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.

中文翻译：

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LAMOST望远镜揭示，热木星的海王星表亲大多是富含重元素的恒星的单个后代

我们发现了一个短周期的海王星大小的行星，它们与热木星有着关键的相似之处：这两个行星群都优先由富含金属的恒星占据，并且都优先在具有单瞬变行星的开普勒系统中找到。我们使用主场恒星的精确大天空区多目标光纤光谱望远镜（LAMOST）数据发布4（DR4）恒星参数来研究短周期<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） 开普勒的分布行星与主恒星金属性的关系。与贫金属的恒星周围的行星相比，富金属的恒星周围的行星的半径分布更“膨胀”。在两个时期半径范围内，行星优先居住在富含金属的恒星周围，而几乎没有行星存在于贫金属的恒星周围。一个是著名的炽热木星，另一个是海王星大小的行星（<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⊕），被称为“合欢”。就像热木星一样，跳频在具有单过渡行星的系统中更频繁地发生，尽管以多种形式出现的跳频的比例要大于热木星。大约<mml:math><mml:mrow><mml:mn>1</mml:mn><mml:mo>%</mml:mo></mml:mrow></mml:math>1％太阳型恒星中有许多恒星伴有跳动，而跳动和木星的频率会随着[Fe / H]的变化而以一致的趋势增加。在行星半径分布中，热木星和合子由一个“谷”隔开，大约为土星大小（在<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⊕范围内），而这个“热土星谷”大约代表一个土星的数量级。与炎热的木星和跳频相比，行星频率的幅度降低。Hoptunes和炎热的木星之间的经验“亲属关系”表明可能存在共同的过程（迁移和/或形成）。