The metallicity of exoplanet systems serves as a critical diagnostic of planet formation mechanisms. Previous studies have demonstrated the planet-metallicity correlation for large planets (R P ≥ 4 R E ); however, a correlation has not been found for smaller planets. With a sample of 406 Kepler Objects of Interest whose stellar properties are determined spectroscopically, we reveal a universal planet-metallicity correlation: not only gas-giant planets (3.9 R E < R P ≤ 22.0 R E ) but also gas-dwarf (1.7 R E < R P ≤ 3.9 R E ) and terrestrial planets (R P ≤ 1.7 R E ) occur more frequently in metal-rich stars. The planet occurrence rates of gas-giant planets, gas-dwarf planets, and terrestrial planets are 9.30 +5.62 −3.04 , 2.03 +0.29 −0.26 , and 1.72 +0.19 −0.17 times higher for metal-rich stars than for metal-poor stars, respectively. Subject headings:
Almost half of the stellar systems in the solar neighborhood are made up of multiple stars. In multiple-star systems, planet formation is under the dynamical influence of stellar companions, and the planet occurrence rate is expected to be different from that for single stars. There have been numerous studies on the planet occurrence rate of single star systems. However, to fully understand planet formation, the planet occurrence rate in multiplestar systems needs to be addressed. In this work, we infer the planet occurrence rate in multiple-star systems by measuring the stellar multiplicity rate for planet host stars. For a sub-sample of 56 Kepler planet host stars, we use adaptive optics (AO) imaging and the radial velocity (RV) technique to search for stellar companions. The combination of these two techniques results in high search completeness for stellar companions. We detect 59 visual stellar companions to 25 planet host stars with AO data. Three stellar companions are within 2 ′′ , and 27 within 6 ′′ . We also detect 2 possible stellar companions (KOI 5 and KOI 69) showing long-term RV acceleration. After correcting for a bias against planet detection in multiple-star systems due to flux contamination, we find that planet formation is suppressed in multiple-star systems with separations smaller than 1500 AU. Specifically, we find that compared to single star systems, planets in multiple-star systems occur 4.5 ± 3.2, 2.6±1.0, and 1.7±0.5 times less frequently when a stellar companion is present at a distance of 10, 100, and 1000 AU, respectively. This conclusion applies only to circumstellar planets; the planet occurrence rate for circumbinary planets requires further investigation.
As hundreds of gas giant planets have been discovered, we study how these planets form and evolve in different stellar environments, specifically in multiple stellar systems. In such systems, stellar companions may have a profound influence on gas giant planet formation and evolution via several dynamical effects such as truncation and perturbation. We select 84 Kepler Objects of Interest (KOIs) with gas giant planet candidates. We obtain highangular resolution images using telescopes with adaptive optics (AO) systems. Together with the AO data, we use archival radial velocity data and dynamical analysis to constrain the presence of stellar companions. We detect 59 stellar companions around 40 KOIs for which we develop methods of testing their physical association. These methods are based on color information and galactic stellar population statistics. We find evidence of suppressive planet formation within 20 AU by comparing stellar multiplicity. The stellar multiplicity rate for planet host stars is 0 +5 −0 % within 20 AU. In comparison, the stellar multiplicity rate is 18%±2% for the control sample, i.e., field stars in the solar neighborhood. The stellar multiplicity rate for planet host stars is 34%±8% for separations between 20 and 200 AU, which is higher than the control sample at 12%±2%. Beyond 200 AU, stellar multiplicity rates are comparable between planet host stars and the control sample. We discuss the implications of the results to gas giant planet formation and evolution.
The census of exoplanets is incomplete for orbital distances larger than 1 AU. Here, we present 41long-period planet candidates in 38systems identified by Planet Hunters based on Kepler archival data (Q0-Q17). Among them, 17exhibit only one transit, 14have two visible transits, and 10have more than three visible transits. For planet candidates with only one visible transit, we estimate their orbital periods based on transit duration and host star properties. The majority of the planet candidates in this work (75%) have orbital periods that correspond to distances of 1-3 AU from their host stars. We conduct follow-up imaging and spectroscopic observations to validate and characterize planet host stars. In total, we obtain adaptive optics images for 33stars to search for possible blending sources. Six stars have stellar companions within 4″. We obtain high-resolution spectra for 6stars to determine their physical properties. Stellar properties for other stars are obtained from the NASA Exoplanet Archive and the Kepler Stellar Catalog by Huber et al. We validate 7 planet candidates that have planet confidence over 0.997 (3σ level). These validated planets include 3 single-transit planets (KIC-3558849b, KIC5951458b, and KIC-8540376c), 3 planets with double transits (KIC-8540376b, KIC-9663113b, and KIC10525077b), and 1 planet with four transits (KIC-5437945b). This work provides assessment regarding the existence of planets at wide separations and the associated false positive rate for transiting observation (17%-33%). More than half of the long-period planets with at least three transits in this paper exhibit transit timing variations up to 41 hr, which suggest additional components that dynamically interact with the transiting planet candidates. The nature of these components can be determined by follow-up radial velocity and transit observations.
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