M Dwarf Singles and Multis Have Dissimilar Metallicities Indicating Different Origins

Abstract

It is unclear if host star metallicities differ between systems with one detected transiting planet ("singles") or multiple detected transiting planets ("multis") around FGK dwarfs. Here, we find strong evidence that they do for M dwarfs. We use a homogeneous sample of FGK and M dwarf chemical abundances to demonstrate that M dwarf multis are significantly more metal-poor than singles ($p$ = 0.00079), even when we only consider small (<4 $R_{\oplus}$) planets ($p$ = 0.0013). This result emerges at the late-K to M dwarf transition. We observe that M dwarf multis become more metal-poor as a function of increasing planet multiplicity. Additionally, planets in singles have significantly higher eccentricities and shorter orbital periods compared to planets in multis. We interpret these findings as evidence that M dwarf singles lead relatively active dynamical lives, and the different planetary architectures of singles and multis arise from distinct dynamics influenced by metallicity. This may not occur in FGK systems because their more massive protoplanetary disks may supply enough planet-forming material to instigate dynamics that lead to only one transiting planet, even when the host star metallicity is low. Another possibility is that multis are less compact around FGK dwarfs because they form from larger disks, which may often cause them to be observed as singles in transit data.