Final answer:
Exoplanets, including "hot Jupiters," can be closer to their stars than Mercury due to planetary migration from where they originally formed. The discovery of various-sized exoplanets has broadened our understanding of planet formation and distribution.
Step-by-step explanation:
The presence of large exoplanets close to their stars, such as "hot Jupiters," and the occurrence of planetary migration are distinctive aspects of some exoplanetary systems. Our understanding has evolved with the discovery of such systems, leading to the conclusion that giant planets may migrate inward from their colder, ice-forming regions of origin. Additionally, the discovery of a variety of exoplanets including Earth-sized and those larger than Neptune has expanded our knowledge of planet formation and distribution in the galaxy. Planetary migration and the interaction with a star's gravitational field are primary mechanisms explaining why exoplanets can be much closer to their stars than Mercury is to the Sun.