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The ubiquity of planets poses an interesting question: when first planets areformed in galaxies. We investigate this problem by adopting a theoretical modeldeveloped for understanding the statistical properties of exoplanets. Our modelis constructed as the combination of planet traps with the standard coreaccretion scenario in which the efficiency of forming planetary cores directlyrelates to the dust density in disks or the metallicity ([Fe/H]). Westatistically compute planet formation frequencies (PFFs) as well as theorbital radius ($<R_{rapid}>$) within which gas accretion becomes efficientenough to form Jovian planets. The three characteristic exoplanetarypopulations are considered: hot Jupiters, exo-Jupiters densely populated around1 AU, and low-mass planets such as super-Earths. We explore the behavior of thePFFs as well as $<R_{rapid}>$ for the three different populations as a functionof metallicity ($-2 \leq$[Fe/H]$\leq -0.6$). We show that the total PFFsincrease steadily with metallicity, which is the direct outcome of the coreaccretion picture. For the entire range of the metallicity considered here, thepopulation of the low-mass planets dominates over the Jovian planets. TheJovian planets contribute to the PFFs above [Fe/H]-1. We find that the hotJupiters form at lower metallcities than the exo-Jupiters. This arises from theradially inward transport of planetary cores by their host traps, which is moreeffective for lower metallicity disks due to the slower growth of the cores.The PFFs for the exo-Jupiters exceed those for the hot Jupiters around[Fe/H]-0.7. Finally, we show that the critical metallicity for forming Jovianplanets is [Fe/H]-1.2, which is evaluated by comparing the values of$<R_{rapid}>$ between the hot Jupiters and the low-mass planets. The comparisonintrinsically links to the different gas accretion efficiency between them.

Planet Traps and First Planets: the Critical Metallicity for Gas Giant Formation