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We present a statistical study of the planet-metallicity (P-M) correlation,by comparing the 744 stars with candidate planets (SWPs) in the Kepler fieldwhich have been observed with LAMOST, and a sample of distance-independent,fake "twin" stars in the Kepler field with no planet reported (CKSNPs) yet.With the well-defined and carefully-selected large samples, we find for thefirst time a turn-off P-M correlation of Delta [Fe/H]_(SWPs-SNPs), which inaverage increases from ~0.00+-0.03 dex to 0.06+-0.03 dex, and to 0.12+-0.03 forstars with Earth, Neptune, Jupiter-sized planets successively, and thendeclines to ~-0.01+-0.03 dex for more massive planets or brown dwarfs.Moreover, the percentage of those systems with positive Delta[Fe/H] has thesame turn-off pattern. We also find FG-type stars follow this general trend,but K-type stars are different. Moderate metal enhancement (~0.1-0.2 dex) forK-type stars with planets of radii between 2 to 4 Earth radius as compared toCKSNPs is observed, which indicates much higher metallicities are required forSuper-Earths, Neptune-sized planets to form around K-type stars. We point outthat the P-M correlation is actually metallicity-dependent, i.e., thecorrelation is positive at solar and super-solar metallicities, and negative atsubsolar metallicities. No steady increase of Delta[Fe/H] against planet sizesis observed for rocky planets, excluding the pollution scenario as a majormechanism for the P-M correlation. All these clues suggest that giant planetsprobably form differently from rocky planets or more massive planets/browndwarfs, and the core-accretion scenario is highly favoured, and highmetallicity is a prerequisite for massive planets to form.

Giant planets around FGK stars form probably through core accretion