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We investigate and compare the composition of M-dwarf planets in systems withonly one known planet (``singles") to those residing in multi-planet systems(``multis") and the fundamental properties of their host stars. We restrict ouranalysis to planets with directly measured masses and radii, which comprise atotal of 70 planets: 30 singles and 40 multis in 19 systems. We compare thebulk densities for the full sample, which includes planets ranging in size from$0.52 R_{\oplus}$ to $12.8R_\oplus$, and find that single planets havesignificantly lower densities on average than multis, which we cannot attributeto selection biases. We compare the bulk densities normalized by an Earth modelfor planets with $R_{p} < 6R_{\oplus}$, and find that multis are also denserwith 99\% confidence. We calculate and compare the core/water mass fractions(CMF/WMF) of low-mass planets ($M_p <10 M_{\oplus}$), and find that the likelyrocky multis (with $R_p <1.6 R_{\oplus}$) have lower CMFs than singles. We alsocompare the [Fe/H] metallicity and rotation period of all single versusmulti-planet host stars with such measurements in the literature and find thatmulti-planet hosts are significantly more metal-poor than those hosting asingle planet. Moreover, we find that host star metallicity decreases withincreasing planet multiplicity. In contrast, we find only a modest differencein the rotation period. The significant differences in planetary compositionand metallicity of the host stars point to different physical processesgoverning the formation of single- and multi-planet systems in M dwarfs.

A Comparison of the Composition of Planets in Single- and Multi-Planet Systems Orbiting M dwarfs