We present a statistical study of the planet-metallicity ( P-M ) correlation , by comparing the 744 stars with candidate planets ( SWPs ) in the Kepler field which 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 the first time a turn-off P-M correlation of \Delta [ Fe/H ] _ { SWPs - SNPs } , which in average increases from \sim 0.00 \pm 0.03 dex to 0.06 \pm 0.03 dex , and to 0.12 \pm 0.03 for stars with Earth , Neptune , Jupiter-sized planets successively , and then declines to \sim - 0.01 \pm 0.03 dex for more massive planets or brown dwarfs .
Moreover , the percentage of those systems with positive \Delta [ Fe/H ] has the same turn-off pattern .
We also find FG-type stars follow this general trend , but K-type stars are different .
Moderate metal enhancement ( \sim 0.1 - 0.2 dex ) for K-type stars with planets of radii between 2 to 4 R _ { \oplus } as compared to CKSNPs is observed , which indicates much higher metallicities are required for Super-Earths , Neptune-sized planets to form around K-type stars .
We point out that the P-M correlation is actually metallicity-dependent , i.e. , the correlation is positive at solar and super-solar metallicities , and negative at subsolar metallicities .
No steady increase of \Delta [ Fe/H ] against planet sizes is observed for rocky planets , excluding the pollution scenario as a major mechanism for the P-M correlation .
All these clues suggest that giant planets probably form differently from rocky planets or more massive planets/brown dwarfs , and the core-accretion scenario is highly favoured , and high metallicity is a prerequisite for massive planets to form .