Relationship between central densities of open clusters (ρ_c) and their ages (t_age). The data are from Lada & Lada (2003) for left-pointing triangles, Bonatto & Bica (2005) for down-pointing triangles, and Piskunov et al. (2007) and Meibom et al. (2002) for circles and red diamonds (planet-harboring clusters). Each color represents the total mass of each open cluster: blue (100–300 M_⊙), green (300–600 M_⊙), and pink (600–1000 M_⊙). Our target clusters and planet-harboring clusters are shown by filled black squares and red diamonds, respectively.