An upper boundary in the mass-metallicity plane of exo-Neptunes

With the progress of detection techniques, the number of low-mass and small-size exoplanets is increasing rapidly. However their characteristics and formation mechanisms are not yet fully understood. The metallicity of the host star is a critical parameter in such processes and can impact the occurrence rate or physical properties of these planets. While a frequency-metallicity correlation has been found for giant planets, this is still an ongoing debate for their smaller counterparts. Using the published parameters of a sample of 157 exoplanets lighter than 40 M-circle plus, we explore the mass-metallicity space of Neptunes and super-Earths. We show the existence of a maximal mass that increases with metallicity, that also depends on the period of these planets. This seems to favour in situ formation or alternatively a metallicity-driven migration mechanism. It also suggests that the frequency of Neptunes (between 10 and 40M(circle plus)) is, like giant planets, correlated with the host star metallicity, whereas no correlation is found for super-Earths (< 10 M-circle plus).