Formation theory for the observed high-density exoplanets. (a) Super-Earths are predicted to complete their formation through giant impacts whose energies are too low to strip their rocky mantles and form metal-rich planets (Scora et al. 2020, 2022; Poon et al. 2020; Esteves et al. 2022; Goldberg & Batygin 2022). (b) After their formation ceases, super-Earths can remain on closely spaced orbits typical of observed compact systems (Weiss et al. 2023) and can experience instabilities over timescales comparable to the main-sequence lifetime of their host stars (Volk & Gladman 2015; Pu & Wu 2015; Tamayo et al. 2020; Volk & Malhotra 2020). (c) If a compact system becomes unstable, super-Earths can experience a second stage of giant impacts that are more energetic than those that occurred during their formation (Volk & Gladman 2015). These late giant impacts may erode the silicate mantles of the super-Earths to form metal-rich, high-density planets. (d) The post-instability planetary systems may have a metal-rich world with density akin to the measured densities of observed high-density exoplanets (Table 1). Colors indicate different planetary compositions: orange is metal-rich, blue indicates planets with a terrestrial composition (that is, differentiated in an iron core and a rocky mantle with core-mass fraction ∼30 wt.%).