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, 115 (2), 266-271

LAMOST Telescope Reveals That Neptunian Cousins of Hot Jupiters Are Mostly Single Offspring of Stars That Are Rich in Heavy Elements

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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

Subo Dong et al. Proc Natl Acad Sci U S A.

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 [Formula: see text] 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 ([Formula: see text]), 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 [Formula: see text] 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 [Formula: see text]), 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.

Keywords: exoplanets; metallicity; transit.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Period–radius distribution for short-period Kepler planet candidates hosted by metal-rich (Top) and metal-poor (Bottom) stars. Planets in single- and multiple-transiting planetary systems are plotted in blue and red circles, respectively. The numbers of planets and stars (including nontransiting targets) are shown on the top of each panel. The dark green horizontal line (Rp=10R) denotes the empirical lower boundary of hot Jupiter, and the magenta lines denote the empirical boundaries of Hoptunes (Analysis and Results, iii) Hoptunes and Hot Jupiters Are Separated by a Hot-Saturn Valley). Note that the typical uncertainty in Rp is about 15%.
Fig. 2.
Fig. 2.
Cumulative fractions as a function of host [Fe/H]. Hoptunes (magenta) and hot Jupiters (green) have similar distributions, and using the two-sample K-S test, the two samples are drawn from the same underlying distribution with a probability of 45%. In contrast, the distributions for both Hoptunes and hot Jupiters are different from those of the stellar sample (blue) and other planets (black) at high statistical significance.
Fig. 3.
Fig. 3.
The hot-Saturn valley revealed from the radius distribution of planets in our sample. (Top) Cumulative distribution of planets with radii larger than 4 R. (Middle) Number of detected planets in radius bins with equal size in logarithm. (Bottom) Intrinsic planet frequencies as a function of planetary radius.
Fig. 4.
Fig. 4.
The dependence of planet distribution (Left) and intrinsic frequency (Right) on stellar metallicity ([Fe/H]), using three metallicity subsamples. The frequencies of hot Jupiters and Hoptunes have similar trends as a function of metallicity—for both populations, they increase by a factor of 10 from the “subsolar” metallicity regime ([Fe/H]0.1) to the supersolar regime ([Fe/H]0.1), and the frequencies of Hoptunes are similar (within a factor of 2) to those of hot Jupiters for all subsamples.

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