the First planet in the universe
Planet formation needs raw materials - metals, i.e., elements other than H and He. Metals take time to build since the onset of the first star (see the following models by James Johnson). The two dashed lines roughly mark the time when the first rocky planet and gas-giant planet formed when the metallicity is high enough.
I studied how metallicity affects planet formation and found that more metal-rich stars tend to have more planets. With my graduate students, we are searching for the most metal-poor planet systems and investigating how metallicity affects their formation and habitability.