="+1.7pt"><mml:mn>1</mml:mn></mml:mpadded><mml:mpadded width="+1.7pt"><mml:mi mathvariant="normal">d</mml:mi></mml:mpadded></mml:mrow><mml:mo><</mml:mo><mml:mpadded width="+1.7pt"><mml:mi>P</mml:mi></mml:mpadded><mml:mo><</mml:mo><mml:mrow><mml:mpadded width="+1.7pt"><mml:mn>10</mml:mn></mml:mpadded><mml:mi mathvariant="normal">d</mml:mi></mml:mrow></mml:mrow><mml:mo stretchy="false">)</mml:mo></mml:mrow></mml:math>(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 (<mml:math><mml:mrow><mml:mn>2</mml:mn><mml:mpadded width="+1.7pt"><mml:msub><mml:mi>R</mml:mi><mml:mo>⊕</mml:mo></mml:msub></mml:mpadded><mml:mpadded width="+1.7pt"><mml:mo>≲</mml:mo></mml:mpadded><mml:mpadded width="+1.7pt"><mml:msub><mml:mi>R</mml:mi><mml:mi>p</mml:mi></mml:msub></mml:mpadded><mml:mo>≲</mml:mo><mml:mn>6</mml:mn><mml:msub><mml:mi>R</mml:mi><mml:mo>⊕</mml:mo></mml:msub></mml:mrow></mml:math>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 <mml:math><mml:mrow><mml:mn>1</mml:mn><mml:mo>%</mml:mo></mml:mrow></mml:math>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 <mml:math><mml:mrow><mml:mn>6</mml:mn><mml:mpadded width="+1.7pt"><mml:msub><mml:mi>R</mml:mi><mml:mo>⊕</mml:mo></mml:msub></mml:mpadded><mml:mpadded width="+1.7pt"><mml:mo>≲</mml:mo></mml:mpadded><mml:mpadded width="+1.7pt"><mml:msub><mml:mi>R</mml:mi><mml:mi>p</mml:mi></mml:msub></mml:mpadded><mml:mo>≲</mml:mo><mml:mn>10</mml:mn><mml:msub><mml:mi>R</mml:mi><mml:mo>⊕</mml:mo></mml:msub></mml:mrow></mml:math>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.